Selina Concise Solutions for ICSE Class 7 Physics Chapter 5 Heat

Points to Remember

1. Heat is a form of energy that leads to the sensations of hotness or coldness.
2. Temperature is the degree of hotness and coldness of a body.
3. Thermometer is used to measure temperature.
4. The S.I. unit of temperature is °C.
5. The most common liquid for a thermometer is mercury.
6. The main sources of heat are (i) Fire (ii) Sun (iii) Electricity.
7. Those substances which can easily catch fire are called inflammable substances.
8. Those substances which are fire resistant are called non-inflammable substances.
9. The fixed temperature at which freezing of liquid occurs is known as freezing point.
10. The temperature at which vapourisation occurs is known as the boiling point.
11. Substances through which heat is easily conducted are called good conductors e.g. silver, gold, copper etc.
12. Substances through which heat is not easily conducted are called Insulators.
13. Radiation is the process of transfer of heat from a hot body to a cold body without affecting the intermediate medium.

Test Yourself

A. Objective Questions

1. Write true or false for each statement

(a) On touching a lump of ice, we feel cold because some heat passes from our body to the ice.
Answer: True.
Heat naturally travels from a region of higher temperature to a region of lower temperature. When you touch ice, your body loses thermal energy to the ice, which your brain interprets as a cold sensation.
Teacher's Tip: Remember that "cold" isn't a thing that flows; it is just the feeling of heat leaving your body.
Exam Tip: Always mention the direction of heat flow (hot to cold) to explain sensations of temperature.

(b) Heat flows from a body at a high temperature to a body at a low temperature when they are kept in contact. .
Answer: True.
This is the fundamental principle of heat transfer known as thermal equilibrium. The energy will continue to flow until both objects reach the exact same temperature.
Teacher's Tip: Think of heat flow like water flowing from a high tank to a low tank.
Exam Tip: Use the term "thermal contact" to describe when two objects are touching and exchanging heat.

(c) All solids expand by the same amount when heated to the same rise in temperature.
Answer: False.
Different materials have different internal atomic structures and bonding strengths. Because of this, a rod of iron will expand a different amount than a rod of aluminum even if they are heated by the same number of degrees.
Teacher's Tip: Every material has a unique "expansion personality" called its coefficient of expansion.
Exam Tip: If a statement says "all" or "always" in physics, check carefully for exceptions like different material properties.

(d) Telephone wires are kept tight between the two poles in summer.
Answer: False.
Materials expand when they get hot in the summer and contract when they get cold in the winter. If wires were kept perfectly tight in summer, they would snap when they tried to shrink during a cold winter night.
Teacher's Tip: Engineers always leave "slack" in wires to allow for seasonal changes.
Exam Tip: Remember that "tight in summer" leads to "breaking in winter."

(e) Equal volumes of different liquids expand by different amounts when they are heated to the same rise in temperature.
Answer: True.
Just like solids, different liquids such as water, oil, and mercury have different rates of thermal expansion. This property is why mercury is chosen for thermometers while water is not.
Teacher's Tip: This is the liquid version of the rule for solids; material type matters!
Exam Tip: Note that liquids generally expand much more than solids do for the same temperature change.

(f) Solids expand the least and gases expand the most on being heated.
Answer: True.
In solids, atoms are tightly bound, while in gases, particles are far apart and move freely. This freedom allows gas particles to push away from each other much more significantly when they gain kinetic energy from heat.
Teacher's Tip: Use the order: Gas > Liquid > Solid for expansion rates.
Exam Tip: Mention the "inter-molecular force" being weakest in gases to explain why they expand the most.

(g) A mercury thermometer makes use of the property of expansion of liquids on heating.
Answer: True.
When the thermometer bulb touches a warm object, the mercury inside gains heat and increases in volume. Since it has nowhere else to go, it rises up the narrow capillary tube to show the temperature.
Teacher's Tip: Mercury is used because its expansion is very uniform and easy to see.
Exam Tip: Describe the "rise of the liquid column" as the visible evidence of thermal expansion.

(h) Kerosene contracts on heating.
Answer: False.
Almost all substances, including kerosene, expand when they are heated. Water between 0°C and 4°C is a rare exception that actually contracts, but kerosene follows the normal rule.
Teacher's Tip: Heating usually means "growing" (expansion).
Exam Tip: Don't let trick questions about specific liquids confuse you; expansion is the general rule for heating.

(i) Water is a bad conductor of heat.
Answer: True.
While water can transfer heat very efficiently through the movement of its particles (convection), the particles do not pass energy well from one to another while staying still (conduction). This is why you can boil water at the top of a tube while the bottom remains icy.
Teacher's Tip: Think of water as a great "mover" (convection) but a poor "passer" (conduction).
Exam Tip: Distinguish between the "medium" (water) and the "method" (conduction vs. convection).

(j) Medium is necessary for the transfer of heat by radiation.
Answer: False.
Radiation travels as electromagnetic waves, which do not need any atoms or molecules to move through. This is how the Sun's heat reaches Earth through millions of miles of empty vacuum.
Teacher's Tip: Radiation is the only mode of heat transfer that can travel through a "nothing" (vacuum).
Exam Tip: Always use the Sun-to-Earth example to prove that radiation doesn't need a medium.

(k) Land and sea breezes are convection currents of cold and warm air.
Answer: True.
These breezes occur because land heats up and cools down faster than water. The resulting temperature difference causes air to rise in one place and sink in another, creating a circular flow.
Teacher's Tip: Remember: Sea breeze happens during the day; Land breeze happens at night.
Exam Tip: Mention that air pressure differences caused by temperature are the root cause of these winds.

(l) Liquids are heated by conduction and radiation.
Answer: False.
While conduction and radiation play minor roles, the primary and most effective way liquids heat up is through convection. Convection involves the actual movement of hot, less dense liquid rising and cool, denser liquid sinking.
Teacher's Tip: Liquids and gases are "Fluids," and fluids prefer Convection!
Exam Tip: If asked for the main mode of heating in fluids, always specify Convection.

(m) Black surfaces are the poor absorbers of heat radiations.
Answer: False.
Dark and rough surfaces, especially black ones, are excellent at soaking up radiant energy. This is why black objects get much hotter in the sun compared to white or shiny objects.
Teacher's Tip: Black is "greedy" for heat; it absorbs almost everything that hits it.
Exam Tip: Remember the rule: Black is a good absorber AND a good radiator of heat.

2. Fill in the blanks

(a) Heat is a form of energy.
Answer: energy.
Energy is the ability to do work, and heat can definitely do work, like moving a steam engine. It is the total internal kinetic energy of all the molecules in a substance.
Teacher's Tip: Just like light and sound, heat is one of the many "flavors" of energy.
Exam Tip: Always capitalize "S.I. unit" if you are explaining heat as energy in a definition.

(b) Temperature determines the degree of hotness or coldness of a body.
Answer: Temperature.
Temperature is a measurement that tells us how fast the molecules in an object are moving on average. It doesn't tell us the total heat, but rather the "intensity" of the heat.
Teacher's Tip: Think of heat as the "total amount" of soup and temperature as "how spicy" it is.
Exam Tip: Don't confuse Heat (total energy) with Temperature (average energy).

(c) On heating a body, its temperature rises.
Answer: rises.
Adding thermal energy makes the particles inside a body vibrate or move faster. This increased molecular speed is what we measure as a higher temperature reading.
Teacher's Tip: Heat IN = Temp UP. Heat OUT = Temp DOWN.
Exam Tip: Note that during a "change of state" (like melting), the temperature stays the same even if you add heat.

(d) We use a thermometer for measuring the temperature of a body.
Answer: thermometer.
Thermometers are designed with materials that react predictably to heat, like expanding mercury or changing electrical resistance. They provide a standardized scale so everyone can agree on how hot an object is.
Teacher's Tip: "Thermo" means heat and "meter" means to measure.
Exam Tip: Name specific types like "Clinical" or "Laboratory" thermometers if the question asks for details.

(e) The S.I. unit of temperature is kelvin.
Answer: kelvin.
The Kelvin scale is used by scientists because it starts at "Absolute Zero," where all molecular motion stops. Unlike Celsius, the Kelvin scale does not use "degrees" in its name, just the word Kelvin.
Teacher's Tip: Remember: Kelvin is for the lab; Celsius is for the kitchen.
Exam Tip: 0 Kelvin is equal to -273.15°C.

(f) In a thermometer, the commonly used liquid is mercury.
Answer: mercury.
Mercury is a liquid metal that stays liquid over a very wide temperature range. It also doesn't stick to the glass walls of the thermometer tube, making readings very accurate.
Teacher's Tip: Mercury is silver and shiny, making it easy to read against a scale.
Exam Tip: List "uniform expansion" and "good conductor" as reasons why mercury is used.

(g) The temperature of a normal human body is 37°C.
Answer: 37°C.
Our bodies work hard to maintain this specific internal temperature so our organs can function correctly. If this number goes significantly higher or lower, it usually indicates an illness.
Teacher's Tip: 37°C is about 98.6°F.
Exam Tip: Memorize this exact number as it is a very common "general knowledge" science question.

(h) A person is said to have fever if his body temperature is more than 98.6
Answer: 98.6.
This value is on the Fahrenheit scale, which is commonly used in clinical thermometers at home. Any reading above this normal baseline indicates that the body is fighting an infection.
Teacher's Tip: On the Celsius scale, a fever is usually anything above 38°C.
Exam Tip: Always include the unit (°F or °C) when writing temperature values.

(i) A hot metallic piece is placed in tap water contained in a bucket. Heat will flow from metallic piece to water.
Answer: metallic piece to water.
Since the metal is hot and the tap water is relatively cool, the energy moves from the higher temperature object to the lower one. This process continues until both the metal and the water are at the same lukewarm temperature.
Teacher's Tip: Heat is like a "downhill" flow from high temp to low temp.
Exam Tip: Use the phrase "direction of heat flow" when explaining these scenarios.

(j) The temperature of boiling water is 100°C.
Answer: 100°C.
At standard sea-level air pressure, water will always turn into steam at exactly this temperature. This fixed point is used to calibrate the upper end of the Celsius scale.
Teacher's Tip: 100°C is the "Steam Point."
Exam Tip: Specify "at sea level" if you want to show advanced knowledge, as pressure affects boiling point.

(k) Liquids expand more than the solids.
Answer: more.
The particles in a liquid have weaker bonds than those in a solid, so they can push apart more easily when heated. This difference in expansion rates is exactly how a glass thermometer works without the glass breaking.
Teacher's Tip: Imagine solid particles are "holding hands" while liquid particles are just "rubbing shoulders."
Exam Tip: This fact is why we leave space at the top of soda bottles; the liquid inside might expand!

(l) Gases expand more than the liquids.
Answer: more.
Gas particles have almost no forces holding them together, so heat makes them zoom apart very quickly. A small temperature change in a gas causes a much larger volume change than in a liquid.
Teacher's Tip: Air is a gas, and a balloon grows much faster when heated than a bottle of water does.
Exam Tip: Remember the sequence: Gas > Liquid > Solid for expansion magnitude.

(m) Heat transfer in solids is by conduction.
Answer: conduction.
In a solid, the particles are fixed in one spot, so they can only vibrate and bump into their neighbors to pass energy along. This "chain reaction" of vibrations is the definition of conduction.
Teacher's Tip: Think of conduction as a "bucket brigade" where people pass buckets without moving their feet.
Exam Tip: Mention that conduction requires "direct contact" between the heat source and the object.

(n) Heat transfer in liquids and gases is by convection.
Answer: convection.
Because particles in fluids (liquids and gases) can move around, the hot particles actually travel and carry their heat with them. This creates circular patterns called convection currents.
Teacher's Tip: Convection is like people carrying buckets of water across a room themselves.
Exam Tip: Use the keyword "fluid" to refer to both liquids and gases in your answers.

(o) Metals are conductors of heat.
Answer: conductors.
Metals have "free electrons" that can zip through the material, carrying thermal energy much faster than simple atomic vibrations can. This makes them ideal for things like cooking pots and heat sinks.
Teacher's Tip: Most good conductors of electricity (like copper) are also good conductors of heat.
Exam Tip: Give examples like silver, copper, and iron to support this statement.

(p) Still air is an insulator of heat.
Answer: insulator.
Air molecules are very far apart, which makes it extremely difficult for them to pass heat through conduction. If the air is trapped and cannot move (still air), it also prevents convection, making it a great heat barrier.
Teacher's Tip: This is why fluffy blankets and woolen clothes keep you warm; they trap "still air."
Exam Tip: Use the word "trapped air" when explaining how insulators like wool or foam work.

(q) Black and dull surfaces are good absorbers of heat.
Answer: good absorbers.
Light and heat radiation that hit a black surface are mostly soaked up rather than bounced away. This absorbed energy is converted into heat, making the surface temperature rise quickly.
Teacher's Tip: Don't wear a black t-shirt on a sunny day if you want to stay cool!
Exam Tip: Mention that black surfaces are also "good radiators," meaning they lose heat quickly in the shade.

3. Match the following

Column A
(a) mercury
(b) wood
(c) aluminum
(d) contracts
(e) black surface
Column B
(i) insulator
(ii) water from 0 ^∘ C to 4 ^∘ C
(iii) absorbs
(iv) conductor
(v) thermometer

Column A - Column B
(a) mercury - (v) thermometer
(b) wood - (i) insulator
(c) aluminum - (iv) conductor
(d) contracts - (ii) water from 0°C to 4°C
(e) black surface - (iii) absorbs
Answer: (a)-(v), (b)-(i), (c)-(iv), (d)-(ii), (e)-(iii)
This exercise connects specific materials and behaviors to their scientific roles in heat study. For example, mercury is matched to a thermometer because its expansion property makes it a perfect tool for measurement.
Teacher's Tip: "Water from 0 to 4" is the weird one; it's the only liquid that shrinks when you warm it up in that range!
Exam Tip: When matching, draw straight lines and double-check that every item has a unique pair.

4. Select the correct alternative

(a) If we add a lump of ice to a tumbler containing water,
1. heat flows from water to ice
2. heat flows from ice to water
3. heat flows from water to ice if water is more
4. heat flows from ice to water if ice is more
Answer: 1. heat flows from water to ice.
Heat always moves from the object with the higher temperature to the one with the lower temperature. Since liquid water is always warmer than frozen ice, the energy moves into the ice to melt it.
Teacher's Tip: The amount of water doesn't matter; only the temperature difference dictates the flow direction.
Exam Tip: Always identify which object is "hotter" to find the starting point of the heat flow.

(b) The temperature of pure melting ice is
1. 0°C
2. 100°C
3. 95°C
4. 98.6°F
Answer: 1. 0°C.
This is the "Ice Point" or freezing point of water on the Celsius scale. As long as there is a mix of ice and water, the temperature will stay exactly at 0°C.
Teacher's Tip: 0°C is 32°F.
Exam Tip: If the question says "pure" ice, it means there are no impurities like salt that could lower the freezing point.

(c) A thermometer uses
1. water
2. mercury
3. air
4. none of the above
Answer: 2. mercury.
Mercury is preferred because it is a liquid metal with a very uniform expansion rate. It is also shiny, which makes it much easier to read through the glass of the thermometer.
Teacher's Tip: Some modern thermometers use alcohol (red color), but mercury is the traditional lab choice.
Exam Tip: List "opaque" and "non-stick" as two reasons why mercury is better than water for thermometers.

(d) Which of the statement is correct
1. Iron rims are cooled before they are placed on cart wheels
2. A glass stopper gets tight on warming the neck of the bottle
3. Telephone wires sag in winter, but become tight in summer
4. A little space is left between two rails on a railway track
Answer: 4. A little space is left between two rails on a railway track.
Railway tracks are made of steel which expands during hot summer days. If there were no gaps, the expanding rails would push against each other and buckle or bend sideways.
Teacher's Tip: These gaps are called "expansion joints."
Exam Tip: Explain that these gaps prevent train accidents by keeping the tracks straight all year round.

(e) Heat in a liquid is transferred by
1. conduction
2. convection
3. radiation
4. conduction and radiation
Answer: 2. convection.
In liquids, heat causes the molecules to move further apart, making the hot parts less dense so they rise. This movement creates a current that spreads the heat throughout the entire liquid volume.
Teacher's Tip: Convection only happens in liquids and gases because their particles are free to move.
Exam Tip: Do not choose conduction for liquids unless it is specifically mentioned as a very slow secondary effect.

(f) In the process of convection, heat travels
1. sideways
2. downwards
3. upwards
4. in all directions
Answer: 3. upwards.
Hot fluids expand and become lighter (less dense), which naturally causes them to float to the top. Cold, heavier fluids then sink down to the bottom to take their place.
Teacher's Tip: Remember the phrase: "Heat rises!"
Exam Tip: Specify that it is "vertically upwards" for the most accurate description of convection currents.

(g) The vacuum kept in between the walls of a thermos flask reduces the heat transfer by
1. conduction only
2. convection only
3. radiation only
4. conduction and convection
Answer: 4. conduction and convection.
Since conduction and convection both require particles of matter to move heat, a vacuum (which has no particles) blocks them both completely. Radiation is the only mode that can still pass through a vacuum.
Teacher's Tip: A vacuum is the ultimate "no-entry" zone for most heat transfer.
Exam Tip: Remember that silvering the walls is what handles the radiation part of the flask's design.

B. Short/Long Answer Questions

Question 1: What is heat ? State its S.I. unit.
Answer: Heat is a form of energy which flows. It is the energy of motion of molecules constituting the body. The unit of heat is same as that of energy, The S.I. unit of heat is joule (abbreviated as J) and other common units of heat are calorie and kilo calorie, where 1 kcal = 1000 cal.
Heat is essentially the total kinetic energy of all the tiny particles that make up an object. When we add heat to something, we are just making its molecules zoom around faster.
Teacher's Tip: Heat is the "Total" energy, while temperature is the "Average" energy.
Exam Tip: Always provide the symbol (J) alongside the word Joule for a complete answer.

Question 2: What is meant by the term temperature.
Answer: Temperature is a quantity which tells the thermal state of a body (i.e. the degree of hotness or coldness). It determines the direction of flow of heat when the two bodies at different temperatures are placed in contact.
Think of temperature as a reading that tells you how "intense" the thermal energy is in an object. It is the scale we use to compare different objects to see which one will give heat and which one will take it.
Teacher's Tip: Temperature is like the "pressure" of heat; higher pressure flows to lower pressure.
Exam Tip: Mention that temperature is an "intensive property," meaning it doesn't change based on the amount of substance.

Question 3: State the three units of temperature.
Answer: The S.I. unit of temperature is kelvin or K. The other most common unit of temperature is degree Celsius (°C) and degree Fahrenheit (°F).
The Celsius scale is used for daily life in most countries, while Fahrenheit is common in the USA. Kelvin is the scientific standard because it has no negative numbers.
Teacher's Tip: Scientists love Kelvin because "0 K" means absolutely no movement at all.
Exam Tip: Do not use the degree symbol (°) with Kelvin; write it as just "K".

Question 4: Name the instrument used to measure the temperature of a body.
Answer: To measure the temperature of a body with the help of a thermometer.
There are various types of thermometers, such as clinical ones for the body and laboratory ones for experiments. Digital thermometers use electronic sensors to give even faster and more accurate readings.
Teacher's Tip: Modern thermometers use "thermistors" which are electronic parts that change with heat.
Exam Tip: If asked for an example, always mention the "mercury-in-glass" thermometer as the classic scientific tool.

Question 5: Name two scales of temperature. How are they inter-related?
Answer: Two scales of temperature are (i) Celsius (ii) Fahrenheit.
Relation:
Water freezes at 0°C or 32°F and boils at 100°C or 212°F
therefore (212 - 32) °F = (100 - 0) °C
180°F = 100°C
1°F = 5/9 °C
or 1°C = 9/5°F
or C - 0/100 - 0 = F - 32/2 12 - 32
or C/100 = F - 32/180
or C/5 = F - 32/9
This mathematical formula allows us to convert a temperature reading from one system to the other. It accounts for the different starting points (0 vs 32) and the different sized "steps" or degrees in each scale.
Teacher's Tip: Use the simplified formula C/5 =F - 32/9 for all your conversion homework.
Exam Tip: Always show the full substitution of values into the formula to avoid calculation errors.

Question 6: How is the size of a degree defined on a Celsius scale ?
Answer: The interval between the ice point and steam point divided by 100 (hundred) equal parts is called a degree on the Celsius scale.
Each of these parts represents a change of one degree Celsius. This scale is very logical because it is based on the behavior of water at sea level.
Teacher's Tip: Think of Celsius as a "metric" scale for heat since it uses base 100.
Exam Tip: Clearly state that the two fixed points are "ice point" (0°C) and "steam point" (100°C).

Question 7: How is the size of a degree defined on a Fahrenheit scale?
Answer: The interval between the ice point and steam point divided into 180 equal parts is called a degree on the Fahrenheit scale.
Because there are 180 parts instead of 100, a single degree Fahrenheit is smaller than a degree Celsius. This allows for more precise readings of small temperature changes without using decimals.
Teacher's Tip: 180 is the same number of degrees in a semi-circle; use that to remember it!
Exam Tip: Mention that the Fahrenheit scale starts at 32 for ice and ends at 212 for steam.

Question 8: State the temperature of (i) ice point and (ii) steam point, on the Celsius scale.
Answer: (i) Ice point. Is the mark on Celsius scale at which ice melts. Ice point on the Celsius scale is 0°C.
(ii) Steam point. On the Celsius scale is the mark at which water changes into steam at normal atmospheric pressure. On Celsius scale it is 100°C.
These two points are essential for building any thermometer. They are called "fixed points" because they never change as long as the water is pure and the air pressure is normal.
Teacher's Tip: Pure water is the secret ingredient for an accurate scale.
Exam Tip: Use the exact terms "Ice point" and "Steam point" instead of just saying "freezing" and "boiling."

Question 9: Write down the temperature of (i) lower fixed point, and (ii) upper fixed point, on the Fahrenheit scale.
Answer: Lower fixed point: On the Fahrenheit scale is the mark at which pure ice melts. It is 32°F on Fahrenheit scale.
Upper fixed point: On the Fahrenheit scale is the mark at which water starts changing into steam at normal atmospheric pressure. It is 212°F.
These numbers might seem random, but they were chosen by Daniel Gabriel Fahrenheit when he invented the scale in 1724. The lower point represents a mixture of ice and water, while the upper point is for boiling water.
Teacher's Tip: Subtract the two points: 212 - 32 = 180, which is the number of divisions.
Exam Tip: If a question asks for "lower fixed point," always define it as the melting point of pure ice.

Question 10: What is the Celsius scale of temperature ?
Answer: Celsius scale is that which has ice point as 0°C and steam point marked as 100°C.
Originally called the centigrade scale, it was renamed in 1948 to honor Anders Celsius. It is the most widely used temperature scale in the world for daily activities like weather and cooking.
Teacher's Tip: Centi-grade means "100 steps."
Exam Tip: Mention the two fixed points (0 and 100) whenever you define this scale.

Question 11: What is the Fahrenheit scale of temperature ?
Answer: Fahrenheit scale is that which has ice point as 32°F and the steam point marked as 212°F.
The total distance between freezing and boiling is split into 180 equal segments. While it's less common globally, it remains the standard for air temperature and body temperature in some regions like the United States.
Teacher's Tip: Think of 100°F as a very hot summer day, while 100°C is a pot of boiling water!
Exam Tip: Remember that 1°C change is almost twice as big as 1°F change.

Question 12: What is the Kelvin scale of temperature ?
Answer: On Kelvin scale of temperature zero mark is when no molecular motion occurs. Ice point is at 273 and steam point is at 373 K. Thus 0 K = - 273°C and one degree on Kelvin scale is same as one degree on Celsius scale.
This scale is used in thermodynamics because it has no negative values. If an object is at 0 K, its atoms have completely stopped moving, which is the coldest possible temperature in the universe.
Teacher's Tip: Kelvin is just "Celsius plus 273."
Exam Tip: Note that we write "K" but never say "degree Kelvin"—it is just called a "Kelvin."

Question 13: The fig. shows a glass tumbler containing hot milk which is placed in a tub of cold water. State the direction in which heat will flow.
Answer: When we bring two objects of different temperature together, energy will always be transferred from hotter to the cooler object. Here, also heat will flow from hot milk tumbler to tub of cold water.
This transfer happens because the fast-moving milk molecules crash into the glass walls, which then crash into the water molecules. This continues until the milk is cool and the water is warm, reaching a state of thermal balance.
Teacher's Tip: Heat "descends" from hot to cold just like gravity.
Exam Tip: In your answer, identify the two bodies and their relative temperatures clearly.

Question 14: Draw a neat labelled diagram of a laboratory thermometer.
Answer: (A diagram showing a glass stem with a mercury bulb at the bottom, a capillary tube inside, and markings from -10°C to 110°C).
The laboratory thermometer is much longer than a clinical one because it needs to measure a wider range of temperatures. It does not have a "kink" in the tube, so the mercury will fall back down as soon as it is removed from the heat.
Teacher's Tip: "Lab = Long" - remember the lab thermometer covers a larger range.
Exam Tip: Label the "Bulb," "Stem," "Capillary Tube," and "Mercury Column" for full marks.

Question 15: Write down the body temperature of a healthy person.
Answer: The temperature of a healthy persons is 98.6 degrees fahrenheit or 37.0 degree Celsius or 310 k.
These three numbers represent the same level of heat measured on different scales. Our bodies use energy from food to maintain this warmth, which is necessary for our chemical reactions to happen properly.
Teacher's Tip: 37 and 98.6 are the two numbers you should memorize for health.
Exam Tip: If the question doesn't specify a scale, providing both Celsius and Fahrenheit is the safest way to answer.

Question 16: What do you understand by thermal expansion of a substance ?
Answer: The expansion of a substance when, heated, is called thermal expansion. Or Thermal expansion is the tendency of matter to change in shape, area and volume in response to a change in temperature.
When atoms get hot, they vibrate more violently and push their neighbors away, needing more room to move. This collective pushing makes the entire object grow slightly larger in all directions.
Teacher's Tip: Heat makes molecules "antisocial"—they push each other away!
Exam Tip: Use the terms "shape," "area," and "volume" to describe the types of expansion.

Question 17: Name two substances which expand on heating.
Answer: Mercury and Aluminium wire.
Almost every metal, such as iron, copper, and silver, will expand when its temperature rises. This principle is why mercury is used in thermometers and why bridges have expansion gaps.
Teacher's Tip: Metals are usually the best examples of thermal expansion.
Exam Tip: Providing one liquid example (mercury) and one solid example (aluminium) shows a well-rounded understanding.

Question 18: Why do telephone wires sag in summer ?
Answer: The telephone wires will sag in summer due to expansions and will break in winter due to contraction. Therefore, while putting up the wires between the poles, care is taken that in summer they are kept slightly loose so that they may not break in winter due to contraction. While in winter they are kept light so that they may not sag too much in summer due to expansion.
Heat causes the metal in the wires to stretch and become longer, making them droop or sag toward the ground. If they were stretched perfectly tight on a hot day, they would surely snap when the freezing winter air makes them shrink.
Teacher's Tip: Think of it as "summer growth" and "winter shrinking."
Exam Tip: Always explain the winter "breaking" risk when discussing why summer sagging is allowed.

Question 19: Iron rims are heated before they are fixed on the wooden wheels. Explain the reason.
Answer: The wooden wheels of a bullock-cart are fitted with iron tyres. To ensure a tight fit, the tyre is made slightly smaller in diameter than the wheel. The tyre is first heated due to which it expands. The heated tyre is then fitted on the wheel. When the tyre cools, it contracts and makes a tight fit on the wheel.
By making the iron rim slightly too small, craftsmen use the physics of heat to create a powerful grip. As the hot metal cools and tries to shrink back, it squeezes the wooden wheel so tightly that it will never fall off.
Teacher's Tip: This is a practical use of "Expansion" followed by "Contraction."
Exam Tip: Use the word "diameter" to describe the size change of the circular rim.

Question 20: Why are gaps left between successive rails on a railway track ?
Answer: The rails of railway track are made of steel. While laying the railway track, a small gap is left between the two successive length of rails. The reason is that the rails expand in summer. The gap is provided to allow for this expansion. If no gap is left, the expansion in summer will cause the rails to bend sideways. This may result in a train accidents.
Steel rails can grow significantly on a 40°C summer day. Without these tiny gaps, the rails would have no room to grow and would push against each other with enough force to twist the entire track out of shape.
Teacher's Tip: These gaps are literal "lifesavers" for the trains!
Exam Tip: Connect the physical "expansion" to the practical "safety" risk of derailment.

Question 21: A glass stopper stuck in the neck of a bottle can be removed by pouring hot water on the neck of the bottle. Explain why ?
Answer: When hot water is poured over the neck of the bottle, it expands. As a result the stopper gets loosened and can be removed easily.
The heat from the water causes the glass molecules in the bottle's neck to move apart, widening the opening. Since the bottle neck grows faster than the heat can reach the internal stopper, the fit becomes loose.
Teacher's Tip: This trick works because the outside part (neck) expands before the inside part (stopper).
Exam Tip: Be specific that the *neck* expands to allow the stopper to move.

Question 22: Why is a cement floor laid in small pieces with gaps in between?
Answer: The floor is laid in small pieces with gaps in between to allow for the expansion during summer. However glass strips can be placed in the gaps.
A single giant sheet of concrete would eventually crack as it expands and contracts with the changing seasons. By breaking it into squares with flexible gaps, we give each piece its own "breathing room" to grow and shrink safely.
Teacher's Tip: These are called "control joints"—they control where the floor moves.
Exam Tip: Mention that the gaps prevent "cracking" or "buckling" of the floor.

Question 23: One end of a steel girder in a bridge is not fixed, but is kept on rollers. Give the reason.
Answer: In the construction of a bridge, steel girders are used. One end of the girder is fixed into the concrete or brick pillars and its other end is not fixed, but it is placed on rollers. The reason is that if there is any rise (or fall) in temperature of atmosphere, the girder can freely expand (or contract) without affecting the pillars.
Bridges are massive structures that can grow by several inches in the heat. Rollers allow the bridge to "slide" back and forth smoothly so it doesn't push over the support pillars and collapse the entire structure.
Teacher's Tip: Think of rollers like "wheels" that let the bridge move in its sleep.
Exam Tip: Use the terms "free expansion" and "avoiding stress on pillars" in your explanation.

Question 24: Describe one experiment to show that liquids expand on heating.
Answer: (i) Take an empty bottle with a tight fitting cork having a hole drilled in its middle, a drinking straw, two bricks, a wire gauze and a burner.
(ii) Fill the bottle completely with water and add few drops of ink in it to make it coloured.
(iii) Fix the cork in the mouth of the bottle and pass the drinking straw through the cork. Put some molten wax around the hole so as to avoid the leakage of water.
(iv) Pour some more water into the drinking straw so that water level in the straw can be seen. Mark the water level in the straw as shown in Figure.
(v) Place the bottle on the wire gauze kept over the two bricks as shown in Figure. Then heat the bottle by means of a burner.
(vi) Look at the level of water in the straw. You will notice that as the water is heated more and more, the level of water in the drinking straw rises. This shows that water expands on heating.
As the water molecules get hotter, they move faster and need more space, pushing up into the narrow straw where we can easily see the change. This is exactly how a clinical thermometer works, just using colored water instead of mercury.
Teacher's Tip: Adding ink is just to make the "show" easier to see!
Exam Tip: List the materials (apparatus) clearly at the beginning of your experiment description.

Question 25: State one application of thermal expansion of liquids.
Answer: Mercury is a metal found in liquid state. It expands more and uniformly over a wide range of temperature. So mercury is used as thermometric liquid.
Because liquid mercury expands exactly the same amount for every degree of heat, it creates a perfectly accurate ruler for temperature. This allows us to rely on thermometers for everything from weather reports to checking for a fever.
Teacher's Tip: Mercury is the only metal that is liquid at room temperature!
Exam Tip: Mention "uniform expansion" as the key reason why it is a good application.

Question 26: Describe an experiment to show that air expands on heating.
Answer: (i) Take an empty bottle. Actually the empty bottle contains air. Attach a rubber balloon to its neck as shown in Figure. Initially, the balloon is deflated.
(ii) Place the bottle in a water bath containing boiling water. After some time you will notice that the balloon gets inflated as shown in Figure. The reason is that the air inside the bottle expands on heating and it fills the balloon.
(iii) Take the bottle out of the water bath and allow it to cool by itself. We will notice that the balloon gets deflated and it collapses. This is because the air inside the balloon and the bottle, has contracted on cooling. The air from balloon passes to the bottle, so the balloon gets deflated.
When we heat the air, the gas molecules push against each other with more force, needing more volume. Since they are trapped, they push into the balloon and stretch it out, showing us the power of thermal expansion in gases.
Teacher's Tip: Air is invisible, but the balloon helps us "see" it growing.
Exam Tip: Mention both "inflation on heating" and "deflation on cooling" to provide a complete answer.

Question 27: An empty glass bottle is fitted with a narrow tube at its mouth. The open end of the tube is kept in a beaker containing water. When the bottle is heated, bubbles of air are seen escaping into the water. Explain the reason.
Answer: When the bottle is heated, bubbles of air are seen escaping into the water. This happens because the air present in glass bottle expands on heating and tries to escape out through the tube into the water.
As the air inside the bottle gets hot, it needs more space than the bottle provides. It pushes its way out through the only available path—the tube—and we see this as bubbles popping out under the water surface.
Teacher's Tip: Bubbles are just "parcels" of air that have been pushed out by expansion.
Exam Tip: The term for this movement is "thermal expansion of gas."

Question 28: State which expands more, when heated to the same temperature : solid, liquid or gas ?
Answer: Gases expand much more than the liquids and the solids. Like liquids, the gases do not have a definite shape, so they also have only the cubical expansion.
In a gas, there are almost no attractive forces keeping the molecules together, so they fly apart instantly when heated. Solids are the most "stubborn" and expand the least because their particles are locked in place.
Teacher's Tip: Remember the sequence: Gas > Liquid > Solid.
Exam Tip: Explain that gases have the most expansion because of "weak inter-molecular forces."

Question 29: Name the three modes of transfer of heat.
Answer: There are three modes of transfer of heat (i) Conduction (ii) Convection (iii) Radiation.
(i) Conduction “is that mode of transfer of heat, when heat travels from hot end to cold end from particle to particle of the medium, without actual movement of particles.”
(ii) Convection. “Is a process of transfer of heat by actual movement of the medium particles.”
(iii) Radiation. “Is that mode of transfer of heat in which heat directly passes from one body to the other body without heating the medium.”
Every way that heat moves falls into one of these three categories. Whether it's a metal spoon getting hot, water boiling, or sunlight warming your skin, one of these modes is at work.
Teacher's Tip: Conduction = Touching; Convection = Currents; Radiation = Rays.
Exam Tip: Be ready to identify which mode is active in daily life examples like a campfire or a radiator.

Question 30: Name the mode of transfer of heat in the following : (a) solid, (b) liquid, (c) gas (d) vacuum.
Answer: (a) Conduction (b) Convection (c) Convection (d) Radiation.
Solids must use conduction because their atoms can't move away from each other. Fluids like liquids and gases use convection because their particles are free to travel, and the vacuum can only use radiation because there are no particles at all.
Teacher's Tip: Radiation is the "loner" mode—it doesn't need anyone (particles) to help it travel.
Exam Tip: If a question mentions "vacuum," the answer is almost always radiation.

Question 31: What are the good and bad conductors of heat ? Give two examples of each.
Answer: Good conductors. “The substances through which heat is easily conducted are called good conductors of heat.” Example : Copper, iron.
Bad conductors. “The substances through which heat is not conducted easily are called bad conductors of heat or poor conductors of heat.” Example : Wood, cloth.
Good conductors are like fast highways for heat, letting energy zip through quickly. Bad conductors (also called insulators) are like traffic jams, slowing down the heat and keeping it trapped on one side.
Teacher's Tip: Most metals are "Good" and most non-metals are "Bad" at conducting heat.
Exam Tip: Use the term "Insulator" interchangeably with "Bad Conductor" in your answers.

Question 32: Name a liquid which is a good conductor of heat.
Answer: Mercury is good conductor of heat.
Most liquids are very poor at conducting heat, which is why we usually heat them through convection. Mercury is special because it is a metal, allowing its free electrons to pass thermal energy along very quickly.
Teacher's Tip: Mercury is the "odd one out" in the world of liquids.
Exam Tip: This is a favorite trick question for teachers; remember that mercury is the metallic liquid exception.

Question 33: Name a solid which is a good conductor of heat.
Answer: Aluminium is a good conductor of heat.
Silver is technically the best conductor of all, but aluminum is much more common and cheaper to use. Metals like aluminum are used for everything from car radiators to laptop cases to help move heat away quickly.
Teacher's Tip: If you need to cool something fast, wrap it in metal!
Exam Tip: You can also use Copper or Silver as correct examples for this question.

Question 34: Select good and bad conductors of heat from the following : copper, mercury, wood, iron, air, saw-dust, cardboard, silver, plastic, wool.
Answer: Good conductors — Mercury, copper, silver, iron.
Bad conductors — Wood, air, saw dust, plastic, wool, cardboard.
The good conductors are all metals, which have a special structure that lets heat flow like water through a pipe. The bad conductors are mostly porous or organic materials that trap air, acting as thermal barriers.
Teacher's Tip: Metals = Good; Non-metals = Bad. It's a simple rule of thumb!
Exam Tip: Group your answers clearly into two lists to make it easy for the teacher to grade.

Question 35: Why is an oven made of double walls with the space in between filled with cork ?
Answer: An oven is made of double walls and the space between them is filled with wool, cork etc. because the wool and cork are the insulator of heat. They prevent the heat of the oven to escape.
By placing a "bad conductor" like cork in between the metal walls, we create a barrier that heat cannot easily cross. This keeps the inside of the oven hot for cooking while the outside remains safe to touch.
Teacher's Tip: This is like putting a winter coat on your oven to keep its warmth inside.
Exam Tip: Use the term "thermal insulation" to describe the purpose of the double walls.

Question 36: Why do we use cooking utensils made up of copper.
Answer: Cooking utensils are made of metals such as copper, aluminium, brass, steel etc., so that heat is easily conducted through the base to their contents. But they are provided with handles of bad conductors (such as ebonite or wood) to hold them easily as handles will not conduct heat from the utensil to our hand.
Copper is used for the bottom because it spreads heat extremely fast and evenly, preventing "hot spots" that burn food. The wooden or plastic handles stay cool because they are insulators, protecting your fingers from the intense heat of the metal.
Teacher's Tip: It's a "team effort": Copper does the cooking, and the insulator handles do the protecting.
Exam Tip: Mention both the "good conductor base" and the "bad conductor handle" for a full marks answer.

Question 37: Why is a tea kettle provided with an ebonite handle ?
Answer: Tea kettles are provided with wooden or ebonite handles. The wood or the ebonite being the insulators of heat, does not pass heat from the utensils to our hand. Thus, we can hold the hot utensils or pans comfortably by their handles.
Ebonite is a very hard type of rubber that is a terrible conductor of heat but can withstand being near a hot stove. It acts as a safety shield, keeping the thermal energy inside the metal kettle and away from your skin.
Teacher's Tip: Ebonite is a classic "vintage" insulator used before modern plastics were common.
Exam Tip: Always specify that the handle is an "insulator" or "bad conductor" in your explanation.

Question 38: In summer, ice is kept wrapped in a gunny bag. Explain the reason.
Answer: In summer, the ice is kept wrapped in a gunny bag or it is covered with saw dust. The air filled in the fine pores of the gunny bag or saw dust, is the insulator of heat. The air does not allow heat from outside to pass through it to the ice. Thus, the ice is prevented from melting rapidly.
The rough fibers of the gunny bag trap millions of tiny pockets of air, and "still air" is one of the best insulators found in nature. This invisible shield stops the hot summer air from touching and melting the cold ice inside.
Teacher's Tip: The gunny bag isn't "cooling" the ice; it's just blocking the "warmth thieves" from getting in.
Exam Tip: Be sure to mention that "trapped air" is the actual reason for the insulation.

Question 39: Explain why (a) we wear woolen clothes in winter. (b) the water pipes are covered with cotton during very cold weather.
Answer: (a) Woolen clothes have fine pores filled with air. Wool and air both are bad conductors of heat. Therefore in winter, we wear woolen clothes as they check the conduction of heat from the body to the surroundings and thus keeps the body warm.
(b) During very cold weather, the water pipes are covered with cotton. The cotton has air trapped in its fine pores. The cotton and air are the insulators of heat. They do not pass heat from water inside the pipes to the outside atmosphere. Thus, cotton prevents the water in the pipes from freezing.
In both cases, we are using the insulating power of trapped air to stop heat from escaping. Wool keeps your body heat inside your clothes, while cotton keeps the water's heat inside the pipes so it doesn't turn into ice and burst them.
Teacher's Tip: Trapped air is the "secret ingredient" in both wool and cotton insulation.
Exam Tip: Use the phrase "check the conduction of heat" to explain how insulators work.

Question 40: Why are quilts filled with fluffy cotton ?
Answer: Quilts are filled with fluffy cotton. Air is trapped in the fine pores of cotton. Cotton and air are the insulators of heat. They check heat from our body to escape and thus keep us warm. The newly made quilts are warmer than the old ones because in the old quilts, there is no air trapped in the cotton.
As a quilt gets used, the cotton gets squashed down and loses those vital air pockets. "Fluffiness" is actually the measure of how much air a quilt can hold, which determines how warm it will keep you.
Teacher's Tip: Fluff your pillows and quilts to let the insulating air back in!
Exam Tip: Contrast "new quilts" and "old quilts" to show how compressed cotton loses its insulating ability.

Question 41: State the direction of heat transfer by way of convection.
Answer: By the process of convection, heat is always transferred vertically upwards. The reason is that the medium particles near the source of heat absorb heat from the source and they start moving faster. As a result, the medium at this place becomes less dense so it rises up and the medium from above being denser, moves down to take its place. Thus, current is set up in the medium which is called a convection current. The current continues till the entire medium acquires the same temperature.
Convection is a "circular dance" where hot, light particles move up and cold, heavy particles sink down. This vertical movement is why the air above a candle is much hotter than the air to the side of it.
Teacher's Tip: Use a "U-turn" hand gesture to remember how convection currents circulate.
Exam Tip: Mention the change in "density" as the reason why hot fluids move upwards.

Question 42: Why is a ventilator provided in a room ?
Answer: Ventilators and windows are provided in rooms for proper ventilation. The reason is that when we breathe out in a room, the air in the room becomes warm and impure. The warm air is less dense i.e. lighter, so it rises up and moves out through the ventilators. Then the cold fresh air comes in the room through the windows to take its place. Thus the continuous circulation of fresh air keeps the air in the room fresh.
Without ventilators near the ceiling, the used, warm air from our breath would get stuck at the top of the room. This natural convection "pump" works 24/7 to keep our rooms full of fresh, breathable air without needing a fan.
Teacher's Tip: This is why ventilators are always high up, while windows can be lower down.
Exam Tip: Explain the cycle: "Warm air rises out, Cold air rushes in."

Question 43: Why are chimneys provided over furnace in factories ?
Answer: Chimneys are provided over the furnace in factories. This is because the hot gases coming out of the furnace are less dense than the air. They rise up through the chimney. The smoke, fumes etc. around the furnace rush in so as to take their place and they are sucked out. Thus, the chimney helps to remove the undesired fumes, smoke etc. from the premises.
A chimney creates a "suction" effect by using the natural tendency of hot smoke to rise vertically. It acts as a giant straw, pulling dirty air up and out of the factory so workers can breathe safely.
Teacher's Tip: A taller chimney creates a stronger "draft" or pull.
Exam Tip: Link the "less dense" property of hot gas to the action of "rising up" the chimney.

Question 44: What are the land and sea breezes ? Explain their formation.
Answer: LAND BREEZE : Blowing of breeze (air) from land towards sea is called land breeze. During night land and sea water both lose heat. Specific heat capacity of land being very low as compared to that of sea water, land loses heat energy fast and cools more rapidly as compared to sea. Sea water being at higher temperature, the air above it becomes lighter and rise up. Air from land being at higher pressure. So air from land starts blowing towards sea and gives rise to Land Breeze.
SEA BREEZE : Blowing of breeze (cold air) from sea towards land during the day is called the SEA BREEZE. During day time land and sea both are heated equally by the sun, but land has very low specific heat capacity as compared to sea, is heated up more quickly. Thus air above land due to heat becomes lighter and rises up. Thus pressure decreases and cold and humid air above the sea starts blowing towards land, thereby giving rise to SEABREEZE.
These coastal winds are giant convection currents driven by the Sun's daily cycle. Land is like a fast-heating pan, while the sea is like a slow-boiling pot, and air always moves toward the "rising" warm spots.
Teacher's Tip: Breeze always comes FROM the place it's named after (Sea breeze comes FROM the sea).
Exam Tip: Draw a small diagram for each, showing the loop of air and labeling "Warmer" and "Cooler."

Question 45: Why is the freezing chest in a refrigerator fitted near its top?
Answer: Freezing chest in a refrigerator is fitted near the top, because it cools the remaining space of the refrigerator by convection current. Air near the top comes in contact with the freezing chest gets cooled, becomes denser and therefore descends while the hot air from the lower part rises and hence convection currents produced cool the whole space inside.
If the freezer were at the bottom, the cold air would stay stuck at the bottom and never reach your milk on the top shelf. By putting it at the top, we let nature's "sinking" cold air do the work of circulating coolness through the entire fridge.
Teacher's Tip: Cold air is heavy and wants to "fall" down your fridge shelves.
Exam Tip: Use the keyword "convection current" to explain how the entire fridge gets cooled.

Question 46: Explain briefly the process of heat transfer by radiation.
Answer: RADIATION. “The transfer of heat energy from a hot body to cold body directly, without heating the medium between two bodies is called RADIATION.” The radiant heat or thermal radiation is of the form of ELECTROMAGNETIC WAVES. These waves can travel even in vacuum in all directions in straight line with the speed of light. They do not heat the medium through which they pass. Heat radiations are also called INFRA-RED RADIATIONS because the wavelength of heat radiations is longer than that of visible light. These radiations can cause heating effect only if they are absorbed by some material.
Radiation is like a light beam that carries heat instead of just color. It doesn't need to touch you or blow air on you; it simply "beams" energy through space until it hits a surface and soaks in.
Teacher's Tip: Radiation is the "remote control" of heat transfer.
Exam Tip: Mention that radiation travels at the "speed of light" for a high-scoring answer.

Question 47: Give one example of heat transfer by radiation.
Answer: When we sit in the sun, we feel warm. We cannot get heat from the sun by the process of conduction or convection because most of the space between the sun and the earth is a vacuum and both of these modes of heat transfer require medium. Hence, one must be getting heat from the sun by the mode of radiation.
Another common example is feeling the heat from a bonfire even if you aren't touching it and the wind is blowing away from you. This "invisible" warmth hitting your face is pure infrared radiation traveling in straight lines from the fire.
Teacher's Tip: If you can block the heat with a piece of cardboard, it's definitely radiation!
Exam Tip: Use the "vacuum of space" as proof that only radiation can bridge the gap between Sun and Earth.

Question 48: Why do we prefer to wear white or light coloured clothes in summer and black or dark coloured clothes in winter ?
Answer: We prefer to wear white clothes in summer. The reason is that the white clothes reflects most of the sun’s heat and absorb very little of the sun’s heat, thus they keep our body cool. We prefer to wear black and dark coloured clothes in winter. The reason is that the black or dark colour clothes absorb most of the sun’s heat and keep our body warm.
White surfaces are "bouncy" and send heat rays away, while black surfaces are "soaky" and pull heat rays in. By choosing the right color, we can use the Sun's energy to help our bodies stay at a comfortable 37°C.
Teacher's Tip: White is a "mirror" for heat; Black is a "sponge" for heat.
Exam Tip: Use the words "reflects" for white and "absorbs" for black in your explanation.

Question 49: The bottom of a cooking utensil is painted black. Give the reason.
Answer: The bottom part of the cooking utensil or pan is painted black. The reason is that the black surface absorbs more heat and so the contents of utensil or pan get cooked rapidly if its bottom part is painted black.
By making the base black, we ensure that as much energy as possible from the stove flame is soaked into the metal rather than being bounced away. This makes the stove more efficient and helps your dinner cook much faster.
Teacher's Tip: Shiny pots might look pretty, but black-bottomed pots are better cooks!
Exam Tip: Connect "black surface" to "better absorption" to "rapid cooking" in your logic chain.

Question 50: Draw a labelled diagram of a thermo flask. Explain how the transfer of heat by conduction, convection and radiation is reduced to a minimum in it.
Answer: (A diagram showing a double-walled glass bottle with a vacuum in between, a cork stopper, and silvered surfaces).
Heat transfer is minimised because of:
(1) The vacuum between the two walls, rubber, glass, cork and air do not allow the loss of heat by conduction.
(2) Cork in the neck of flask and the cup over it prevent loss of heat by convection.
(3) Heat cannot be lost by conduction or convection because of vacuum between the two walls.
(4) Heat loss is also minimised by radiation, by making outer surface of inner wall and inner surface of outer wall silvered. The inner wall is a BAD RADIATOR and the outer wall is a GOOD REFLECTOR of radiation.
A thermos is like a fortress designed to stop heat from escaping in any direction. The vacuum is the main defense against conduction/convection, while the shiny silver walls act as mirrors to reflect radiation back where it came from.
Teacher's Tip: The silver walls are basically "thermal mirrors."
Exam Tip: List each mode of heat transfer and specifically explain how the flask blocks it (Vacuum = No conduction/convection; Silvering = No radiation).

C. Numericals

Question 1: The temperature of a body rises by 1°C. What is the corresponding rise on the (a) Fahrenheit scale (b) Kelvin scale?
Answer: (a) Since 100 divisions on Celsius scale = 180 divisions on the Fahrenheit scale
1 division on Celsius scale ∴ 1 division on Celsius scale = 1.80 / 1.00 × 1 = 1.8 divisions in the Fahrenheit scale.
For 1°C rise corresponding rise in Fahrenheit = 1.8°F
(b) Since 100 divisions in the Celsius scale = 100 divisions in the Kelvin scale
1 division on Celsius scale = 100 / 100 × 1 = 1 division on Kelvin scale
For 1°C rise corresponding rise in Kelvin is 1 K.
This shows that a Kelvin "step" is exactly the same size as a Celsius "step," but a Fahrenheit "step" is much smaller. You need almost two Fahrenheit steps to match just one Celsius step.
Teacher's Tip: Kelvin and Celsius are "size twins" but Fahrenheit is the "smaller sibling."
Exam Tip: Remember that "rise in temperature" is about the size of the division, not the absolute reading on the thermometer.

Question 2: The temperature rises by 18°F. What is the rise on the Celsius scale ?
Answer: Since 100 divisions on the Celsius scale = 180 divisions on the Fahrenheit scale
therefore 18 divisions on Fahrenheit scale.
= 100/180 × 18
= 10 divisions in the Celsius scale = 10°C
To find the equivalent rise, we use the ratio of their total divisions (100:180). Since the Fahrenheit rise is 18, which is one-tenth of 180, the Celsius rise is one-tenth of 100, which is 10.
Teacher's Tip: The ratio 100180 simplifies to 59. Use that for faster math!
Exam Tip: Always clarify that you are calculating a "rise" or "change," not a specific point like freezing.

Question 3: Convert 5°F to the Celsius scale.
Answer: C/5 = F - 32/9
Given, F = 5°F
C/5 = 5 - 32/9
Substitute the value of F in above equation
C/5 = -27/9
C/5 = -3
C = -3 × 5
C = -15
Solution = -15°C.
This negative result tells us that 5°F is extremely cold, well below the freezing point of water. By using the formula, we can accurately pinpoint that it corresponds to exactly fifteen degrees below zero on the Celsius scale.
Teacher's Tip: Be careful with the minus sign—if your result is negative, it just means it's below freezing.
Exam Tip: Show every step of your algebra, especially the subtraction and division, to get full method marks.

Question 4: Convert 40°C to the (a) Fahrenheit scale (b) Kelvin Scale.
Answer: (a) Fahrenheit scale
C = 40°C
Substitute value of C = 40° in below equation
C/5 =F - 32/9
40/5 = F - 32/9
8 = F - 32/9
8 × 9 = F - 32
72 = F - 32
F = 72 + 32 = 104
Solution = 104°F
(b) t°C = (273 + t) K
Given t = 40°C
Substitute the value of t in above equation
40°C = (273 + 40) K
40°C = 313  K
Solution = 313 K
This calculation shows that a warm day of 40°C is equal to a very hot 104°F or a scientific reading of 313 K. Converting between all three scales helps scientists around the world share data easily regardless of their local units.
Teacher's Tip: Adding 273 is the easiest way to go from Celsius to Kelvin!
Exam Tip: Remember that Kelvin values are always much larger numbers because they start from absolute zero.

Question 5: Convert - 40°F to the Celsius scale.
Answer: F = - 40°F
Substitute the value of F in below equation
C/5 = F - 32/9
C/5 = -40 - 32/9
C/5 = -72/9
C/5 = -8
C = -8 x 5
C = -40
Solution = - 40°C
This is a famous "magic number" in physics because it is the only point where the Celsius and Fahrenheit scales meet. At this extreme cold temperature, it doesn't matter which thermometer you use; they will both show the exact same number.
Teacher's Tip: -40 is the "cross-over" point for these two scales.
Exam Tip: If you get -40 as your answer, don't worry that you made a mistake—it is a real and unique result!