Samacheer Kalvi Class 9 Science Solutions Chapter 7 Heat

Tamilnadu Samacheer Kalvi 9th Science Solutions Chapter 7 Heat

9th Science Guide Heat Text Book Back Questions And Answers

I. Choose The Correct Answer:

 

Question 1. Calorie is the unit of
(a) heat
(c) temperature
(d) food
Answer: (a) heat
In simple words: A calorie is a unit used to measure heat energy. It tells us how much heat is needed to warm up a certain amount of water.

🎯 Exam Tip: Remember that "calorie" is a unit of energy, specifically heat, while "temperature" measures the degree of hotness or coldness.

 

Question 2. SI unit of temperature is
(a) fahrenheit
(b) joule
(c) Celsius
(d) kelvin
Answer: (d) kelvin
In simple words: The standard unit for measuring temperature in science is Kelvin. While Celsius and Fahrenheit are also used, Kelvin is the official SI unit.

🎯 Exam Tip: Know the SI units for basic physical quantities. For temperature, Kelvin is crucial in scientific contexts, especially for absolute temperature measurements.

 

Question 3. Two cylindrical rods of same length have the area of cross section in the ratio 2:1. If both the rods are made up of same material, which of them conduct heat faster?
(a) Both rods
(b) Rod-2
(c) Rod-1
(d) None of the options
Answer: (c) Rod-1
In simple words: A rod with a larger cross-sectional area will conduct heat faster because there is more space for heat to travel through. Since Rod-1 has a cross-section ratio of 2 compared to Rod-2's 1, Rod-1 is wider and conducts heat better.

🎯 Exam Tip: Heat conduction rate is directly proportional to the cross-sectional area of the material. A larger area means more paths for heat transfer.

 

Question 4. In which mode of transfer of heat, molecules pass on heat energy to molecules without actually moving from their positions?
(a) Radiation
(b) Conduction
(c) Convection
(d) Both B and C
Answer: (b) Conduction
In simple words: In conduction, heat moves from one particle to the next without the particles themselves changing their location. This usually happens in solids.

🎯 Exam Tip: Distinguish clearly between conduction (vibration of fixed particles), convection (movement of fluid particles), and radiation (energy transfer through electromagnetic waves).

 

Question 5. A device in which the loss of heat due to conduction, convection and radiation is minimized is
(a) Solar cell
(b) Solar cooker
(c) Thermometer
(d) Thermos flask
Answer: (d) Thermos flask
In simple words: A thermos flask is designed to keep things hot or cold by stopping heat from moving in or out through conduction, convection, and radiation. It uses vacuum and shiny surfaces to block all three types of heat transfer.

🎯 Exam Tip: Understand the role of vacuum (stops conduction/convection) and silvered surfaces (stops radiation) in a thermos flask for effective heat insulation.

II. Fill In The Blanks:

 

Question 1. The fastest mode of heat transfer is ....................
Answer: radiation
In simple words: Radiation is the quickest way heat can travel, as it moves through waves and does not need any material to pass through.

🎯 Exam Tip: Remember that radiation travels at the speed of light and can pass through a vacuum, making it the fastest method of heat transfer.

 

Question 2. During day time, air blows from .................... to....................
Answer: sea to land
In simple words: During the day, the land gets hotter than the sea. This makes the air above the land rise, and cooler air from over the sea moves in to take its place. This movement is called sea breeze.

🎯 Exam Tip: The direction of local breezes (sea breeze, land breeze) is determined by temperature differences between land and water, which cause air pressure changes.

 

Question 3. Liquids and gases are generally .................... conductors of heat.
Answer: poor
In simple words: Liquids and gases are not very good at letting heat pass through them by conduction. This is because their particles are far apart and move freely, so they don't easily bump into each other to pass heat along.

🎯 Exam Tip: While liquids and gases are poor conductors, they are excellent at transferring heat through convection due to the movement of their particles.

 

Question 4. The fixed temperature at which matter changes state from solid to liquid is called.
Answer: melting point
In simple words: The melting point is the specific temperature at which a solid substance turns into a liquid. At this point, the substance absorbs heat without its temperature rising.

🎯 Exam Tip: Remember that during a phase change (like melting), the temperature remains constant as latent heat is absorbed to change the state, not the temperature.

III. Assertion And Reason Type Questions:

Mark the correct choice as:
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of assertion.
(c) If assertion is true but reason is false.
(d) If assertion is false but reason is true.

 

Question 1. Assertion : Food can be cooked faster in vessels with copper bottom. Reason: Copper is the best conductor of heat.
Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion.
In simple words: Copper spreads heat very well, so pots with copper bottoms get hot quickly and evenly. This helps food cook faster because heat reaches all parts of the food efficiently.

🎯 Exam Tip: When evaluating Assertion-Reason questions, first check if both statements are individually true. Then, determine if the reason directly explains the assertion.

 

Question 2. Assertion : Maximum sunlight reaches earth's surface during the noon time. Reason: Heat from the sun reaches earth's surface by radiation.
Answer: (b) If both assertion and reason are true but reason is not the correct explanation of assertion.
In simple words: It's true that most sunlight hits the Earth at noon, and heat from the sun travels by radiation. However, the way heat travels (radiation) doesn't explain *why* more sunlight reaches us at noon; that's due to the sun's direct angle.

🎯 Exam Tip: Differentiate between a true statement and a true explanation. An accurate reason must directly support and clarify the assertion, not just be a related fact.

 

Question 3. Assertion : When water is heated up to 100° C, there is no raise in temperature until all water gets converted into water vapour. Reason: Boiling point of water is 10° C.
Answer: (c) Assertion is true but the reason is false
In simple words: The assertion is true because water's temperature stays at 100°C while it boils and turns into steam, as it absorbs latent heat. The reason is false because water's boiling point is 100°C, not 10°C.

🎯 Exam Tip: Understand that during a phase transition (like boiling), the temperature of the substance remains constant as it absorbs latent heat to change state, not to increase temperature.

IV. Answer Briefly:

 

Question 1. Define conduction.
Answer: Conduction is how heat moves through solid materials. It happens when heat goes from a hotter part to a colder part, without the actual particles of the material moving from their places. The heat energy just passes from one vibrating particle to the next.
In simple words: Conduction is the way heat travels in solids. Hot particles vibrate and pass energy to their cooler neighbors without moving themselves.

🎯 Exam Tip: Remember that conduction primarily occurs in solids, where particles are closely packed and transfer energy through vibrations, not bulk movement.

 

Question 2. Ice is kept in a double-walled container. Why?
Answer: An ice box has two walls, and the space between them is filled with materials that do not conduct heat well. This design helps to block heat from getting in or out. This way, the ice stays cold for a longer time because heat loss is minimized.
In simple words: Ice is kept in a double-walled container with insulating material in between. This stops heat from entering or leaving, keeping the ice cold for longer.

🎯 Exam Tip: Insulating materials and double walls (often with a vacuum or air gap) are crucial for minimizing heat transfer by conduction, convection, and radiation, thus maintaining temperature.

 

Question 3. How does the water kept in an earthen pot remain cool?
Answer: Earthen pots have tiny holes through which some water slowly leaks out to the surface. This water then evaporates because of the heat in the air around the pot. Evaporation takes heat from the pot and the water inside it, making both cooler. This is why water in earthen pots stays cool.
In simple words: Water seeps out of tiny holes in earthen pots and evaporates. This evaporation process takes heat away from the pot and the water, keeping it cool.

🎯 Exam Tip: The principle of evaporative cooling is key here; it's the same reason we feel cool after sweating, as the evaporation of sweat removes heat from our skin.

 

Question 4. Differentiate convection and radiation.
Answer:
Convection:
Convection is when heat transfers through a liquid or gas by the actual movement of its particles. The heated, lighter particles move away from the heat source and are replaced by cooler, denser particles, which then get heated. This creates a current. For example, land and sea breezes are caused by convection currents. Convection always needs a material medium (liquid or gas) to happen.

Radiation:
Radiation is a way heat travels without needing any material in between. Heat is directly transferred from a hot object to a colder one as electromagnetic waves. For instance, the sun's heat reaches Earth through radiation, even across empty space. Radiation can easily occur even in a vacuum.
In simple words: Convection moves heat by making hot liquids or gases move around. Radiation sends heat as waves, like sunlight, and doesn't need air or water to travel.

🎯 Exam Tip: To differentiate, remember: Conduction uses vibrations in solids. Convection uses fluid movement. Radiation uses electromagnetic waves and can travel through empty space.

 

Question 5. Why do people prefer wearing white clothes during summer?
Answer: People choose white or light-colored clothes in summer because these colors reflect a lot of sunlight and heat away. Dark colors, however, absorb more heat. By reflecting heat, light clothes help to keep the body cooler and more comfortable.
In simple words: White clothes reflect heat instead of absorbing it. This keeps people cooler during hot summer days.

🎯 Exam Tip: Good reflectors are poor absorbers and poor emitters of heat, while good absorbers are also good emitters. This principle applies to clothing color for comfort.

 

Question 6. What is specific heat capacity?
Answer: Specific heat capacity is the amount of heat energy needed to raise the temperature of 1 kilogram of a substance by 1 degree Celsius or 1 Kelvin. Every substance has its own specific heat capacity, which tells us how much heat it can store.
In simple words: Specific heat capacity is how much heat you need to add to 1 kg of a material to make its temperature go up by 1 degree.

🎯 Exam Tip: Pay attention to the units (per kg, per °C or K) in the definition of specific heat capacity, as it refers to a unit mass of the substance.

 

Question 7. Define thermal capacity.
Answer:
Thermal capacity, also called heat capacity, is the total amount of heat energy required to increase the temperature of an entire object by 1 degree Celsius. It is shown by the symbol 'C'. This property considers the whole body, not just a unit mass.
\( C = \frac{Q}{\Delta T} \) , where C is the heat capacity, Q is the amount of heat needed, and \( \Delta T \) is the change in temperature.
The SI unit for heat capacity is J/K (Joules per Kelvin). It can also be expressed as J/°C, cal/°C, or kcal/°C.
In simple words: Thermal capacity is the heat needed to warm up a whole object by one degree. It's about the whole thing, not just a small part.

🎯 Exam Tip: Distinguish between specific heat capacity (per unit mass) and thermal capacity (for the entire body). Thermal capacity equals mass times specific heat capacity.

 

Question 8. Define specific latent heat capacity.
Answer: Specific latent heat capacity is the amount of heat energy that a substance either takes in or gives out when it changes its state (like melting or boiling) without its temperature changing. This energy is measured per unit mass of the substance.
In simple words: Specific latent heat is the hidden heat absorbed or released when a material changes from one state to another, like ice to water, without changing temperature.

🎯 Exam Tip: Latent heat refers to energy involved in phase change, where temperature remains constant. Specific latent heat makes it about a unit mass of the substance.

V. Answer In Detail:

 

Question 1. Explain convection in daily life.
Answer: Convection is a process where heat moves through a fluid (liquid or gas) because the fluid itself moves from warmer areas to cooler areas. This is a common way heat transfers around us.

Hot Air Balloons: When a heat source warms the air at the bottom of a hot air balloon, this warm air becomes lighter and rises. Cooler, denser air then sinks down to take its place and gets heated too. This cycle continues, and when enough hot air is collected inside the balloon, it gains lift and rises.

Breeze (Sea and Land Breezes):
During the day, land heats up faster than the sea. The warm air above the land rises, and cooler, denser air from over the sea moves in to replace it. This moving air is known as a sea breeze. At night, the land cools faster than the sea. The air over the warmer sea rises, and cooler air from the land moves towards the sea, creating a land breeze. This constant circulation is a form of convection.

Chimneys: In kitchens and factories, tall chimneys are used to let out hot gases and smoke. These hot gases are lighter than the surrounding air, so they naturally rise up the chimney and escape into the atmosphere. This upward movement is driven by convection, effectively clearing the air indoors.
In simple words: Convection moves heat by making warm liquids or gases rise, and cooler ones sink. This happens in hot air balloons, causes sea and land breezes, and helps smoke go up chimneys.

🎯 Exam Tip: When explaining convection, always mention the density changes of the fluid (hotter fluid becomes less dense and rises, cooler fluid becomes denser and sinks) and its application in real-world scenarios.

 

Question 2. What are the changes of state in water? Explain.
Answer: A change of state happens when a substance goes from one physical form to another at a specific temperature. Here's how water changes its state:

• At normal temperatures, water is a liquid.
• When liquid water is heated to 100°C, it turns into steam, which is a gas. If this steam is cooled down, it turns back into liquid water.
• If liquid water is cooled further to 0°C, it turns into ice, which is a solid. If ice is heated, it turns back into liquid water.
• So, water changes its state when its temperature changes, or when it absorbs or releases heat at a constant temperature (latent heat).

Common changes of state are:

• Melting or Fusion: A solid changing into a liquid by absorbing heat. For example, ice melts into water.
• Freezing: A liquid changing into a solid by giving off heat. For example, water freezes into ice.
• Boiling or Vaporization: A liquid changing into a gas (vapor) by absorbing heat. For example, water boils into steam.
• Condensation: A gas (vapor) changing back into a liquid by giving off heat. For example, steam condenses into water droplets.
• Sublimation: A solid changing directly into a gas without first becoming a liquid. While less common for water, this process exists for other substances like dry ice.

In simple words: Water can be solid (ice), liquid (water), or gas (steam). It changes between these states by heating up or cooling down, like ice melting into water or water boiling into steam.

🎯 Exam Tip: For phase changes, remember the specific names for each transition (melting, freezing, boiling, condensation, sublimation) and whether heat is absorbed or released in each process.

 

Question 3. How can you experimentally prove water is a bad conductor of heat? How is it possible to heat water easily while cooking?
Answer:
(a) To prove water is a bad conductor of heat (Ice Cube in Test Tube):
Fill a test tube with cold water and place a piece of ice, wrapped in wire gauze, at the bottom so it sinks. Then, heat the top part of the test tube with a flame. You will observe that the water at the top begins to boil quickly, but the ice at the bottom takes a long time to melt completely. This shows that heat from the top does not easily travel downwards through the water by conduction.
The heat is transferred mainly through the water by convection (at the top), but conduction is very poor, especially downwards.

(b) To heat water easily while cooking (Convection):
To heat water quickly, you should apply heat from below. In cooking, placing a pot of water over a flame heats the bottom layers of water. This heated water becomes less dense and rises, while cooler, denser water from the top sinks to the bottom to be heated. This continuous movement of water, called convection, efficiently distributes heat throughout the entire volume of water, allowing it to heat up quickly.
In simple words: To show water is a bad heat conductor, you can boil the top of a test tube while ice stays frozen at the bottom. To cook quickly, heat water from below, which makes the hot water rise and cold water sink, spreading the heat fast.

🎯 Exam Tip: This experiment clearly demonstrates that water transfers heat poorly by conduction but efficiently by convection. Applying heat from below for cooking maximizes convection currents, speeding up heating.

VI. Numerical Problems.

 

Question 1. What is the heat in joules required to raise the temperature of 25 grams of water from 0°C to 100°C? What is the heat in Calories? (Specific heat of water = 4.18 J/g°C)
Answer:
Given:
Mass of water \( m = 25 \, \text{g} \)
Initial temperature \( T_1 = 0^\circ\text{C} \)
Final temperature \( T_2 = 100^\circ\text{C} \)
Change in temperature \( \Delta T = T_2 - T_1 = (100 - 0)^\circ\text{C} = 100^\circ\text{C} \)
Specific heat of water \( c = 4.18 \, \text{J/g}^\circ\text{C} \)

Solution for Heat in Joules:
The formula for heat required is \( H = m \times c \times \Delta T \)
\( H = 25 \, \text{g} \times 4.18 \, \text{J/g}^\circ\text{C} \times 100^\circ\text{C} \)
\( H = 25 \times 4.18 \times 100 \)
\( H = 25 \times 418 \)
\( H = 10450 \, \text{J} \)

Solution for Heat in Calories:
We know the conversion factor: \( 1 \, \text{calorie} = 4.18 \, \text{J} \)
To convert 10450 J to calories, we divide by 4.18:
Heat in calories \( = \frac{10450 \, \text{J}}{4.18 \, \text{J/calorie}} \)
Heat in calories \( = 2497.60 \, \text{calories} \)
Therefore, the heat required is \( 10450 \, \text{J} \) or \( 2497.60 \, \text{calories} \).
In simple words: We calculated how much heat energy is needed to warm up 25 grams of water by 100 degrees Celsius using a specific formula. Then, we changed that amount from Joules to Calories using a known conversion rate.

🎯 Exam Tip: Always pay attention to units (g vs. kg, °C vs. K) and ensure they are consistent throughout your calculations. Remember the conversion factor between Joules and calories.

 

Question 2. What could be the final temperature of a mixture of 100 g of water at 90°C and 600 g of water at 20°C.
Answer:
Given:
For hot water (1):
Mass \( m_1 = 100 \, \text{g} = 0.1 \, \text{kg} \)
Initial temperature \( T_1 = 90^\circ\text{C} \)

For cold water (2):
Mass \( m_2 = 600 \, \text{g} = 0.6 \, \text{kg} \)
Initial temperature \( T_2 = 20^\circ\text{C} \)

Specific heat capacity of water \( c = 4186 \, \text{J/kg}^\circ\text{C} \)
Let \( T_F \) be the final temperature of the mixture.

Solution:
According to the principle of calorimetry, Heat lost by hot water = Heat gained by cold water.
\( m_1 \times c \times (T_1 - T_F) = m_2 \times c \times (T_F - T_2) \)
Since 'c' (specific heat capacity of water) is the same on both sides, it cancels out.
\( m_1 \times (T_1 - T_F) = m_2 \times (T_F - T_2) \)
Now, substitute the given values:
\( 0.1 \, \text{kg} \times (90^\circ\text{C} - T_F) = 0.6 \, \text{kg} \times (T_F - 20^\circ\text{C}) \)
\( 0.1(90 - T_F) = 0.6(T_F - 20) \)
\( 9 - 0.1 T_F = 0.6 T_F - 12 \)
Now, gather the \( T_F \) terms on one side and constant terms on the other:
\( 9 + 12 = 0.6 T_F + 0.1 T_F \)
\( 21 = 0.7 T_F \)
\( T_F = \frac{21}{0.7} \)
\( T_F = 30^\circ\text{C} \)
The final temperature of the mixture is \( 30^\circ\text{C} \).
In simple words: When hot and cold water mix, the hot water loses heat and the cold water gains heat until they reach the same temperature. We used a formula to find this final temperature, which turned out to be 30°C.

🎯 Exam Tip: The principle of calorimetry (heat lost = heat gained) is fundamental for these types of problems. Remember to use consistent units and set up the equation carefully based on temperature changes.

 

Question 3. How much heat energy is required to change 2 kg of ice at 0°C into water at 20°C? (Specific latent heat of fusion of water = 3,34,000J/kg, Specific heat capacity of water = 4200 JKg\(^{-1}\)K\(^{-1}\))
Answer:
Given:
Mass of ice \( m = 2 \, \text{kg} \)
Specific latent heat of fusion of water \( L = 3,34,000 \, \text{J/kg} \)
Specific heat capacity of water \( c = 4200 \, \text{J/kg}^\circ\text{C} \) (or J/kg K)

The process involves two steps:
1. Melting the ice at \( 0^\circ\text{C} \) into water at \( 0^\circ\text{C} \).
2. Raising the temperature of this water from \( 0^\circ\text{C} \) to \( 20^\circ\text{C} \).

Solution:
Step 1: Heat required to melt ice (latent heat of fusion)
Heat \( Q_1 = m \times L \)
\( Q_1 = 2 \, \text{kg} \times 3,34,000 \, \text{J/kg} \)
\( Q_1 = 6,68,000 \, \text{J} \)

Step 2: Heat required to raise temperature of water
Change in temperature \( \Delta T = (T_2 - T_1) = (20 - 0)^\circ\text{C} = 20^\circ\text{C} \)
Heat \( Q_2 = m \times c \times \Delta T \)
\( Q_2 = 2 \, \text{kg} \times 4200 \, \text{J/kg}^\circ\text{C} \times 20^\circ\text{C} \)
\( Q_2 = 1,68,000 \, \text{J} \)

Total Heat Energy Required:
Total Heat \( Q = Q_1 + Q_2 \)
\( Q = 6,68,000 \, \text{J} + 1,68,000 \, \text{J} \)
\( Q = 8,36,000 \, \text{J} \)
Therefore, \( 8,36,000 \, \text{J} \) of heat energy is required.
In simple words: First, we calculated the heat needed to turn 2 kg of ice into water without changing its temperature. Then, we calculated the heat needed to warm up that water from 0°C to 20°C. Adding these two amounts gave us the total heat energy required.

🎯 Exam Tip: When a substance undergoes both a phase change and a temperature change, calculate the latent heat and specific heat components separately, then add them for the total heat energy.

Intext Activities

Activity - 1

 

Question 1. Describe the activity of putting ice cubes in water. What happens to the ice cubes, and why?
Answer:
Aim: To show how heat transfers.
Materials Required: A glass of water, ice cubes.
Procedure: Put some ice cubes into a glass of water and watch them for a while.
Observation: The ice cubes slowly melt and seem to vanish into the water.
Explanation: This happens because the water, which is warmer than the ice, transfers its heat energy to the ice. The ice absorbs this heat energy, causing it to change its state from solid to liquid, thus melting and disappearing into the water. This process demonstrates heat transfer from the warmer water to the colder ice.
In simple words: When ice is put in water, it melts because the warmer water gives its heat to the colder ice. This shows how heat moves from a warm thing to a cold thing.

🎯 Exam Tip: This activity illustrates the natural flow of heat from a warmer body to a colder body until thermal equilibrium is reached.

Activity - 2

 

Question 1. Describe an experiment to compare the heat conducting powers of different metals. What are the observations and conclusions?
Answer:
Aim: To compare how well different metals conduct heat.
Materials Required: Rods made of copper, aluminum, brass, and iron; matchsticks; and melted wax.
Procedure: Attach a matchstick to one end of each metal rod using a small amount of melted wax. Then, heat the other end of each rod. Observe what happens.
Observation: You will notice that the matchstick on the copper rod drops off first. After that, the matchsticks on the aluminum, brass, and iron rods will fall in that order. This indicates how quickly heat travels through each metal.
Conclusion: Metals are generally good conductors of heat. Among the tested metals, copper is the best conductor of heat, followed by aluminum, then brass, and finally iron. The rate at which the matchsticks fall shows their relative ability to conduct heat.
In simple words: We attached matchsticks with wax to different metal rods and heated the other end. The matchstick on the copper rod fell first, showing copper is the best heat conductor, then aluminum, brass, and iron.

🎯 Exam Tip: This experiment visually demonstrates the difference in thermal conductivity among metals. Faster melting of wax signifies better heat conduction.

Activity -3

Drop a few crystals of potassium permanganate down to the bottom of a beaker containing water. When the beaker is heated just below the crystals, by a small flame, purple streaks of water rise upwards and fan outwards.

Aim: To demonstrate transfer of heat through convection in liquids.

Materials required: Crystals of potassium permanganate, beaker containing water.

Procedure: Drop a few crystals of potassium permanganate down to the bottom of a beaker containing water, heat it with a small flame.

Observation: When the beaker is heated, just below the crystals, purple streaks of water rise upwards and fan outward.

Conclusion: Water molecules at the bottom of the beaker receive heat energy and move upward, replacing the molecules at the top. This activity shows that heat flows through a fluid from hotter to colder places by the movement of the fluid itself.

 

Activity -4

Take some crushed ice cubes in a beaker and note down the temperature using a thermometer. It will be 0°C. Now heat the ice in the beaker. You can observe that ice is melting to form water. Record the temperature at regular intervals and it will remain at 0°C until all the ice is converted to liquid. Now heat the beaker again and record the temperature. You can notice that the temperature will rise up to 100°C and it will retain the same even after continuous heating until the whole mass of water in the beaker is vaporized.

Aim: To understand the latent heat of water.

Materials Required: Crushed ice cubes, beaker, and thermometer.

Procedure: Take some crushed ice cubes in a beaker and note down the temperature using a thermometer. It will be 0°C. Now heat the ice in the beaker, observe, and record the temperature at regular intervals. Heat the beaker again and record the temperature.

Observation:

• Ice is melting to form water.
• Water will remain at 0°C until all the ice is converted to liquid.
• On further heating, we can observe that the temperature will rise up to 100°C, and the temperature will stay at 100°C even after continuous heating until all the water in the beaker is vaporized.

Conclusion: In this activity, the temperature stays constant at 0°C until all the ice becomes liquid. It then stays constant again at 100°C until all the water turns into vapor. This happens because a substance absorbs or releases a lot of heat energy when it changes state, which is called latent heat.

 

9th Science Guide Heat Additional Important Questions and Answers

I. Choose The Correct Answer :

 

Question 1. Water is used as a coolant because it ....................
(a) is inexpensive
(b) is easily available
(c) is a good conductor of heat
(d) has a high specific heat capacity
Answer: (d) has a high specific heat capacity
In simple words: Water is very good at absorbing a lot of heat without getting much hotter itself. This makes it perfect for cooling things down, like in car engines.

🎯 Exam Tip: Remember that coolants work by absorbing excess heat, and a high specific heat capacity means a substance can absorb a lot of heat without a large temperature increase.

 

Question 2. The amount of heat required to raise the temperature through 1°C is called...................
(a) thermal energy
(b) calorie
(c) heat capacity
(d) specific heat capacity
Answer: (c) heat capacity
In simple words: Heat capacity tells us how much heat energy is needed to warm up a substance by one degree Celsius. It's a way to measure how well something stores heat.

🎯 Exam Tip: Distinguish between heat capacity (for a specific mass) and specific heat capacity (per unit mass). The question doesn't specify unit mass, implying total heat capacity.

 

Question 3. The temperature at which a liquid gets converted into its vapour state is called its...................
(a) melting point
(b) boiling point
(c) dew point
(d) freezing point
Answer: (b) boiling point
In simple words: When a liquid gets hot enough to turn into a gas, that special temperature is called its boiling point. Think of water turning into steam.

🎯 Exam Tip: Clearly understand the difference between boiling point (liquid to gas) and melting point (solid to liquid), and their reverse processes.

 

Question 4. Sweating causes cooling because water has a...................
(a) high specific heat
(b) low specific heat
(c) high latent heat of fusion
(d) high latent heat of vaporisation
Answer: (d) high latent heat of vaporisation
In simple words: When sweat evaporates from your skin, it takes a lot of heat from your body to change from liquid to vapor, which makes you feel cooler.

🎯 Exam Tip: Latent heat of vaporization is key here: a significant amount of heat is absorbed by water molecules when they transition from liquid sweat to water vapor, leading to a cooling effect on the skin.

 

Question 5. Which of the following is true?
(a) 1 J = 412 calorie
(b) 1 J = 0.24 calorie
(c) 1 calorie = 4.2 J
Answer: (c) 1 calorie = 4.2 J
In simple words: A calorie is a unit of energy, and it's equal to about 4.2 Joules. This helps us change between different ways of measuring energy.

🎯 Exam Tip: Remember the conversion factor between Joules (the SI unit of energy) and calories, as it's a fundamental concept in thermodynamics.

 

Question 6. Ice does not melt rapidly because of
(a) high specific heat capacity
(b) high latent of fusion
(c) high heat capacity
(d) high latent heat of fusion
Answer: (d) high latent heat of fusion
In simple words: Ice needs to absorb a lot of heat to change from solid ice to liquid water, even without its temperature rising. This is called high latent heat of fusion, and it's why ice takes time to melt.

🎯 Exam Tip: The latent heat of fusion is the energy required to change a substance from solid to liquid at a constant temperature. For ice, this value is quite high, making it melt slowly.

 

Question 7. Which one of the following scales has a lower fixed point at 0°C?
(a) Kelvin scale
(b) Fahrenheit scale
(c) Celsius scale
(d) All of these
Answer: (c) Celsius scale
In simple words: The Celsius scale is set up so that the freezing point of water is 0 degrees. Other scales have different starting points.

🎯 Exam Tip: Know the fixed points for different temperature scales. For Celsius, 0°C is the freezing point of water and 100°C is its boiling point.

 

Question 8. When we heat one end of an iron rod, its other end also gets heated. Can you say, Which one of the following is behind this?
(a) Convection of heat
(b) Radiation of heat
(c) Insulation of heat
(d) Conduction of heat
Answer: (d) Conduction of heat
In simple words: Heat moves through the iron rod by conduction. This means the heat travels from one particle to the next without the particles themselves moving from their spots.

🎯 Exam Tip: Conduction is the primary mode of heat transfer in solids, where heat energy is passed through direct contact between vibrating particles.

 

Question 9. In which of the following, chemical energy is converted into heat energy?
(a) Heater
(b) Refrigerators
(c) Candle
(d) Motor
Answer: (c) Candle
In simple words: When a candle burns, the wax undergoes a chemical reaction with oxygen, and this reaction releases energy mostly in the form of heat and light.

🎯 Exam Tip: Chemical energy is stored in the bonds of molecules and is released or absorbed during chemical reactions. Burning is a classic example of chemical energy converting to heat and light.

 

Question 10. On a cold day, it is hard to open the lid of a tight container. But when you gently heat the neck you can easily open the lid. why?
(a) On heating glass expands and lid contracts
(b) On heating lid expands more than the neck and thus slides easily
(c) Neck becomes slippery on heating
(d) Lid of the bottle cannot bear the heat.
Answer: (b) On heating lid expands more than the neck and thus slides easily
In simple words: When you heat the lid, it gets bigger than the bottle's neck, so it becomes loose and easy to twist open. This is because metals expand more than glass when heated.

🎯 Exam Tip: Different materials expand at different rates when heated. This principle of thermal expansion is useful for loosening tight lids or fixing components.

 

Question 11. Warm air is .......................
(a) lighter than cold air
(b) heavier than cold air
(c) both have equal weights
Answer: (a) lighter than cold air
In simple words: Hot air is less dense than cold air, which means it rises. This is why hot air balloons float and why smoke goes up.

🎯 Exam Tip: The principle that warmer air is less dense and rises is fundamental to understanding convection currents in the atmosphere and in liquids.

 

Question 12. The phenomenon involved in the sea breeze and the land breeze is
(a) convection
(b) conduction
(c) radiation
(d) none of these
Answer: (a) convection
In simple words: Sea breeze and land breeze happen because warm air rises and cooler air moves in to take its place. This movement of air, which transfers heat, is called convection.

🎯 Exam Tip: Convection currents are crucial for weather patterns. Remember that convection involves the actual movement of fluid (liquid or gas) particles to transfer heat.

 

Question 13. A liquid changes into a gas at a constant temperature known as its...................
(a) absolute zero
(b) boiling point
(c) evaporation point
(d) dew point
Answer: (b) boiling point
In simple words: The specific temperature where a liquid turns into a gas, like water turning into steam, is called its boiling point. During this change, the temperature stays the same.

🎯 Exam Tip: Phase changes (like boiling) occur at constant temperatures as long as the pressure is constant. The energy added during this time is latent heat.

 

Question 14. Copper and Iron are good conductors of heat. Which one of the following is not a good conductor of heat?
(a) Soil
(c) Tungsten
(d) Steel
Answer: (a) Soil
In simple words: Soil is not a good conductor of heat because it's made of many small particles with air gaps between them. Air is a poor conductor, so soil doesn't transfer heat well.

🎯 Exam Tip: Metals are generally good conductors due to free electrons, while materials with trapped air, like soil or wool, are poor conductors (insulators).

 

Question 15. The specific heat capacity of water is
(a) \( 4200 Jkg^{-1}K^{-1} \)
(b) \( 420 Jg^{-1}K^{-1} \)
(c) \( 0.42 Jg^{-1}K^{-1} \)
(d) \( 4.2 Jkg^{-1}K^{-1} \)
Answer: (a) \( 4200 Jkg^{-1}K^{-1} \)
In simple words: Water needs a lot of energy to heat up. Its specific heat capacity is 4200 Joules for every kilogram to raise its temperature by one Kelvin (or Celsius degree).

🎯 Exam Tip: Remember the value of specific heat capacity of water, as it's a common constant in heat calculations and explains many of water's thermal properties.

 

Question 16. Two cylinders of equal height and radius are made of copper and aluminum. Which of them conducts heat faster?
(a) Copper rod
(b) Aluminium rod
(c) Both of them
(d) None of them
Answer: (a) Copper rod
In simple words: Copper is a better conductor of heat than aluminum. This means heat moves through copper faster, so a copper rod will transfer heat quicker.

🎯 Exam Tip: Different metals have different thermal conductivities. Copper is generally known to have higher thermal conductivity than aluminum.

 

II. Fill In The Blanks:

 

Question 1. ..................... is a process which is just reverse of melting.
Answer: Freezing
In simple words: When a liquid turns into a solid, like water turning into ice, that process is called freezing. It's the opposite of melting.

🎯 Exam Tip: Understand phase changes: melting (solid to liquid), freezing (liquid to solid), boiling (liquid to gas), condensation (gas to liquid).

 

Question 2. While a substance is undergoing a change of state, the temperature of the body remains ....................
Answer: same
In simple words: During a phase change, like ice melting into water, all the heat energy goes into changing its state, not into making it hotter. So, the temperature stays fixed.

🎯 Exam Tip: This constant temperature during a phase change is due to latent heat being absorbed or released, which changes the potential energy of molecules, not their kinetic energy (which determines temperature).

 

Question 3. A change of state is a change of a substance from...................
Answer: one physical state to another
In simple words: A change of state means a substance transforms from a solid to a liquid, or a liquid to a gas, or vice-versa. It's a physical change, not a chemical one.

🎯 Exam Tip: Recognize that changes of state are physical processes where the chemical identity of the substance remains the same, only its arrangement of molecules changes.

 

Question 4. ....................is the degree of hotness or coldness of a body.
Answer: Temperature
In simple words: Temperature tells us how hot or cold something is. It's a way to measure the average kinetic energy of the tiny particles inside an object.

🎯 Exam Tip: Temperature is a measure of the average kinetic energy of particles, while heat is the transfer of thermal energy between objects.

 

Question 5. The solid, liquid, gaseous phases of water can coexist in equilibrium at...................
Answer: 273.16K
In simple words: At exactly 273.16 Kelvin (which is 0.01 degrees Celsius), water can exist as solid ice, liquid water, and water vapor all at once in a balanced state. This unique point is called the triple point.

🎯 Exam Tip: The triple point is a specific temperature and pressure at which all three phases (solid, liquid, gas) of a substance can coexist in thermodynamic equilibrium. For water, this occurs at 273.16 K (0.01 °C) and 611.657 Pa.

 

Question 6. The sum of the kinetic and potential energy is called the ...................of the molecules.
Answer: internal energy
In simple words: The total energy stored inside a substance, including the movement and position energy of its tiny particles, is known as its internal energy.

🎯 Exam Tip: Internal energy is a state function that represents the total energy content of a thermodynamic system, comprising both kinetic and potential energy of its constituent particles.

 

Question 7. ................... is greater for liquids than that for solids and maximum in case of gases.
Answer: Kinetic energy
In simple words: The tiny particles in gases move around much more freely and quickly than in liquids, and even more than in solids. This is why gases have the highest kinetic energy.

🎯 Exam Tip: The kinetic energy of particles increases as a substance moves from solid to liquid to gas phases, reflecting the increased freedom and speed of molecular motion.

 

Question 8. When heat energy is added to a substance, the kinetic energy of its particles and so the particles ................... move at a higher speed.
Answer: increase
In simple words: Adding heat makes the tiny particles inside a substance move faster and bump into each other more often. This causes the substance to get hotter.

🎯 Exam Tip: Heat energy directly correlates with the kinetic energy of particles. More heat means more vigorous motion and higher temperature.

 

Question 9. When a dog keeps out its tongue and breathes hard, the moisture on the tongue turns into ...................and it evaporates.
Answer: water
In simple words: Dogs pant to cool down. When the water on their tongue turns into vapor, it takes heat away from their body, helping them stay cool.

🎯 Exam Tip: Panting is a natural cooling mechanism that utilizes the latent heat of vaporization of water to remove excess body heat.

 

Question 10. Black marks appearing on the ceiling above a lamp or fan caused by dust being carried upwards in the air are due to...................
Answer: convection currents
In simple words: The warm air from the lamp rises, carrying tiny dust particles with it. As this air moves, it forms patterns on the ceiling where the dust settles, creating black marks.

🎯 Exam Tip: Convection causes the movement of heated fluids (like air), which can carry particles like dust or smoke and deposit them in specific patterns.

 

Question 11. ...................is the method of heat transfer that does not require particles to carry the heat energy.
Answer: Radiation
In simple words: Radiation is a way heat travels, like from the sun to Earth, without needing air or any other material to carry it. It travels as waves.

🎯 Exam Tip: Radiation is unique because it can transfer heat through a vacuum, unlike conduction and convection which require a medium.

 

Question 12. Radiation consists of ...................waves travelling at the speed of light.
Answer: electromagnetic
In simple words: Heat radiation is made of electromagnetic waves, just like light. These waves can travel through empty space very fast.

🎯 Exam Tip: All objects above absolute zero emit electromagnetic radiation. The type and intensity of radiation depend on the object's temperature.

 

Question 13. We can observe all the three ways of heat transfer while...................
Answer: burning wood
In simple words: When wood burns, you feel the heat in front (radiation), the air above gets hot (convection), and if you touch the wood, it's hot (conduction). All three types of heat transfer happen together.

🎯 Exam Tip: A burning fire is a great example to illustrate all three modes of heat transfer: conduction through the wood, convection of heated air, and radiation of infrared waves.

 

Question 14. ................... is known as an absolute scale.
Answer: Kelvin
In simple words: The Kelvin scale is called an absolute scale because its zero point (absolute zero) is the lowest possible temperature where particles have the least amount of energy.

🎯 Exam Tip: The Kelvin scale is crucial in science as it's directly proportional to the kinetic energy of particles, making it ideal for thermodynamic calculations.

 

Question 15. Specific latent heat L =...................
Answer: Q/m
In simple words: Specific latent heat is calculated by dividing the total heat energy (Q) needed for a phase change by the mass (m) of the substance. It tells us how much heat is needed per unit mass.

🎯 Exam Tip: The formula \( L = \frac{Q}{m} \) is fundamental for calculating the heat involved in phase transitions at constant temperature.

 

III. Match The Following :

Answer: 1-c, 2-a, 3-e, 4-b, 5-d
In simple words: This match helps us connect the basic ideas about heat and temperature with how we measure them and the energy involved in different processes. Knowing these pairs helps in understanding the concepts better.

🎯 Exam Tip: For matching questions, systematically go through each item in Column A and find its corresponding match in Column B. Understanding the definitions of each term is key.

 

Answer: 1-d, 2-e, 3-b, 4-a, 5-c
In simple words: This helps us remember important numbers for water and ice, like how much energy they need to change temperature or state, and at what specific temperatures these changes happen.

🎯 Exam Tip: Memorize the specific values for water's specific heat capacity, latent heat of fusion, latent heat of vaporization, and its key phase change temperatures (melting and boiling points).

 

IV. Assertion And Reason Type Questions :

 

Question 1. Assertion (A) : Temperature is the measure of heat energy. Reason (R) : Energy is the capacity to do work.
(a) If both assertion and reason are true and the reason is the correct explanation of assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of assertion.
(c) If assertion is true but reason is false.
(d) If assertion is false but reason is true.
Answer: (b) Both assertion and reason are true but reason is not the correct explanation of the assertion
In simple words: Both statements are true on their own. Temperature does measure how hot or cold something is, and energy is indeed the ability to do work. However, the reason given doesn't explain why temperature is a measure of heat energy.

🎯 Exam Tip: For Assertion-Reason questions, first determine if both statements are individually true. Then, check if the Reason logically explains the Assertion. If not, option (b) is usually correct.

 

Question 2. Assertion (A): Radiation is a process of transfer of heat in which a material medium is not necessary. Reason (R): The heat from the sun reaches us through millions of miles of empty space by convection.
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of assertion.
(c) If assertion is true but reason is false.
(d) If assertion is false but reason is true.
Answer: (c) Assertion is true but reason is false
In simple words: Radiation can transfer heat through empty space without needing anything to carry it, which makes Assertion (A) true. However, the sun's heat reaches us by radiation, not convection, because there's no air in space for convection to work. So, Reason (R) is false.

🎯 Exam Tip: Clearly differentiate between conduction, convection, and radiation. Radiation is unique in not requiring a medium, making it the mode of heat transfer from the sun to Earth.

 

Question 3. Assertion (A) : Heat energy is transferred from one body to another due to a temperature difference between them. Reason (R) : Heating a substance causes a rise in temperature.
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of assertion.
(c) If assertion is true but reason is false.
(d) If assertion is false but reason is true.
Answer: (b) If both assertion and reason are true but reason is not the correct explanation of assertion.
In simple words: Both statements are correct: heat does move from hotter to colder things, and adding heat usually makes something hotter. But the second statement doesn't explain *why* heat moves. It just states another effect of heat.

🎯 Exam Tip: The transfer of heat is governed by temperature differences. While heating often causes a temperature rise, this effect isn't the underlying reason for heat transfer itself.

 

Question 4. Assertion (A) : When a very hot liquid is poured into a thick glass tumbler it cracks. Reason (R) : Unequal expansion of the inner and outer glass walls causes the glass to crack.
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of assertion.
(c) If assertion is true but reason is false.
(d) If assertion is false but reason is true.
Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion
In simple words: When a hot liquid hits a thick glass, the inside surface heats up and expands quickly, but the outside stays cool. This uneven expansion creates stress and can cause the glass to break.

🎯 Exam Tip: Thermal stress occurs when different parts of an object expand or contract at different rates due to uneven heating or cooling. This is particularly problematic in materials like thick glass that don't conduct heat well.

 

V. Very Short Answer Type Questions.

 

Question 1. What is the other name of heat capacity?
Answer: Thermal capacity.
In simple words: Heat capacity is also known as thermal capacity. It tells you how much heat energy a substance can hold for a certain temperature change.

🎯 Exam Tip: These terms are often used interchangeably. Make sure to define it as the amount of heat energy required to change the temperature of a body by one degree Celsius (or Kelvin).

 

Question 2. Define one calorie.
Answer: One calorie is the amount of heat energy needed to raise the temperature of 1 gram of water by 1°C. This is a common unit for measuring heat, especially in nutrition.
In simple words: A calorie is a small amount of heat that can make a tiny bit of water (1 gram) get 1 degree Celsius warmer.

🎯 Exam Tip: The definition of a calorie is specific to water and its temperature change. Be precise with "1 gram of water" and "1°C".

V. Very Short Answer Type Questions

 

Question 2. Define one calorie.
Answer: A calorie is the amount of heat energy needed to raise the temperature of 1 gram of water by 1 degree Celsius. This unit helps us measure how much heat is involved in different processes. It is a common unit for energy in many contexts.
In simple words: One calorie is the heat energy required to warm up 1 gram of water by 1 degree Celsius.

🎯 Exam Tip: Remember the specific conditions (1 gram of water, 1°C rise) when defining a calorie to ensure full accuracy.

 

Question 3. What is the relation between calorie and Joule?
Answer: The relation between calorie and Joule is that 1 calorie is equal to 4.186 Joules, which is often rounded to 4.2 Joules. Both are units of energy, with Joules being the SI unit.
In simple words: One calorie is the same as about 4.2 Joules of energy.

🎯 Exam Tip: Always use the conversion factor 4.186 J/calorie for precise calculations, but 4.2 J/calorie is acceptable for estimates.

 

Question 4. Name a device that prevents loss of energy (or gain) by conduction, convection and radiation.
Answer: A thermos flask is a device designed to minimize heat transfer through conduction, convection, and radiation. It keeps hot things hot and cold things cold for a longer time by creating a vacuum and using reflective surfaces.
In simple words: A thermos flask stops heat from moving in or out, keeping things hot or cold.

🎯 Exam Tip: Mentioning the three modes of heat transfer (conduction, convection, radiation) is crucial when describing the function of a thermos flask.

 

Question 5. Which factor determines the direction of flow of heat from one body to another?
Answer: The temperature of a body determines the direction of heat flow. Heat always moves from a region of higher temperature to a region of lower temperature until both reach thermal equilibrium.
In simple words: Heat always moves from a hotter place to a colder place.

🎯 Exam Tip: Understand that temperature is the driving force for heat transfer, not the amount of heat itself.

 

Question 6. Who introduced the term latent heat?
Answer: The term "latent heat" was introduced by Joseph Black in 1750. He was a Scottish chemist and physician who made significant discoveries in the field of thermodynamics.
In simple words: Joseph Black first used the term "latent heat" in 1750.

🎯 Exam Tip: Knowing the names of key scientists and their contributions can earn you extra marks in science questions.

 

Question 7. What is the minimum possible temperature? Is there also a maximum possible temperature?
Answer: The minimum possible temperature is absolute zero, which is 0 Kelvin or \( -273.15^{\circ}\text{C} \). At this temperature, particles have the lowest possible energy. There is no theoretical limit to the maximum possible temperature, as temperature relates to kinetic energy, which can continuously increase.
In simple words: The lowest temperature is absolute zero (0 Kelvin). There is no highest temperature.

🎯 Exam Tip: Clearly state the value of absolute zero in both Kelvin and Celsius and explain why a maximum temperature doesn't exist.

 

VI. Answer Briefly:

 

Question 1. Heat gained by a body depends upon which factors?
Answer: The amount of heat energy gained by a body depends on three main factors:

• Mass of the body: A heavier body needs more heat to change its temperature than a lighter body.
• Change in temperature of the body: The larger the temperature change needed, the more heat must be added.
• Nature of the material of the body: Different materials have different specific heat capacities, meaning some need more heat than others to warm up by the same amount.

The specific heat capacity of a material determines how easily its temperature changes when heat is added.
In simple words: How much heat an object gains depends on its weight, how much its temperature changes, and what material it's made of.

🎯 Exam Tip: Remember the formula \( Q = mc\Delta T \) to easily recall the three factors: mass (m), specific heat capacity (c), and change in temperature (\( \Delta T \)).

 

Question 2. Water is used as a coolant in car radiators. Why?
Answer: Water is used as a coolant in car radiators because it has a very high specific heat capacity. This means water can absorb a large amount of heat energy from the car engine without its own temperature rising significantly. This property helps to keep the engine cool and prevents it from overheating.
In simple words: Water is good for cooling car engines because it can soak up a lot of heat without getting hot itself.

🎯 Exam Tip: The key phrase here is "high specific heat capacity," which is the main reason water is an effective coolant.

 

Question 3. What do you mean by thermal equilibrium?
Answer: Thermal equilibrium is a state where two or more bodies in contact with each other have reached the same temperature. When objects with different temperatures are placed together, heat energy flows from the hotter body to the colder body until they both achieve a common, stable temperature. At this point, there is no net transfer of heat between them.
In simple words: Thermal equilibrium means all connected objects have the same temperature, and heat has stopped moving between them.

🎯 Exam Tip: Explain that heat flow stops, and temperatures become equal, to accurately define thermal equilibrium.

 

Question 4. Define latent heat of fusion?
Answer: Latent heat of fusion is the amount of heat energy a solid substance absorbs when it changes into a liquid, or the amount of heat a liquid releases when it changes back into a solid. This change happens at a constant temperature (its melting or freezing point) without any change in the substance's temperature. For example, ice melts into water at \( 0^{\circ}\text{C} \) by absorbing latent heat.
In simple words: Latent heat of fusion is the hidden heat absorbed or released when a solid turns into a liquid, or vice versa, without the temperature changing.

🎯 Exam Tip: Emphasize that latent heat is absorbed/released *without* a change in temperature during a phase transition (like melting or freezing).

 

Question 5. Why are burns caused by steam more painful than those caused by boiling water at the same temperature?
Answer: Burns from steam are more painful than those from boiling water at the same temperature ( \( 100^{\circ}\text{C} \) ) due to the release of latent heat. Steam at \( 100^{\circ}\text{C} \) contains extra energy (latent heat of vaporization) compared to boiling water at the same temperature. When steam touches the skin, it condenses into water, releasing this large amount of latent heat. This extra heat energy, transferred quickly and to a small area, causes much more severe tissue damage and pain than boiling water, which only transfers its sensible heat.
In simple words: Steam burns hurt more because steam has extra "hidden" heat that it gives off when it turns into water on your skin, causing more damage.

🎯 Exam Tip: The key to this answer is explaining the concept of "latent heat of vaporization" and how its release upon condensation causes greater energy transfer to the skin.

 

Question 6. What do you mean by solidification or deposition?
Answer:
Solidification: This is the process where a liquid changes into a solid state by losing heat. For example, water freezing into ice.
Deposition: This is the process where a gas changes directly into a solid state without first becoming a liquid. For example, carbon dioxide gas can be converted directly into dry ice (solid carbon dioxide). This phase change also involves the release of heat energy.
In simple words: Solidification is when a liquid turns into a solid. Deposition is when a gas turns straight into a solid, skipping the liquid stage.

🎯 Exam Tip: Provide a clear definition and a relevant example for both solidification and deposition to show a complete understanding.

 

Question 7. Define absolute zero.
Answer: Absolute zero is the lowest theoretically possible temperature, defined as 0 Kelvin (\( 0 \text{ K} \)) or approximately \( -273.15^{\circ}\text{C} \). At this temperature, the particles (atoms and molecules) of a substance have the minimum possible kinetic energy and are in their lowest energy state. This means their motion almost completely stops, and both the pressure and volume of an ideal gas would theoretically become zero.
In simple words: Absolute zero is the coldest possible temperature where particles have almost no movement.

🎯 Exam Tip: Remember to mention both the Kelvin and Celsius values for absolute zero and briefly explain what happens to particle motion at this temperature.

 

Question 8. Give some practical applications of conduction in daily life.
Answer: Here are some practical applications of conduction in daily life:

• Cooking Utensils: Metals like aluminum are good conductors of heat, so cooking utensils are made from them to transfer heat efficiently to food.
• Thermometers: Mercury is used in traditional thermometers because it is a good conductor of heat, allowing it to quickly absorb heat and expand to show temperature changes.
• Woolen Clothes: We wear woolen clothes in winter because wool traps air, which is a poor conductor of heat (an insulator). This prevents our body heat from escaping, keeping us warm.

Conduction is important in how heat moves through materials that are in direct contact.
In simple words: Cooking pots work by conduction. Thermometers use it. Woolen clothes keep us warm by stopping heat conduction.

🎯 Exam Tip: When listing applications, describe *how* conduction is involved in each example, explaining whether the material is a good conductor or an insulator.

 

Question 9. Give some practical applications of radiation.
Answer: Here are some practical applications of radiation:

• Light-Colored Clothes in Summer: People wear white or light-colored clothes in summer because these colors are good reflectors of heat radiation. This helps to keep the body cool by reflecting sunlight away.
• Blackened Cooking Utensils: The bottom surface of cooking utensils is often blackened because black surfaces are good absorbers of heat radiation. This helps the utensil absorb more heat from the flame efficiently.
• Polished Airplane Surfaces: The surface of an airplane is highly polished to reflect most of the heat radiation from the sun. This prevents the aircraft from getting too hot during flight.

Radiation is unique because it doesn't require any material medium for heat transfer.
In simple words: Light clothes reflect heat. Black pans absorb heat. Polished airplanes reflect heat. These are all examples of radiation.

🎯 Exam Tip: Focus on the properties of surfaces (color, texture) and their interaction with radiant heat (absorption, reflection) when explaining these applications.

 

Question 10. Can convection take place in solids? Why?
Answer: No, convection cannot take place in solids. Convection requires the actual movement of particles (molecules) to carry heat from one place to another. In solids, molecules are tightly packed in fixed positions and can only vibrate about these positions; they cannot move freely. Therefore, the bulk movement of matter necessary for convection is not possible in solids. Heat transfer in solids primarily occurs through conduction.
In simple words: No, convection cannot happen in solids. This is because the tiny parts (molecules) in solids are stuck in place and cannot move around to carry heat.

🎯 Exam Tip: The key reason for convection not occurring in solids is the lack of free movement of their constituent particles.

 

Question 11. In winters, when the sun suddenly goes behind the clouds we feel cold, can you say why?
Answer: In winter, when the sun suddenly goes behind the clouds, we feel cold because the clouds block the radiant heat coming directly from the sun. The sun's warmth reaches Earth primarily through radiation, which does not require a medium. When clouds obstruct this radiation, the amount of heat energy reaching us decreases rapidly, causing us to feel colder almost immediately.
In simple words: When clouds hide the sun in winter, we feel colder because the clouds stop the sun's direct heat (radiation) from reaching us.

🎯 Exam Tip: The core concept here is that clouds block solar *radiation*, which is the primary mode of heat transfer from the sun.

 

VII. Answer in Detail:

 

Question 1. Give the difference between heat and temperature.
Answer: Here are the differences between heat and temperature:
Heat:
1. Heat is a form of energy that makes us feel hot or cold.
2. Its SI unit is the Joule (J).
3. It depends on the mass, nature, and temperature of the body.
4. Heat is transferred from one object to another.
5. It is measured using a calorimeter.

Temperature:
1. Temperature is a measure of the degree of hotness or coldness of a body.
2. Its SI unit is Kelvin (K).
3. It does not directly depend on the mass or nature of the body, but on the average kinetic energy of its particles.
4. Temperature determines the direction of heat flow (from high to low temperature).
5. It is measured using a thermometer.
In simple words: Heat is a type of energy, while temperature tells us how hot or cold something is. Heat is measured in Joules, temperature in Kelvin or Celsius. Heat depends on the object's size and material, but temperature does not.

🎯 Exam Tip: For differentiation questions, use clear, distinct points for each concept, ideally presented in a comparative manner or as two separate lists.

 

Question 2. Give some practical applications of specific latent heat of ice.
Answer: The specific latent heat of fusion for ice is very high (approximately 336 J/g or 334,000 J/kg), which has several practical applications:

• Gradual Snowmelt: Due to its high latent heat, snow on mountains melts slowly into water using the sun's heat. If its latent heat were low, all the snow would melt quickly, causing severe floods.
• Moderation of Water Bodies: Large bodies of water, like lakes and ponds, freeze slowly during cold weather and keep the surrounding environment warmer. This slow freezing, due to the release of latent heat, moderates the local climate.
• Cooling Drinks: Ice pieces at \( 0^{\circ}\text{C} \) are more effective at cooling drinks than water at \( 0^{\circ}\text{C} \). This is because 1 gram of ice takes away 336 Joules of heat from the drink as it melts into water at the same temperature.

The high latent heat allows ice to absorb or release a lot of energy during phase changes without temperature changes.
In simple words: Ice melts slowly, stopping big floods. Water bodies freeze slowly, making the air around them warmer. Ice cubes make drinks colder because they soak up a lot of heat as they melt.

🎯 Exam Tip: When discussing latent heat, clearly explain that it involves energy absorption/release during a phase change *without* a temperature change, and how this property is beneficial in each application.

 

Question 3. 600 g of copper at \( 50^{\circ}\text{C} \) is mixed with 1000g water at \( 20^{\circ}\text{C} \). Find the final temperature of the mixture. Specific heat capacity of copper is \( 0.4 \text{ Jg}^{-1}{^{\circ}\text{C}}^{-1} \) and that of water is \( 4.2 \text{ Jg}^{-1}{^{\circ}\text{C}}^{-1} \).
Answer:
Let the final temperature of the mixture be \( x^{\circ}\text{C} \).

For copper:
Mass of copper \( m_1 = 600 \text{ g} \)
Specific heat capacity of copper \( c_1 = 0.4 \text{ Jg}^{-1}{^{\circ}\text{C}}^{-1} \)
Initial temperature of copper \( t_1 = 50^{\circ}\text{C} \)
Final temperature of copper \( t_2 = x^{\circ}\text{C} \)
Fall in temperature \( \Delta t_1 = (t_1 - t_2) = (50 - x)^{\circ}\text{C} \)
Heat lost by copper \( Q_1 = m_1 c_1 \Delta t_1 = 600 \times 0.4 \times (50 - x) \)

For water:
Mass of water \( m_2 = 1000 \text{ g} \)
Specific heat capacity of water \( c_2 = 4.2 \text{ Jg}^{-1}{^{\circ}\text{C}}^{-1} \)
Initial temperature of water \( t_1 = 20^{\circ}\text{C} \)
Final temperature of water \( t_2 = x^{\circ}\text{C} \)
Rise in temperature \( \Delta t_2 = (t_2 - t_1) = (x - 20)^{\circ}\text{C} \)
Heat gained by water \( Q_2 = m_2 c_2 \Delta t_2 = 1000 \times 4.2 \times (x - 20) \)

According to the principle of calorimetry, Heat lost by copper = Heat gained by water:
\( 600 \times 0.4 \times (50 - x) = 1000 \times 4.2 \times (x - 20) \)
\( 240 \times (50 - x) = 4200 \times (x - 20) \)
\( 12000 - 240x = 4200x - 84000 \)
Now, group terms with \( x \):
\( 12000 + 84000 = 4200x + 240x \)
\( 96000 = 4440x \)
\( x = \frac{96000}{4440} \)
\( x \approx 21.62 \)
So, the final temperature of the mixture of water and copper is approximately \( 21.6^{\circ}\text{C} \).
In simple words: When hot copper is mixed with cold water, the heat moves from the copper to the water until they both reach the same temperature. We calculate this final temperature by making sure the heat lost by the copper is equal to the heat gained by the water.

🎯 Exam Tip: Always state the principle of calorimetry (Heat lost = Heat gained) and clearly show each step of the calculation, ensuring units are consistent.

 

Question 4. Define heat capacity and specific heat capacity.
Answer:
Specific Heat Capacity:
1. This is the amount of heat energy required to raise the temperature of 1 gram of a substance by \( 1^{\circ}\text{C} \) or 1 Kelvin.
2. It depends only on the nature of the substance, not its mass. This property helps identify different materials.
3. Its unit is \( \text{Jg}^{-1}{^{\circ}\text{C}}^{-1} \) (Joules per gram per degree Celsius) or \( \text{Jkg}^{-1}\text{K}^{-1} \) (Joules per kilogram per Kelvin).

Heat Capacity (or Thermal Capacity):
1. This is the total amount of heat energy required to raise the temperature of a *given mass* of a substance by \( 1^{\circ}\text{C} \) or 1 Kelvin.
2. It depends on both the mass of the body and the nature of the substance. A larger object needs more heat for the same temperature rise.
3. Its unit is \( \text{J}^{\circ}\text{C}^{-1} \) (Joules per degree Celsius) or \( \text{JK}^{-1} \) (Joules per Kelvin).
In simple words: Specific heat capacity is how much heat 1 gram of a material needs to warm up by 1 degree. Heat capacity is how much heat a whole object needs to warm up by 1 degree.

🎯 Exam Tip: The main difference lies in whether the quantity refers to a unit mass (specific heat capacity) or the total mass of the object (heat capacity).

 

Question 5. Explain the following effects of heat.
(i) Expansion
(ii) Change in temperature
(iii) Change in state
(iv) Chemical changes.
Answer:
(i) Expansion: When heat is added to a substance, its molecules gain energy and start to vibrate more vigorously. This increased vibration forces the molecules further apart, causing the substance to increase in size, which is called expansion. For instance, gaps are left in railway tracks to allow for expansion in hot weather. Expansion is generally greater in gases, followed by liquids, and is least in solids.

(ii) Change in temperature: Adding heat energy to a substance increases the kinetic energy of its particles, making them move faster. This increased average kinetic energy is observed as a rise in the substance's temperature. Conversely, when heat is removed from a substance, its molecules lose energy, slow down, and its temperature falls.

(iii) Change in state: Heat can cause a substance to change from one physical state to another (e.g., solid to liquid, liquid to gas). When ice cubes are heated, they first turn into water (solid to liquid). With further heating, the water turns into vapor (liquid to gas). These changes happen at specific temperatures, and the reverse processes (gas to liquid, liquid to solid) occur when heat is removed.

(iv) Chemical changes: Heat energy plays a crucial role in many chemical reactions. Sometimes, heat is needed to start a reaction, like lighting a fire. Heat can also influence how fast a chemical reaction happens. For example, when we cook food, applying heat causes chemical changes that make the food soft and digestible. These processes involve breaking and forming chemical bonds.
In simple words: Heat can make things bigger (expansion), hotter or colder (temperature change), turn from solid to liquid or gas (change of state), and make new things by changing chemicals (chemical changes).

🎯 Exam Tip: For each effect, explain the underlying molecular behavior (e.g., increased vibration for expansion, kinetic energy for temperature, bond changes for chemical reactions) and provide a clear example.

 

VIII. Numerical Problems

 

Question 1. What is the amount of heat required to raise the temperature of 5 kg of iron from \( 30^{\circ}\text{C} \) to \( 130^{\circ}\text{C} \)? Specific heat capacity of iron = \( 483 \text{ Jkg}^{-1}{^{\circ}\text{C}}^{-1} \).
Answer:
Given:
Mass of iron \( m = 5 \text{ kg} \)
Initial temperature \( t_1 = 30^{\circ}\text{C} \)
Final temperature \( t_2 = 130^{\circ}\text{C} \)
Change in temperature \( \Delta t = t_2 - t_1 = 130^{\circ}\text{C} - 30^{\circ}\text{C} = 100^{\circ}\text{C} \)
Specific heat capacity of iron \( c = 483 \text{ Jkg}^{-1}{^{\circ}\text{C}}^{-1} \)

The amount of heat required \( Q \) is calculated using the formula:
\( Q = m \times c \times \Delta t \)
\( Q = 5 \text{ kg} \times 483 \text{ Jkg}^{-1}{^{\circ}\text{C}}^{-1} \times 100^{\circ}\text{C} \)
\( Q = 2415 \times 100 \)
\( Q = 2,41,500 \text{ J} \)
Therefore, \( 2,41,500 \) Joules of heat are required.
In simple words: To find out how much heat is needed to warm up the iron, we multiply its mass by how much its temperature changes and by its specific heat capacity.

🎯 Exam Tip: Always ensure that all units are consistent (e.g., kg and Jkg\(^{-1}\) for mass and specific heat) before performing calculations.

 

Question 2. Calculate the amount of heat required to convert 200g of ice at \( 0^{\circ}\text{C} \) into water at \( 0^{\circ}\text{C} \). Specific latent heat of fusion of ice = \( 336 \text{ Jg}^{-1} \).
Answer:
Given:
Mass of ice \( m = 200 \text{ g} \)
Specific latent heat of fusion of ice \( L = 336 \text{ Jg}^{-1} \)

The heat required to change the state of a substance without changing its temperature is given by the formula:
\( Q = m \times L \)
\( Q = 200 \text{ g} \times 336 \text{ Jg}^{-1} \)
\( Q = 67,200 \text{ J} \)
So, \( 67,200 \) Joules of heat are required to convert 200g of ice at \( 0^{\circ}\text{C} \) into water at \( 0^{\circ}\text{C} \).
In simple words: To melt the ice without changing its temperature, we multiply its weight by the "hidden heat" needed to turn ice into water.

🎯 Exam Tip: Remember that during a phase change (like melting ice into water at \( 0^{\circ}\text{C} \)), the temperature remains constant, and the heat exchanged is solely latent heat.

 

Question 3. 2875 J of heat is required to melt 115 g of lead at its melting point. Calculate the specific latent heat capacity of fusion of lead.
Answer:
Given:
Heat required \( Q = 2875 \text{ J} \)
Mass of lead \( m = 115 \text{ g} \)
Specific latent heat of fusion of lead \( L = ? \)

We know the formula relating heat, mass, and specific latent heat:
\( Q = m \times L \)
To find \( L \), we rearrange the formula:
\( L = \frac{Q}{m} \)
\( L = \frac{2875 \text{ J}}{115 \text{ g}} \)
\( L = 25 \text{ Jg}^{-1} \)
Thus, the specific latent heat capacity of fusion of lead is \( 25 \text{ Jg}^{-1} \). This means 25 Joules are needed to melt one gram of lead.
In simple words: To find the "hidden heat" needed to melt one gram of lead, we divide the total heat used by the amount of lead melted.

🎯 Exam Tip: Ensure that the units of heat (Joules) and mass (grams) are consistent with the required unit for specific latent heat (Jg\(^{-1}\)).

 

Question 4. What will be the final temperature if 1,68,000 J of heat is absorbed by 2 kg of water, initially at \( 30^{\circ}\text{C} \)? Specific heat capacity of water \( C = 4200 \text{ J kg}^{-1}{^{\circ}\text{C}}^{-1} \).
Answer:
Given:
Heat absorbed \( Q = 1,68,000 \text{ J} \)
Mass of water \( m = 2 \text{ kg} \)
Initial temperature \( t_1 = 30^{\circ}\text{C} \)
Specific heat capacity of water \( C = 4200 \text{ J kg}^{-1}{^{\circ}\text{C}}^{-1} \)
Let the final temperature be \( t_2 \).
The rise in temperature \( \Delta t = (t_2 - t_1) = (t_2 - 30)^{\circ}\text{C} \)

We use the formula for heat absorbed:
\( Q = m \times C \times \Delta t \)
Substitute the given values:
\( 1,68,000 = 2 \times 4200 \times (t_2 - 30) \)
\( 1,68,000 = 8400 \times (t_2 - 30) \)
Divide both sides by 8400:
\( \frac{1,68,000}{8400} = t_2 - 30 \)
\( 20 = t_2 - 30 \)
Now, solve for \( t_2 \):
\( t_2 = 20 + 30 \)
\( t_2 = 50^{\circ}\text{C} \)
So, the final temperature of the water is \( 50^{\circ}\text{C} \).
In simple words: We know how much heat the water absorbed and its starting temperature. Using a formula, we can find out how much its temperature went up, and then calculate its new, final temperature.

🎯 Exam Tip: Clearly identify all knowns and unknowns, then apply the formula \( Q = mc\Delta T \) and rearrange it to solve for the required variable, showing each algebraic step.

 

Question 5. A metal ball of heat capacity \( 50 \text{ J/}^{\circ}\text{C} \) loses 2000 J of heat. By how much will its temperature fall?
Answer:
Given:
Heat capacity of the metal ball \( C_{\text{heat}} = 50 \text{ J/}^{\circ}\text{C} \)
Heat lost \( Q = 2000 \text{ J} \)
Let the fall in temperature be \( \Delta t \).

The relationship between heat lost/gained, heat capacity, and change in temperature is:
\( Q = C_{\text{heat}} \times \Delta t \)
To find the fall in temperature, rearrange the formula:
\( \Delta t = \frac{Q}{C_{\text{heat}}} \)
\( \Delta t = \frac{2000 \text{ J}}{50 \text{ J/}^{\circ}\text{C}} \)
\( \Delta t = 40^{\circ}\text{C} \)
The temperature of the metal ball will fall by \( 40^{\circ}\text{C} \).
In simple words: We can find how much an object's temperature drops by dividing the heat it loses by its heat capacity.

🎯 Exam Tip: Distinguish between specific heat capacity and heat capacity; heat capacity is for the entire object, while specific heat capacity is per unit mass.

 

IX. Convert The Following:

 

Question 1. 100°F to °C
Answer:
To convert Fahrenheit to Celsius, use the formula:
\( \text{T}(^{\circ}\text{C}) = (\text{T}(^{\circ}\text{F}) - 32) / 1.8 \)
Given \( \text{T}(^{\circ}\text{F}) = 100^{\circ}\text{F} \)
\( \text{T}(^{\circ}\text{C}) = (100 - 32) / 1.8 \)
\( \text{T}(^{\circ}\text{C}) = 68 / 1.8 \)
\( \text{T}(^{\circ}\text{C}) \approx 37.77^{\circ}\text{C} \)
So, \( 100^{\circ}\text{F} \) is approximately \( 37.7^{\circ}\text{C} \).
In simple words: To change Fahrenheit to Celsius, subtract 32 from the Fahrenheit number and then divide by 1.8.

🎯 Exam Tip: Memorize the conversion formulas for temperature scales to quickly solve these types of problems.

 

Question 2. 40°C to Fahrenheit (°F)
Answer:
To convert Celsius to Fahrenheit, use the formula:
\( \text{T}(^{\circ}\text{F}) = (\text{T}(^{\circ}\text{C}) \times 1.8) + 32 \)
Given \( \text{T}(^{\circ}\text{C}) = 40^{\circ}\text{C} \)
\( \text{T}(^{\circ}\text{F}) = (40 \times 1.8) + 32 \)
\( \text{T}(^{\circ}\text{F}) = 72 + 32 \)
\( \text{T}(^{\circ}\text{F}) = 104^{\circ}\text{F} \)
So, \( 40^{\circ}\text{C} \) is equal to \( 104^{\circ}\text{F} \).
In simple words: To change Celsius to Fahrenheit, multiply the Celsius number by 1.8 and then add 32.

🎯 Exam Tip: Be careful with the order of operations; multiplication must be done before addition in the Celsius to Fahrenheit conversion.

 

Question 3. 35°C to Kelvin
Answer:
To convert Celsius to Kelvin, use the formula:
\( \text{T}(\text{K}) = \text{T}(^{\circ}\text{C}) + 273.15 \)
Given \( \text{T}(^{\circ}\text{C}) = 35^{\circ}\text{C} \)
\( \text{T}(\text{K}) = 35 + 273.15 \)
\( \text{T}(\text{K}) = 308.15 \text{ K} \)
So, \( 35^{\circ}\text{C} \) is equal to \( 308.15 \text{ K} \). Kelvin is the absolute temperature scale.
In simple words: To change Celsius to Kelvin, just add 273.15 to the Celsius temperature.

🎯 Exam Tip: Remember that the Kelvin scale starts at absolute zero, so there are no negative temperatures in Kelvin.

 

Question 4. 80°K to °C
Answer:
To convert Kelvin to Celsius, use the formula:
\( \text{T}(^{\circ}\text{C}) = \text{T}(\text{K}) - 273.15 \)
Given \( \text{T}(\text{K}) = 80 \text{ K} \)
\( \text{T}(^{\circ}\text{C}) = 80 - 273.15 \)
\( \text{T}(^{\circ}\text{C}) = -193.15^{\circ}\text{C} \)
So, \( 80 \text{ K} \) is equal to \( -193.15^{\circ}\text{C} \).
In simple words: To change Kelvin to Celsius, subtract 273.15 from the Kelvin temperature.

🎯 Exam Tip: Be mindful of negative signs when converting from Kelvin to Celsius, especially for temperatures below \( 273.15 \text{ K} \).

 

X. Define The Following:

 

Question 1. Define Heat.
Answer: Heat is a form of energy that moves from an area of higher temperature to an area of lower temperature within or between objects. It represents the total kinetic and potential energy of the molecules in a substance. Heat transfer is a fundamental process in the universe.
In simple words: Heat is energy that moves from hot places to cold places.

🎯 Exam Tip: Emphasize that heat is energy in transit, always flowing from higher to lower temperature.

 

Question 2. Define Conduction.
Answer: Conduction is a process of heat transfer that primarily occurs in solids. In conduction, heat moves from a region of higher temperature to a region of lower temperature through the direct contact and vibration of molecules, without the actual bulk movement of the material itself. It is how heat travels along a metal rod when one end is heated.
In simple words: Conduction is when heat moves through a solid material by tiny parts (molecules) bumping into each other, but the material itself doesn't move.

🎯 Exam Tip: Highlight "direct contact" and "vibration of molecules" as key aspects of conduction, specifically in solids.

 

Question 3. Define Convection.
Answer: Convection is a mode of heat transfer that happens in fluids (liquids and gases). It involves the transfer of heat from hotter regions to colder regions by the actual movement of the fluid particles themselves. Hotter, less dense fluid rises, and colder, denser fluid sinks, creating a convection current that transfers heat. This process is how boiling water heats up in a pot.
In simple words: Convection is when heat moves through liquids or gases because the warmer parts move and carry the heat with them.

🎯 Exam Tip: Key elements of convection are "fluid movement," "density differences," and "convection currents."

 

Question 4. Define Radiation.
Answer: Radiation is a method of heat transfer that does not require any material medium (like air or water) to transfer heat energy. Instead, heat is transferred through electromagnetic waves, such as infrared radiation. This is how heat from the sun travels through the vacuum of space to Earth, and how a glowing fire warms you from a distance.
In simple words: Radiation is when heat moves as invisible waves, and it doesn't need any air or water to travel.

🎯 Exam Tip: Emphasize that radiation does not require a medium and travels via electromagnetic waves.

 

Question 5. Define Temperature.
Answer: Temperature is a physical quantity that measures the degree of hotness or coldness of a body. It indicates the average kinetic energy of the particles within a substance. A higher temperature means the particles are moving faster on average, while a lower temperature means they are moving slower. Temperature is a key indicator for the direction of heat flow.
In simple words: Temperature tells us how hot or cold something is, based on how fast its tiny parts are moving.

🎯 Exam Tip: Relate temperature directly to the average kinetic energy of a substance's particles, not the total energy.

 

Question 6. Define Specific heat capacity.
Answer: Specific heat capacity is the amount of heat energy required to raise the temperature of 1 kilogram (or 1 gram) of a specific substance by \( 1^{\circ}\text{C} \) (or 1 Kelvin). This value is unique for each material and helps predict how easily a substance will heat up or cool down. Water has a high specific heat capacity, meaning it takes a lot of energy to change its temperature.
In simple words: Specific heat capacity is how much heat energy is needed to warm up a small amount (like 1 kg) of a specific material by 1 degree.

🎯 Exam Tip: Always include the unit mass (e.g., 1 kg) and unit temperature change (e.g., \( 1^{\circ}\text{C} \)) in your definition of specific heat capacity.

 

Question 7. Define Heat capacity or thermal capacity.
Answer: Heat capacity, also known as thermal capacity, is the total amount of heat energy required to raise the temperature of an entire object or a given mass of a substance by \( 1^{\circ}\text{C} \) (or 1 Kelvin). Unlike specific heat capacity, which is per unit mass, heat capacity is for the whole body and therefore depends on both the material and its total mass. A larger object of the same material will have a higher heat capacity.
In simple words: Heat capacity is how much heat is needed to warm up a whole object by 1 degree.

🎯 Exam Tip: Differentiate heat capacity by emphasizing it applies to the *entire body* or a *given mass*, contrasting it with specific heat capacity which is per unit mass.

 

Question 8. Define Change of state.
Answer: A change of state, or phase transition, is the physical process where a substance transforms from one state of matter (solid, liquid, gas, plasma) to another at a specific, constant temperature and pressure. For example, melting (solid to liquid) or boiling (liquid to gas) are changes of state. During this process, heat is absorbed or released without a change in temperature.
In simple words: A change of state is when a material turns from a solid to a liquid, or a liquid to a gas, at a certain temperature.

🎯 Exam Tip: Clearly state that changes of state occur at a *definite temperature* and involve the absorption or release of latent heat.

 

Question 9. Define Melting or fusion.
Answer: Melting, also called fusion, is the physical process where a substance changes from a solid state to a liquid state. This transformation occurs when the solid absorbs enough heat energy (latent heat of fusion) at its specific melting point, causing its particles to overcome their rigid structure and move more freely. For example, ice melts into water at \( 0^{\circ}\text{C} \).
In simple words: Melting (or fusion) is when a solid turns into a liquid by taking in heat, like ice turning into water.

🎯 Exam Tip: Always specify that melting involves changing from a *solid* to a *liquid* state and requires *absorbing* heat at a constant temperature.

 

Question 10. Define Boiling or vaporization.
Answer: Boiling, also known as vaporization, is the process where a substance changes from a liquid state to a gaseous (vapor) state. This happens when the liquid absorbs sufficient heat energy (latent heat of vaporization) at its boiling point, allowing its particles to escape as a gas. Water turning into steam at \( 100^{\circ}\text{C} \) is a common example. Evaporation is a similar process but occurs at temperatures below the boiling point.
In simple words: Boiling (or vaporization) is when a liquid turns into a gas by taking in heat, like water becoming steam.

🎯 Exam Tip: Clarify that boiling is a bulk phenomenon occurring at the boiling point, distinct from evaporation which occurs at the surface at any temperature.

 

Question 11. Define Sublimation.
Answer: Sublimation is a unique physical process where a solid substance changes directly into a gaseous state without first passing through the liquid state. This occurs when the solid absorbs enough energy for its particles to escape directly into the gas phase. A common example is dry ice (solid carbon dioxide) turning directly into carbon dioxide gas at room temperature.
In simple words: Sublimation is when a solid turns straight into a gas, without ever becoming a liquid first.

🎯 Exam Tip: The key characteristic of sublimation is the direct transition from solid to gas, bypassing the liquid phase.

 

Question 12. Define Latent heat.
Answer: Latent heat refers to the "hidden" heat energy absorbed or released by a substance during a change of state (e.g., melting, freezing, boiling, condensation) without any corresponding change in its temperature. This energy is used to break or form intermolecular bonds, rather than to increase the kinetic energy of the particles. It's crucial for understanding phase transitions.
In simple words: Latent heat is the energy taken in or given out when a material changes its form (like solid to liquid) without changing its temperature.

🎯 Exam Tip: Stress that latent heat is associated with phase changes and occurs at a constant temperature.

 

Question 13. Define Specific latent heat.
Answer: Specific latent heat is the amount of heat energy absorbed or liberated by a *unit mass* of a substance during a change of state at a constant temperature and pressure. It is specific to the type of substance and the particular phase transition (e.g., specific latent heat of fusion for melting, or specific latent heat of vaporization for boiling). This value tells us how much energy is needed per gram or per kilogram of a substance to change its state.
In simple words: Specific latent heat is the amount of "hidden heat" needed for a small amount (like 1 kg) of a material to change its form (solid to liquid or liquid to gas) without its temperature changing.

🎯 Exam Tip: Remember to specify "unit mass" in your definition to distinguish specific latent heat from total latent heat.

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