Selina Concise Solutions for ICSE Class 8 Physics Chapter 6 Heat Transfer

• Heat Transfer Overview
• Heat is a form of energy. When two bodies are in contact heat flows from body at higher temperature to body at lower temperature till the lower temperature of both is same.
• When a body is heated, its molecules move faster about their means position and kinetic energy increases and with fall in temperature their K.E. decreases.
• When a substance is heated
  (i) It expands i.e. a change in size takes place
  (ii) Change in temperature takes place.
  (iii) Change in state takes place.
• CHANGE OF STATE : “The process of change from one state to another at a constant temperature is called change of state.”
• Solid on heating changes into LIQUID. LIQUID on absorbing heat changes to VAPOURS some SOLIDS on heating DIRECTLY change in vapours called SUBLIMATION. Substance is called SUBLIMATE.
• SOLIDIFICATION on cooling when a vapours change into SOLID. GAS OR VAPOURS on cooling changes to LIQUID also called LIQUIFACTION.
• MELTING: Change of solid into liquid at constant temperature. FUSION → FREEZING is change of LIQUID into SOLID at constant temperature and change of solid into liquid at a constant temperature is called FUSION.
• EVAPORATION: “Change liquid to gas at ALL TEMPERATURES” It is surface phenomenon.
• VAPOURIZATION : “Change of liquid into vapours at fixed temperature”.
• METING POINT: “Is the temperature at which a solid starts melting and remains constant till the whole of solid melts.”
• M.P. is same as freezing point.
• M.P. of ice is 0°C or freezing point, of water is 0°C.
• BOILING POINT: “Is the temperature of a liquid at which it start, boiling i.e. change into vapours or gaseous state.”
• B.P. of pure water is 100°C.
• ABSOLUTE ZERO: “The temperature at which molecular motion completely ceases.”
• FACTORS EFFECTING THE RATE OF EVAPORATION :
  (i) Temperature: Increases with increase in temperature
  (ii) S.A.: Increases with increase in S.A.
  (iii) BLOWING AIR - Renewal of air increases evaporation.
  (iv) NATURE - Some liquids like spirit, alcohol, petrol evaporate easily.
• EVAPOURATION → produces coolness, BOILING produces Hotness.
• LINEAR EXPANSION: When a solid rod (metal) is heated change in length takes place, which depends upon
  (i) original length (L₀)
  (ii) Increase in temperature
  (iii) Material of rod.
• Let L₀ be the original length at 0°C, when heated to T°C final length becomes L_t
• Increase in length (L_t - L₀) propto L₀ (T - 0)
  Or
• Coefficient of linear expansion alpha which depends upon material of rod.
• L_t - LL₀ = LL₀ alpha T
• alpha = {L_t - L₀}/{L₀ T} = increase in length}/original length × Rise in temperature
• When a metal plate is heated, change in area takes place and the expansion is called SUPERFICIAL expansion.
• When a solid of volume V₀ is heated change in volume called cubical expansion takes place.
• alpha : beta : gamma = 1 : 2 : 3

Test your self

A. Objective Questions

1. Write true or false for each statement

(a) Evaporation is rapid on a wet day.
Answer: False.
On a wet day, the air is already saturated with moisture, which leaves little room for more water molecules to escape the liquid. This high humidity significantly slows down the evaporation process compared to a dry day.
Teacher's Tip: Remember that high humidity is like a crowded room where no one else can enter easily.
Exam Tip: Always mention that evaporation rate is inversely proportional to humidity for full marks.

(b) Evaporation takes place only from the surface of liquid.
Answer: True.
Evaporation is a surface phenomenon where only the molecules at the top have enough kinetic energy to break free into the air. Unlike boiling, which happens throughout the liquid, evaporation happens slowly from the topmost layer.
Teacher's Tip: Think of it like people leaving a stadium; only those near the exit doors (the surface) can leave first.
Exam Tip: Use the term "surface phenomenon" to describe evaporation in your answers.

(c) All molecules of a liquid take part in the process of evaporation.
Answer: False.
Only the molecules at the surface that possess higher-than-average kinetic energy can overcome the attractive forces of neighboring molecules and evaporate. The molecules deep inside the liquid remain in the liquid state.
Teacher's Tip: Remember "Surface Only" for evaporation and "Whole Bulk" for boiling.
Exam Tip: Clearly distinguish between the surface molecules and bulk molecules when explaining evaporation.

(d) Temperature of a liquid rises during boiling or vaporization
Answer: False.
During boiling, the heat energy supplied is used to change the state of the liquid into gas rather than increasing the temperature. This heat is known as latent heat, and the temperature remains constant at the boiling point.
Teacher's Tip: During a "change of state," the thermometer "takes a break" and stays still.
Exam Tip: Mention "Latent Heat" to explain why temperature stays constant during state changes.

(e) All molecules of a liquid take part in boiling.
Answer: True.
Boiling is a bulk phenomenon where molecules from all parts of the liquid gain enough energy to turn into vapor. This is why you see bubbles forming throughout the liquid, including at the bottom and sides of the container.
Teacher's Tip: Boiling is a "Full Team Effort" where every molecule is involved.
Exam Tip: Use the phrase "bulk phenomenon" to describe the process of boiling.

(f) Boiling is a rapid phenomenon.
Answer: True.
Boiling happens quickly once the liquid reaches its specific boiling point temperature under constant heating. It is a much faster process compared to evaporation, which can occur slowly over a long period.
Teacher's Tip: Think of boiling as "Fast and Furious" and evaporation as "Slow and Steady".
Exam Tip: Contrast the speed of boiling with the slow nature of evaporation to earn marks.

(g) All solids expand by the same amount when heated to the same rise in temperature.
Answer: False.
Different solids have different molecular structures and strengths of intermolecular forces, meaning they expand at different rates. For example, brass expands more than iron when both are heated by the same amount.
Teacher's Tip: Every material has its own "personality" or coefficient of expansion.
Exam Tip: Remember that expansion depends on the "nature of the material."

(h) Telephone wires are kept tight between the two poles in winter.
Answer: True.
In winter, metals contract due to the drop in temperature, causing the wires to shorten and become tight. If they were kept tight in summer, they might snap in winter as they try to contract further.
Teacher's Tip: Cold makes things "huddle up" or contract.
Exam Tip: Explain that wires are left sagging in summer to prevent snapping in winter due to contraction.

(i) Equal volumes of different liquids expand by the different amount when they are heated to the same rise in temperature.
Answer: True.
Just like solids, different liquids possess unique coefficients of expansion based on their molecular properties. Even if the initial volume and temperature increase are identical, a liquid like alcohol will expand more than water.
Teacher's Tip: Liquids have their own "expansion rules" just like solids.
Exam Tip: When comparing expansion, always state that it depends on the specific liquid used.

(j) Solids expand the least and gases expand the most on being heated.
Answer: True.
Gases have very weak intermolecular forces, allowing their molecules to move far apart easily when heated. Solids have strong forces that hold molecules tightly, making their expansion much smaller in comparison.
Teacher's Tip: Order of expansion: Gas > Liquid > Solid.
Exam Tip: Mention "weak intermolecular forces" as the reason why gases expand the most.

(k) A mercury thermometer makes use of the property of expansion of liquids on heating.
Answer: True.
Mercury expands uniformly when heated, causing it to rise up the thin capillary tube in the thermometer. This consistent expansion allows us to accurately measure temperature changes by looking at the scale.
Teacher's Tip: Mercury is like a "silver elevator" that goes up as it gets hotter.
Exam Tip: Identify "uniform expansion" as the key property that makes mercury useful in thermometers.

(l) Kerosene contracts on heating.
Answer: False.
Like almost all substances, kerosene expands when heated because its molecules move faster and push further apart. Contraction usually happens during cooling, not heating.
Teacher's Tip: Heating usually means "more space" or expansion.
Exam Tip: Only mention water between 0°C and 4°C as a rare case of contraction on heating.

2. Fill in the blanks

(a) Boiling occurs at a fixed temperature.
Answer: fixed temperature.
Every pure substance has a specific boiling point, such as 100°C for water at standard pressure. This temperature does not change until all the liquid has turned into vapor.
Teacher's Tip: Boiling is like a scheduled appointment at a specific "time" or temperature.
Exam Tip: Distinguish between boiling (fixed temperature) and evaporation (any temperature).

(b) Evaporation takes place at all temperature.
Answer: all temperature.
Evaporation happens even at room temperature or in cold weather, as long as there is a liquid surface. While it is faster at higher temperatures, it never completely stops until the liquid is gone.
Teacher's Tip: Evaporation is a "24/7" process.
Exam Tip: Use the phrase "all temperatures" to define the range of evaporation.

(c) The molecules of liquid absorb heat from surroundings in evaporation.
Answer: absorb.
When molecules escape from the surface, they take energy with them, which is drawn from the remaining liquid or the surroundings. This intake of energy is what allows the phase change to occur.
Teacher's Tip: Evaporation "borrows" heat to turn into vapor.
Exam Tip: Explain that absorbing heat from the surroundings leads to a cooling effect.

(d) Heat is absorbed during boiling.
Answer: absorbed.
To keep a liquid boiling, you must continuously supply heat to break the intermolecular bonds throughout the bulk of the liquid. This heat is used exclusively for the change of state.
Teacher's Tip: No heat = No bubbles.
Exam Tip: Remember that boiling is an endothermic process, meaning it requires heat input.

(e) Cooling is produced in evaporation.
Answer: evaporation.
Since the fastest molecules leave the liquid first, the average kinetic energy (and thus the temperature) of the remaining liquid drops. This is why we feel cold when sweat evaporates from our skin.
Teacher's Tip: Evaporation is nature's own air conditioner.
Exam Tip: Connect evaporation to the "cooling effect" in real-life examples like earthen pots.

(f) A longer rod expands more than a shorter rod on being heated to the same temperature.
Answer: more.
Linear expansion is directly proportional to the original length of the material. Because there are more molecules in a longer rod, the total increase in distance between them results in a greater overall expansion.
Teacher's Tip: More length means more room to grow.
Exam Tip: Remember the formula Delta L = L₀ alpha Delta T, where Delta L depends on L₀.

(g) Liquids expand more than the solids.
Answer: more.
The intermolecular forces in liquids are weaker than those in solids, allowing molecules to move apart more easily when heated. This makes the volumetric expansion of liquids much more noticeable.
Teacher's Tip: Liquids are "looser" than solids, so they expand easier.
Exam Tip: Use this fact to explain why the liquid in a thermometer rises while the glass tube barely changes.

(h) Gases expand more than the liquids.
Answer: more.
Molecules in a gas have almost no attraction to each other, so they spread out tremendously when given extra kinetic energy. This results in the highest rate of thermal expansion among the three states of matter.
Teacher's Tip: Gases are the "expansion champions".
Exam Tip: State the order clearly: Gas > Liquid > Solid expansion.

(i) Alcohol expands more than water.
Answer: more.
Alcohol has a higher coefficient of expansion than water because its internal molecular bonds are easier to stretch. This property makes alcohol very sensitive to temperature changes.
Teacher's Tip: Alcohol is more "reactive" to heat than water.
Exam Tip: Use this to explain why alcohol is used in some specialized thermometers.

(j) Iron expands less than copper.
Answer: less.
Copper has a higher coefficient of linear expansion compared to iron, meaning it grows longer when heated to the same temperature. This difference is utilized in bimetallic strips used in thermostats.
Teacher's Tip: Copper "grows" faster than iron.
Exam Tip: This difference in expansion is the basic principle behind the working of a bimetallic strip.

3. Match the following

Column A                                                   Column B
(a) Blowing air increases                       (i) increase in inter-molecular separation
(b) Increase in pressure increases       (ii) pendulum of a clock
(c) Thermal expansion                         (iii) cooking utensils
(d) Invar                                                (iv) boiling point
(e) Pyrex glass                                     (v) evaporation
Answer: 

Column A                                                  Column B
(a) Blowing air increases                        (v) evaporation
(b) Increase in pressure increases         (iv) boiling point
(c) Thermal expansion                            (i) increase in inter-molecular separation
(d) Invar                                                  (ii) pendulum of a clock
(e) Pyrex glass                                        (iii) cooking utensils

4. Select the correct alternative

(a) In evaporation
1. all molecules of liquid begin to escape out
2. only the molecules at the surface escape out
3. the temperature of liquid rises by absorbing heat from surroundings.
4. the molecules get attracted within the liquid.
Answer: 2. only the molecules at the surface escape out
Evaporation is limited to the top layer of a liquid where molecules have enough energy to break free into the air. This is why a wider container with more surface area speeds up the process.
Teacher's Tip: Evaporation is a "Surface Party."
Exam Tip: Always distinguish evaporation from boiling by highlighting that it's a surface phenomenon.

(b) The rate of evaporation of a liquid increases when:
1. temperature of liquid falls
2. liquid is poured in a vessel of less surface area
3. air is blown above the surface of liquid
4. humidity increases.
Answer: 3. air is blown above the surface of liquid
Moving air carries away the vapor molecules that have just escaped, preventing them from returning to the liquid. This keeps the space above the liquid "fresh" for more molecules to evaporate.
Teacher's Tip: Wind acts like a "broom" for vapor molecules.
Exam Tip: List temperature, surface area, and wind as the three main factors that increase evaporation rate.

(c) During boiling or vaporization
1. all molecules take part
2. temperature rises
3. no heat is absorbed
4. the average kinetic energy of molecules increases.
Answer: 1. all molecules take part
Boiling is a bulk phenomenon, meaning the heat energy reaches every part of the liquid, allowing bubbles to form everywhere. Unlike evaporation, every molecule is eligible to turn into vapor during boiling.
Teacher's Tip: Boiling involves the "whole bottle" of water.
Exam Tip: Remember that temperature stays constant during this process.

(d) The boiling point of a liquid is increased by
1. increasing the volume of liquid
2. increasing the pressure, on liquid
3. adding ice to the liquid
4. decreasing pressure on liquid.
Answer: 2. increasing the pressure, on liquid
Added pressure makes it harder for vapor bubbles to form and grow within the liquid. Therefore, a higher temperature must be reached before the liquid can finally turn into gas.
Teacher's Tip: Pressure "holds down" the liquid molecules.
Exam Tip: Think of a pressure cooker; higher pressure equals faster cooking at higher temperatures.

(e) Two rods A and B of the same metal, but of length 1 m and 2 m respectively, are heated from 0°C to 100°C. Then
1. both the rods A and B elongate the same
2. the rod A elongates more than the rod B
3. the rod B elongates more than the rod A
4. the rod A elongates, but the rod B contracts.
Answer: 3. the rod B elongates more than the rod A
Expansion is directly proportional to the original length of the material. Since rod B is twice as long as rod A, it has more molecules that will move further apart, leading to a larger total increase in length.
Teacher's Tip: Double the length means double the expansion!
Exam Tip: Use the formula Delta L = alpha L₀ Delta T to prove that longer objects expand more.

(f) Two rods A and B of the same metal, same length, but one solid and the other hollow, are heated to the same rise in temperature. Then
1. the solid rod A expands more than the hollow rod B
2. the hollow rod B expands more than the solid rod A
3. the hollow rod B contracts, but the solid rod A expands
4. both the rods A and B expand the same.
Answer: 4. both the rods A and B expand the same.
Thermal expansion depends on the external dimensions and the material, not on whether the interior is solid or empty. Both rods will grow in length by the same amount because their outer structures are identical.
Teacher's Tip: The "middle" doesn't matter for length expansion.
Exam Tip: Remember that expansion is independent of the mass or "solidness" of the rod.

(g) A given volume of alcohol and the same volume of water are heated from the room temperature to the same temperature then.
1. alcohol contracts, but water expands
2. water contracts, but alcohol expands
3. water expands more than alcohol
4. alcohol expands more than water.
Answer: 4. alcohol expands more than water.
Alcohol has weaker intermolecular forces compared to water, making it much easier for its molecules to spread apart when heated. Consequently, alcohol shows a much larger increase in volume for the same temperature rise.
Teacher's Tip: Alcohol is more "heat-sensitive" than water.
Exam Tip: This is why alcohol is preferred in some low-temperature thermometers.

(h) The increase in length of a metal rod depends on
1. the initial length of the rod only
2. the rise in temperature only
3. the material of rod only
4. all the above three factors.
Answer: 4. all the above three factors.
Thermal expansion is a combination of how long the rod was to start with, how much heat you added, and what kind of metal it is made of. Changing any of these three will change the final length of the rod.
Teacher's Tip: Remember "Length, Heat, Material" as the expansion trio.
Exam Tip: For questions about expansion, always check for all three variables.

(i) The correct statement is
1. Iron rims are cooled before they are placed on the cart wheels.
2. A glass stopper gets tighten 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 expand during hot summer days, and the small gaps allow them room to grow without bending or buckling. Without these gaps, the expansion would cause the tracks to warp, leading to dangerous accidents.
Teacher's Tip: Gaps in tracks are "Expansion Rooms."
Exam Tip: This is the most common practical application of thermal expansion in civil engineering.

B. Short/Long Answer Questions

Question 1: What is matter? What is it composed of
Answer: MATTER: Anything around us is a matter. “Anything that has mass, occupies volume and can be felt by our senses.” It is composed of ‘molecules’.
Everything from the air we breathe to the chair we sit on is made of matter. These tiny particles called molecules are always in motion and determine whether a substance is solid, liquid, or gas.
Teacher's Tip: If you can touch it, see it, or weigh it, it's matter!
Exam Tip: Use keywords like "mass" and "volume" to provide a scientific definition of matter.

Question 2: Name the three states of matter and distinguish them on the basis of their (i) volume, and (ii) shape
Answer: THREE STATES OF MATTER:
(i) SOLID (ii) LIQUID (iii) GASEOUS
DISTINCTION BETWEEN THREE STATES ON THE BASES OF .
(i) VOLUME:
SOIDS : have least volume.
LIQUIDS: have definite volume.
GASES: have maximum volume.
(ii) SHAPE:
SOLDIS: Have definite shape.
LIQUIDS : Have no definite shape.
GASES : Have no definite shape.
Solids maintain their form because their molecules are packed tightly together. Liquids and gases take the shape of their container because their molecules are free to move around.
Teacher's Tip: Solids are "stiff", liquids "flow", and gases "fill everything".
Exam Tip: Create a table when asked to distinguish between states of matter for better marks.

Question 3: Distinguish between liquid and vapour (or gas) states of matter on the basis of following factors (a) Arrangement of molecules (b) Inter-molecular separation (c) Inter-molecular force, and (d) Kinetic energy of molecules
Answer: DISTINCTION BETWEEN LIQUID AND VAPOUR ON THE BASES OF:
(a) Arrangement of molecules:
LIQUID: Closely packed
VAPOUR: Very losely packed
(b) Inter molecular separation:
LIQUID: Least
VAPOUR: Maximum
(c) Inter molecular force:
LIQUID: Maximum
VAPOUR: Least
(d) Kinetic energy of molecules:
LIQUID: Least
VAPOUR: Maximum
In liquids, molecules are close and attracted to each other, allowing them to flow but stay together. In gases, the molecules are far apart and move very fast because there is almost no force holding them back.
Teacher's Tip: Liquids are like people in a crowded elevator; gases are like people in an open field.
Exam Tip: Remember that inter-molecular force and separation are always opposite (more force = less separation).

Question 4: What is evaporation ? Explain it on the basis of molecular motion.
Answer: EVAPORATION : “The change of liquid into its vapours at all temperature from its surface is called evaporation.”
EXPLANATION OF EVAPORATION on the bases of molecular motion:
Molecules of liquid have more spaces, less molecular force of attraction and more K.E. than molecules of solids and can move through out the liquid.
While moving they can not escape the surface as they are being pulled inside by other molecules as there are no molecules above the surface. But when some molecules acquire sufficient K.E. (Threshold velocity), they over come the ATTRACTIVE FORCES of other molecules and escape into the open space above the liquid. These escaping molecules form the vapour of the liquid and the process called Evaporation continues till all the liquid evaporates.
Evaporation happens because surface molecules get a "speed boost" from collisions with others. Once they are fast enough, they break away from the liquid's pull and fly off as gas.
Teacher's Tip: Evaporation is like a "breakaway" run in sports where a player escapes the other team.
Exam Tip: Use the term "Threshold velocity" to explain how molecules escape the surface.

Question 5: Do all the molecules of a liquid take part in evaporation ? If not, explain your answer.
Answer: No, all the molecules of the liquid do not take part in evaporation only those molecules near the surface of liquid which acquire sufficient Kinetic energy (Threshold velocity) escape as they over come attractive forces of other molecules. Then other molecules come to the surface of the liquid and acquire more K.E. and escape the surface. This continues till all the liquid evaporates.
Only a small fraction of molecules at the very top have enough energy to leave at any given time. This is why evaporation is a slow process that happens layer by layer.
Teacher's Tip: Evaporation is a "VIP-only" exit for molecules at the surface.
Exam Tip: Emphasize that it is a "surface phenomenon" to contrast it with boiling.

Question 6: No heat is supplied to a liquid during evaporation. How does then the liquid change into its vapours ?
Answer: Though no heat is supplied to the liquid but molecules near the surface of the liquid acquire sufficient kinetic energy by collisions with other liquid molecules and with this K.E. they over come the attractive forces of other molecules and change into vapours.
Molecules are constantly bumping into each other, and some gain extra energy from these hits. This "borrowed" energy is enough to let them escape into the air as vapor without an external fire or heater.
Teacher's Tip: Molecules "steal" energy from their neighbors to evaporate.
Exam Tip: Explain that internal kinetic energy and collisions are the key drivers of evaporation.

Question 7: Comment on the statement ‘evaporation is a surface phenomenon’.
Answer: Change of liquid into vapours at all temperatures from the surface is called evaporation. Evaporation takes place at surface in those molecules which are at surface and have sufficient K.E. to over come attractive force due to inner surrounding molecules.
Because molecules in the middle of the liquid are pulled in all directions by their neighbors, they cannot easily leave. Only the molecules at the top, who have no one pulling them from above, can break away and evaporate.
Teacher's Tip: The "top floor" is the only exit for evaporating molecules.
Exam Tip: Mention that only surface molecules can overcome the "inward pull" of the liquid.

Question 8: Why is cooling produced when a liquid evaporates ?
Answer: For changing liquid into vapours heat is needed this heat is taken from the container or surroundings and temperature of container or body itself fall and cooling is produced.
When the "hottest" or fastest molecules leave during evaporation, they take heat away with them. This lowers the average temperature of whatever they left behind, creating a cooling sensation.
Teacher's Tip: Evaporation "steals" heat, leaving everything else chilly.
Exam Tip: Always link the "absorption of heat from the body/surroundings" to the "drop in temperature."

Question 9: Give reason for the increase in rate of evaporation of a liquid when (a) air is blown above the liquid (b) surface area of liquid is increased (c) temperature of liquid is increased.
Answer: (a) BLOWING AIR ON LIQUID SURFACE INCREASES RATE OF EVAPORATION : Blowing air takes away with it the molecules of liquid escaping out of the surface. To take their place, other molecules escape out from the surface of liquid.
(b) SURFACE AREA INCREASES THE RATE OF EVAPORATION: On increasing the area of the surface, number of molecules escaping out from the surface increases.
(c) INCREASE IN TEMPERATURE INCREASES THE RATE OF EVAPORATION: Increase in temperature increases K.E. (K.E. = 1/2 Mv²). More and more molecules come to the surface of liquid hence the rate of evaporation will increase with increase in temperature.
By providing more space, more energy, or removing vapor quickly, we make it easier for molecules to escape. This is why we spread out wet clothes and use a fan to dry things faster.
Teacher's Tip: Think of these three as "Helpers" that open more doors for molecules.
Exam Tip: List these as the "Factors affecting evaporation" and explain each one clearly.

Question 10: What is boiling ? Explain it on the basis of molecular motion?
Answer: BOILING: “The change of liquid to vapours on heating at a constant temperature is called BOILING”
K.E. = 1/2 Mv² : more the speed of molecules more is the kinetic energy. Heating of the liquid increases the average K.E. of liquid molecules and molecules acquire sufficient K.E. needed to overcome the force of attraction of other molecules. These molecules start leaving the liquid not only at the surface but also near the walls of the containing vessel and bubbles are seen on the walls of vessel. This causes the agitation in the whole of the liquid and this is called boiling.
During boiling, the energy is so high that molecules throughout the liquid can turn into gas and form bubbles. These bubbles rise to the top and burst, creating the vigorous "boiling" motion we see in a pot.
Teacher's Tip: Boiling is like an "all-out party" where every molecule wants to leave.
Exam Tip: Highlight that boiling occurs at a "fixed temperature" and "throughout the bulk."

Question 11: Why does bubbles appear when a liquid is heated ?
Answer: When liquid is heated formation of vapours takes place which appear in the form of bubbles.
As the liquid gets hot, it turns into gas inside the liquid itself. These pockets of gas are lighter than the liquid, so they rise up as bubbles that we see on the surface.
Teacher's Tip: Bubbles are just "gas pockets" trying to reach the top.
Exam Tip: State that bubbles are evidence of a phase change happening within the bulk of the liquid.

Question 12: What is the change in average kinetic energy of molecules of a liquid during boiling at its boiling point ?
Answer: AVERAGE KINETIC ENERGY is the measure of temperature of body. When temperature of body increases the average kinetic energy increases and when Average Kinetic energy decreases there is decrease of body temperature and hence at boiling point average Kinetic energy increases.
Even though the temperature stays at 100°C during boiling, the molecules are highly energetic and use that energy to change state. The molecules that have already turned to vapor possess even more energy than those still in the liquid phase.
Teacher's Tip: More heat = more speed for the molecules.
Exam Tip: Remember that temperature and average kinetic energy are directly related.

Question 13: How is the heat energy supplied to a liquid used during boiling at a fixed temperature ?
Answer: When heat is supplied, temperature of liquid rises continuously till the water starts boiling at 100°C. Once the water starts boiling, its temperature does not rise further, although heat is still being supplied. This heat is being used to change each and every water molecule into vapour.
The energy "hides" inside the vapor molecules as latent heat to keep them in the gaseous state. It doesn't show up on the thermometer because it is busy breaking the bonds between molecules.
Teacher's Tip: The heat is "working" to break bonds, not to raise the temperature.
Exam Tip: Use the term "Latent Heat of Vaporization" for this hidden energy.

Question 14: Name two ways of change of liquid state to the vapour state and distinguish them.
Answer: Two ways of change of liquid state to vapour state are: (i) Evaporation (ii) Boiling.
Evaporation is slow and happens at the surface at any temperature, while boiling is fast and happens throughout the liquid at a specific temperature. Both processes turn liquid into gas but under very different conditions.
Teacher's Tip: Evaporation is "Natural & Slow"; Boiling is "Forced & Fast."
Exam Tip: Creating a side-by-side comparison table is the best way to answer this question.

DIFFERENCE BETWEEN EVAPORATION AND BOILING:
BOILING:
(i) Heat is supplied.
(ii) Fast process.
(iii) Starts from bottom.
(iv) Sound is produced.
(v) Heating effect
(vi) Takes place at a fixed temperature called boiling point.
EVAPORATION:
(i) Heat is absorbed.
(ii) Slow process.
(iii) Starts from surface i.e. surface phenomenon
(iv) Silent process
(v) Cooling effect
(vi) Takes place at all temperatures.

Question 15: What do you understand by thermal expansion of a substance ?
Answer: The expansion of a substance on heating is called the thermal expansion of the substance.
When things get hot, their molecules vibrate more and push each other apart, making the whole object get slightly bigger. This can happen to solids, liquids, and gases alike.
Teacher's Tip: "Thermal" means heat, and "Expansion" means getting bigger.
Exam Tip: Define thermal expansion as an increase in dimensions due to a rise in temperature.

Question 16: Give two examples of the substances which expand on heating.
Answer: Iron, water expand on heating.
Metals like iron are famous for expanding, which is why we leave gaps in railway tracks. Liquids like water also expand when heated above 4°C, as seen in a thermometer or a boiling pot.
Teacher's Tip: Almost everything you see expands when it gets hot!
Exam Tip: Use simple everyday materials like iron, copper, or alcohol as examples.

Question 17: Describe an experiment to demonstrate the thermal expansion in solids.
Answer: Experiment: [Gravesand’s ball and ring experiment]
Take metallic ball and ring so that ball can pass through ring easily. Now heat the ball. We see that heated ball cannot pass through the same ring. This is because on heating the ball increases in size. Now on cooling the ball again it can pass through the ring. This experiment shows that solids expand on heating and contract on cooling.
This classic experiment uses a simple metal ball that fits perfectly through a ring when cold. After heating the ball over a flame, it becomes just a tiny bit too large to fit through the same ring until it cools down again.
Teacher's Tip: It's the "Ball and Ring" trick!
Exam Tip: Draw a simple diagram showing the ball before and after heating to illustrate your answer.

Question 18: State three factors on which depend the linear expansion of a metal rod on heating.
Answer: Factors on which increase in length of a rod depend when heated are:
(1) Length of rod L. Increase in length is directly proportional to original length.
(2) Temperature of rod. It is directly proportional to increase in temperature.
(3) Nature of material of rod.
If L₁ is original length of rod at t₁ °C and on heating to t₂ °C its length becomes L₂.
therefore Increase in length of rod = (L₂ - L₁)
and increase in temperature (t₂ - t₁)
therefore [L₂ - L₁] propto L₁ (t₂ - t₁)
But the increase in length of rod does not depend on whether the rod is solid or hollow.
How much a rod grows depends on how long it was at first, how hot you made it, and what metal it is made from. Every metal has its own "expansion rate" that tells us how much it will grow.
Teacher's Tip: Think of it as "Start Length + Heat + Metal Type".
Exam Tip: State that expansion is "directly proportional" to both length and temperature change.

Question 19: Two iron rods - one 10 m long and the other 5 m long, are heated to the same rise in temperature. Which will expand more ?
Answer: L_t – L₀ ∝ L₀
therefore The rod having greater length will expand more when rods are heated to the same temperature.
therefore Rod with 10 m length will expand more.
Because expansion is directly proportional to the starting length, a longer rod has more "growing room" than a shorter one. The 10m rod has twice as many molecules as the 5m rod, so the total expansion is doubled.
Teacher's Tip: Long rods = Long expansion.
Exam Tip: Always provide the mathematical reason (L_t – L₀ ∝ L₀) to support your choice.

Question 20: Two identical rods of copper are heated to different temperatures - one by 5°C and the other by 10°C. Which rod will expand more ?
Answer: Two rods with same length and material are heated to different temperature.
L_t – L₀ ∝ L₀
therefore Rod with higher temperature i. e. at 10°C will expand more.
More heat means more kinetic energy for the copper molecules, making them vibrate harder and push further apart. Since 10°C is a greater rise than 5°C, that rod will definitely show more expansion.
Teacher's Tip: More "Heat" always equals more "Grow."
Exam Tip: Use the relationship that expansion is proportional to the rise in temperature.

Question 21: One rod of copper and another identical rod of iron are heated to the same rise in temperature. Which rod will expand more? Give reason.
Answer: When two identical rods (same length) are of different material and heated to same rise in temperature, the copper rod will expand more than iron, since expansion depends on nature of material.
(Coefficient of linear expansion of copper 17 × 10^-6 per °C > iron 13 × 10^-6 per °C)
Every material has a unique "coefficient of expansion" that determines how much it grows when heated. Copper molecules are less tightly bound than iron ones in this context, so they spread apart more easily under heat.
Teacher's Tip: Copper is "looser" than iron, so it expands more!
Exam Tip: Mention the "coefficient of expansion" as the scientific reason for the difference.

Question 22: Two identical rods - one hollow and the other solid, are heated to the same rise in temperature. Which will expand more ?
Answer: When two identical rods are heated to the same range of temperature, solid rod will expand more. More heat is present in solid rod.
A solid rod has more molecules than a hollow one, meaning it absorbs more total thermal energy to reach the same temperature. This can lead to a slightly more measurable expansion in the solid version.
Teacher's Tip: Solid = More molecules = More expansion potential.
Exam Tip: Remember that the amount of material (mass) can affect how heat is distributed and used.

Question 23: In the ball and ring experiment, if the ball after heating is left to cool on the ring for some time, the ball again passes through the ring. Explain the reason.
Answer: On heating the ball expands and increases in size and cannot pass the ring when left on it. As the ball cools, it contracts, size becomes less than ring and passes the ring.
Heat made the ball's molecules push apart, making it too big for the ring. When it cools down, the molecules move closer together again, and the ball shrinks back to its original size, allowing it to fall through.
Teacher's Tip: Cooling is like the "undo button" for expansion.
Exam Tip: Use the words "contraction" and "contraction on cooling" to explain the shrinking process.

Question 24: Explain the following:
(a) The telephone wires break in winter.
(b) Iron rims are heated before they are fixed on the wooden wheels.
(c) The gaps are left between the successive rails on a railway track.
(d) A glass stopper stuck in the neck of a bottle can be removed by pouring hot water on the neck of the bottle.
(e) A cement floor is laid in small pieces with gaps in between.
Answer: (a) Metals expand on heating (in summer) and contract on cooling (in winter). Therefore while putting up the wires between two poles, care is taken that they are kept tight while laying them in winter as they sag in summer due to expansion.
(b) Iron rims are made slightly smaller in diameter than the wooden wheel and on heating wheel expands and can easily slip over the wooden wheel and on cooling the rim contracts and MAKES A TIGHT FIT OVER the wooden wheel.
(c) In summer due to considerable rise in temperature, the gaps allow for the expansion of rails, otherwise the rail will bend side ways.
(d) By pouring hot water on the neck of the bottle, the neck expands and stuck glass stopper can easily be removed.
(e) In summer when temperature rises small pieces of cement expand and to allow expansion gaps are left between small pieces.
Thermal expansion and contraction affect almost everything around us in our daily lives. Engineers must plan for these changes so that things like tracks, floors, and wires don't break or warp as the seasons change.
Teacher's Tip: Everything "Grows" in summer and "Shrinks" in winter.
Exam Tip: For each part, mention specifically whether "expansion" or "contraction" is happening.

Question 25: Why is one end of a steel girder in a bridge kept on rollers instead of fixing it in pillar ?
Answer: In summer when temperature increases considerably the bridge made of metal expands and rollers slide to allow for expansion otherwise the bridge may break the pillar.
Metal bridges can grow by several inches in the hot sun. The rollers allow the bridge to "grow" and "shrink" smoothly without putting dangerous pressure on the concrete pillars that hold it up.
Teacher's Tip: Rollers are like "skateboards" for the bridge to move on.
Exam Tip: Explain that rollers prevent "structural damage" or "breaking of pillars."

Question 26: A metal plate is heated. State three factors on which the increase in its area will depend.
Answer: THREE FACTORS ARE:
(i) Original area of plate A₀.
(ii) Rise in temperature t.
(iii) Nature of material of plate.
Just like a rod's length, a plate's area expansion depends on its starting size, how much you heat it, and what it's made of. This is known as superficial expansion because it happens in two dimensions (length and width).
Teacher's Tip: Area expansion is just expansion in "two directions" at once.
Exam Tip: Remember that area expansion is roughly "twice" the linear expansion coefficient (beta = 2alpha).

Question 27: A cubical metal solid block is heated. How will its volume change ?
Answer: When a solid is heated, it expands in all directions. The volume of a cube also increases.
Let V₀ be the volume of cube of side L₀ at 0°C. i.e. V₀ = L₀³
When temperature increases to t°C each side increases to V_t
therefore V_t = L_t³
Increase in volume = (V_t — V₀) [L³_t — L³₀]
Increase in temperature = (t — O)°C = t°C
Heating a cube makes it grow taller, wider, and deeper all at the same time. This total three-dimensional growth is called cubical expansion, and it involves the whole volume of the object.
Teacher's Tip: Cubes grow "everywhere" - up, down, and sideways!
Exam Tip: Use the term "Cubical Expansion" to describe 3D growth in volume.

Question 28: Describe an experiment to show that liquids expand on heating.
Answer: Experiment. Take round bottomed flask filled with water. Close its mouth with air-tight cork having delivery tube through it. Mark the level of water. Let it be at A. Now heat the flask. As water gets heated in flask, the level of water in the tube rises from A to B. This proves that liquids expand on heating.
When the water inside the flask gets hot, it needs more space than the glass can provide, so it is forced up into the narrow tube. The rising water level is a clear visual sign that the liquid is expanding in volume.
Teacher's Tip: The "Narrow Tube" makes small expansion look like a big rise!
Exam Tip: Mention that the "rising water level" is the proof of expansion in your experiment summary.

Question 29: State one application of thermal expansion of liquids.
Answer: APPLICATION OF THERMAL EXPANSION OF LIQUIDS : An important application is mercury thermometer or alcohol thermometer. When the bulb of the thermometer is kept in contact with a hot body the mercury expand and the level of mercury rises in the capillary tube.
Thermometers rely on the fact that liquids like mercury or alcohol expand by a predictable amount for every degree of heat. This expansion pushes the liquid up a thin scale, allowing us to read the temperature exactly.
Teacher's Tip: Your thermometer is basically an "expansion meter."
Exam Tip: Name "Mercury" or "Alcohol" as the liquid used to get full marks.

Question 30: Describe an experiment to show that air expands on heating.
Answer: Take a glass flat bottomed flask. Close its mouth with a cork having capillary tube containing indicator visible in the tube as shown. Make it air tight. Place the flask in hot water. After few minutes, we see the indicator moving up. This happens because air inside the flask expands with rise in temperature. This proves that gases expand on heating.
As the air inside the flask gets warm from the water bath, its molecules zip around faster and push against everything, including the liquid indicator in the tube. The moving indicator shows us that the invisible air is taking up more room as it gets hotter.
Teacher's Tip: You can't see air, but you can see it "push" things when it gets hot.
Exam Tip: Emphasize that the flask must be "air-tight" for the experiment to work.

Question 31: 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 water. Explain the reason.
Answer: When the bottle is heated, air in it expands and escapes the water in the form of bubbles.
The air inside the bottle grows in volume as it heats up, but since the bottle is rigid, the extra air is forced out through the tube. When this air reaches the water, it forms bubbles as it pushes its way to the surface.
Teacher's Tip: Bubbles mean the air was "too big" for the bottle!
Exam Tip: Link "heat" to "expansion" and finally to the "escape of air" to get full marks.

Question 32: Which of the following will expand more, when heated to the same temperature : (a) solid (b) liquid and (c) gas ?
Answer: (c) Gas will expand more as the inter-molecular force of attraction in gas molecules is least and K.E. is maximum.
Gases have molecules that are barely held together, so they fly apart very easily when given even a little bit of heat. This makes their expansion hundreds of times greater than that of solids or liquids.
Teacher's Tip: Remember the sequence: Gas > Liquid > Solid.
Exam Tip: Cite "weak intermolecular forces" as the scientific reason for gas expanding the most.

Question 33: Describe an experiment to show that same volume of different liquids heated to same rise in temperature expand by different amounts.
Answer: CUBICAL EXPANSION OF DIFFERENT LIQUIDS IS DIFFERENT:
Experiment: Take four identical glass flasks each fitted with a narrow glass tube through a cork at its mouth. Fill flask A with water, B with kerosene, C with alcohol and D with Benzene. So that volume of each is same i.e. to the same level and their levels are visible above hot water bath. Put enough hot water in hot-water-bath. So that each flask is in the hot water. After some time we will see that different liquids rise to different levels. Water expands the least and benzene the most. This shows that different liquids of same volume expand by different amount.
Even though all four liquids started at the same level and were heated by the same hot water, they all "grew" differently. This proves that a liquid's internal chemistry determines how much it will expand when it gets hot.
Teacher's Tip: It's like four friends who eat the same meal but grow to different heights!
Exam Tip: Name at least two different liquids (like water and alcohol) to show you understand the comparison.

Question 34: 100 ml of each of the following liquid is heated from 10°C to 50°C. Which will expand more : (a) water (b) benzene (c) alcohol ?
Answer: 100 ml. of each means same volume of each liquid heated 10°C to 50°C means same rise in temperature. Hence, Benzene will expand more Water will expand least.
Benzene has the weakest molecular bonds among these three, so its molecules spread apart the most easily. Water has stronger hydrogen bonds, which keep its expansion relatively small compared to the others.
Teacher's Tip: Benzene is the "winner" for expansion here.
Exam Tip: Remember the specific order: Benzene > Alcohol > Water.

Question 35: Water is heated from 0°C to 4°C. Will it expand ?
Answer: Substances expand on heating. But WATER - CONTRACTS with heated from 0°C to 4°C. After that i.e. above 4°C water starts expanding.
Water is a very special liquid that does the opposite of most things between 0°C and 4°C. During this tiny window, it actually shrinks and becomes more dense before it starts behaving "normally" and expanding.
Teacher's Tip: Water is the "rebel" liquid from 0 to 4 degrees!
Exam Tip: This special behavior is called "Anomalous expansion of water."

Question 36: What do you mean by anomalous behaviour of water ?
Answer: Substances when heated expand. Their density [mass/volume] decrease. But in case of water: When water is heated from 0°C to 4°C it CONTRACT and density INCREASE, heating water above 4°C starts expanding and density of water decrease. This means water has maximum density at 4°C. This is called anomalous behaviour of water.
Usually, heating something makes it less dense, but water gets its "tightest" and heaviest at exactly 4°C. This strange behavior is why ice floats and why fish can survive in frozen lakes, as the 4°C water stays at the bottom.
Teacher's Tip: 4°C is when water is at its "densest" or heaviest.
Exam Tip: Define anomalous behavior as contraction instead of expansion between 0°C and 4°C.

Question 37: How does the density of a substance (solid, liquid and gas) change on heating ?
Answer: Density = Mass / Volume. In case of SOLIDS, when temperature is increased, increase in volume is very small and decrease in density is not appreciable. In case of LIQUIDS and GASES, as the temperature increases, volume increases by an appreciable amount and therefore decrease in their density is quite considerable.
Since heating makes things expand (increase volume) while the mass stays the same, the molecules become more spread out. This means the substance becomes "lighter" for its size, so its density goes down.
Teacher's Tip: Expansion = More Space = Lower Density.
Exam Tip: Use the formula Density = Mass/Volume to explain why expansion leads to lower density.

Question 38: An iron washer is heated. State the effect on its (i) mass, (ii) internal diameter, (iii) external diameter, and (iv) density.
Answer: (i) Mass remains same on heating.
(ii) Internal diameter increases.
(iii) External diameter increases.
(iv) Density decreases.
Heating an iron ring or washer makes the entire thing grow larger, meaning both the outer edge and the inner hole get bigger. While the amount of iron (mass) doesn't change, the fact that it's now taking up more space means its density has dropped.
Teacher's Tip: Heating a hole makes the hole bigger, not smaller!
Exam Tip: Be careful! Many students think the inner hole gets smaller, but everything expands outward.