Frank Brothers Solutions for ICSE Class 9 Physics Chapter 5.2 Heat Thermal Expansion

Question 1. Define the different types of coefficients of thermal expansion.
Answer:

• Coefficient of Linear expansion is equal to the change in length of a rod of length \( 1\text{m} \) when its temperature rises by \( 1^\circ \text{C} \).
• Coefficient of superficial expansion is equal to the change in area of a rod of area \( 1\text{m}^2 \) when its temperature rises by \( 1^\circ \text{C} \).
• Coefficient of volume expansion is equal to the change in volume of a rod of volume \( 1\text{m}^3 \) when its temperature rises by \( 1^\circ \text{C} \).

In simple words: When we heat things up, they grow. These coefficients tell us exactly how much a material stretches (length), spreads out (area), or fills up space (volume) for every degree of temperature increase.

📝 Teacher's Note: Use a simple rod analogy. If a metal rod is 1 meter long and grows by 1 millimeter when heated by 1 degree, that small change represents its expansion coefficient.

🎯 Exam Tip: Remember to mention the standard units: length (\( \text{m} \)), area (\( \text{m}^2 \)), and volume (\( \text{m}^3 \)) along with the temperature rise of \( 1^\circ \text{C} \) for a complete definition.

Question 2. What is the relation between the coefficient of linear (\( \alpha \)), superficial (\( \beta \)) and volume expansion (\( \gamma \))?
Answer: If the Coefficient of Linear expansion is denoted by \( \alpha \)
Coefficient of superficial expansion is denoted by \( \beta \)
And Coefficient of volume expansion is denoted by \( \gamma \)
Then the relation between \( \alpha \), \( \beta \) and \( \gamma \) is stated as
\( \beta = 2\alpha \) and \( \gamma = 3\alpha \)
\( \implies \alpha : \beta : \gamma :: 1 : 2 : 3 \)
In simple words: Superficial expansion (area) is twice as much as linear expansion (length), and volume expansion is three times as much. They always follow the simple ratio 1:2:3.

📝 Teacher's Note: A square has two dimensions (length and width), so its area expands twice as fast as its side. A cube has three dimensions, so its volume expands three times as fast.

🎯 Exam Tip: In multiple-choice questions, the ratio \( \alpha : \beta : \gamma = 1 : 2 : 3 \) is a very common and easy-to-remember fact that scores quick marks.

Question 3. Explain the working of a bimetallic strip and its applications.
Answer: A bimetallic strip consists of two metal strips- one with high coefficient of expansion and the other with low coefficient of expansion. Two different metals are used for regulating temperature in an electrical device as the strip bends due to different coefficient of expansion on excessive heating due to current, thus breaking electrical circuit until the strip cools down to a preset point. Two applications of bimetallic strip are thermostat in electric iron and in balance wheels.

• When boiling water is poured into a glass bottle, it generally cracks because on pouring hot water in the bottle the inner surface heats up and expands more as compared to its outer surface. This unequal expansion between the two surfaces causes a strain and the bottle cracks.
• Telephone wires sag in summer because due to heat of the sun, the wire expands and increases in length, thus they sag in summer.
• In cold countries water pipes burst in winter because the water has maximum density at \( 4^\circ \text{C} \) and, due to anomalous expansion of water, it expands when the water is cooled to a temperature below \( 4^\circ \text{C} \)
• Even when the water in the lakes is frozen, fishes can survive due to anomalous expansion of water, water has maximum density at \( 4^\circ \text{C} \) and this dense water remains at the bottom of the lake and the upper layer of water is less dense and freezes but the temperature of each layer of water below increases by \( 1^\circ \text{C} \). Thus it is warmer in the lakes below the ice layer. This helps the fishes to survive.

In simple words: A bimetallic strip is made of two metals that expand at different rates, causing it to bend and act as a switch. Also, things like glass and water have special expansion rules that explain why bottles crack or why fish can live under ice.

📝 Teacher's Note: Use the "uneven expansion" concept to explain both the bimetallic strip (where it's useful) and the cracking glass bottle (where it's a problem).

🎯 Exam Tip: Always use the phrase "anomalous expansion of water" when explaining why pipes burst in winter or how fish survive in frozen lakes.

Question 4. Show with a diagram how a bimetallic strip bends when heated.
Answer: Diagram is to show how the given strip bends and cools. Due to heating the copper bends more than invar due to high coefficient of linear expansion of copper.
In simple words: Since copper grows more than invar when heated, the copper side becomes longer and pushes the strip into a curve.

📝 Teacher's Note: Illustrate this by having two students walk side-by-side; if the outer student takes longer steps, the pair naturally turns a corner.

🎯 Exam Tip: In diagrams, always label the metal that expands more on the outside of the curve.

Question 5. What is a thermostat and where is it used?
Answer: Thermostat is a device for regulating temperature in electric circuits and it is made up of bimetallic strip. Two applications of thermostat are in electric iron and in refrigerators.
In simple words: A thermostat is like a smart switch that turns a machine on or off to keep the temperature just right.

📝 Teacher's Note: Relate this to an AC or a water heater. When it gets too hot, the bimetallic strip bends away and cuts the power.

🎯 Exam Tip: Mention "bimetallic strip" as the key component for full marks on this question.

Question 6. What do you mean by the anomalous expansion of water?
Answer: When heated, water expands normally but behavior of water is unusual over a small range of temperature between \( 0 \) and \( 4^\circ \text{C} \). This is known as anomalous expansion of water.
In simple words: Usually, things expand when heated, but water actually shrinks as it warms up from \( 0^\circ \text{C} \) to \( 4^\circ \text{C} \). It's a "weird" exception to the rule.

📝 Teacher's Note: Emphasize that "anomalous" means "abnormal." Most liquids expand from the start, but water waits until after \( 4^\circ \text{C} \).

🎯 Exam Tip: State the exact temperature range (\( 0^\circ \text{C} \) to \( 4^\circ \text{C} \)) to define this phenomenon correctly.

Question 7. How does the anomalous expansion of water affect lakes?
Answer: Anomalous expansion of water slows down the complete freezing of water in the lake.
In simple words: Because of this special expansion, the densest water stays at the bottom, which keeps the lake from freezing solid all the way through.

📝 Teacher's Note: This is nature's way of protecting aquatic life. If lakes froze from the bottom up, everything inside would die.

🎯 Exam Tip: Mention that this property prevents the "complete freezing" of water bodies in very cold climates.

Question 8. Explain the experiment where a flask is kept in a hot water bath.
Answer: The level will rise above \( X \) when the flask is kept in hot water bath because the water in the flask will heat up and expand in volume thus raising the level of water in the tube above \( X \).
In simple words: When the water inside the flask gets hot, it needs more room, so it pushes up into the narrow tube.

📝 Teacher's Note: This is a simple thermometer! You can build this in class with a plastic bottle and a straw to show thermal expansion of liquids.

🎯 Exam Tip: Note that the expansion is more visible in a narrow tube because a small increase in volume causes a large change in height.

Question 9. Give scientific reasons for thermal expansion phenomena.
Answer:

• When boiling water is poured into a glass bottle, it generally cracks because on pouring hot water in the bottle the inner surface heats up and expands more as compared to its outer surface. This unequal expansion between the two surfaces causes a strain and the bottle cracks.
• Telephone wires sag in summer because due to heat of the sun, the wire expands and increases in length, thus they sag in summer.
• In cold countries water pipes burst in winter because the water has maximum density at \( 4^\circ \text{C} \) and, due to anomalous expansion of water, it expands when the water is cooled to a temperature below \( 4^\circ \text{C} \).
• Even when the water in the lakes is frozen, fishes can survive due to anomalous expansion of water, water has maximum density at \( 4^\circ \text{C} \) and this dense water remains at the bottom of the lake and the upper layer of water is less dense and freezes but the temperature of each layer of water below increases by \( 1^\circ \text{C} \). Thus it is warmer in the lakes below the ice layer. This helps the fishes to survive.

In simple words: Heat makes things expand, and cold (for water) can also make things expand. These rules explain everyday things like sagging wires or why pipes break in the cold.

📝 Teacher's Note: This set of reasons is fundamental. Focus on the word "strain" for the glass bottle and "density" for the water examples.

🎯 Exam Tip: Use terms like "unequal expansion" for solids and "anomalous expansion" for water to get full credit.

Question 10. Study the diagram of a frozen lake and answer the following.
Answer: The layer at the top level is ice so its temperature is therefore \( 0^\circ \text{C} \) and temperature at every layer of water below ice increases by \( 1^\circ \text{C} \)
(i) At \( X = 1^\circ \text{C} \) since it is the first layer below ice
(ii) At \( Y = 4^\circ \text{C} \) since it is the fourth layer below ice
In simple words: The top is freezing cold ice (\( 0^\circ \text{C} \)), but as you go deeper, it actually gets slightly warmer, reaching \( 4^\circ \text{C} \) at the bottom.

📝 Teacher's Note: Explain that \( 4^\circ \text{C} \) water is the "heaviest" (most dense), which is why it sinks to the very bottom and stays there.

🎯 Exam Tip: Remember the rule: the temperature increases by \( 1^\circ \text{C} \) for every layer you move down from the ice.

Question 11. Interpret the given volume-temperature graph for water.
Answer: (i) At BC temperature is constant because at \( 0^\circ \text{C} \) all of the heat given is used in process of formation of water from ice and no part of heat is used in changing the temperature of ice.
(ii) At point D i.e. at \( 4^\circ \text{C} \) the volume of water is minimum
(iii) At point D i.e. at \( 4^\circ \text{C} \), the density of water is maximum due to anomalous expansion of water in the temperature range of \( 0^\circ \text{C} \) and \( 4^\circ \text{C} \) as volume decreases, density of water increases
In simple words: The graph shows that water is special—it has the smallest volume (takes up the least space) at exactly \( 4^\circ \text{C} \).

📝 Teacher's Note: Point D is the most important part of the graph. It shows the "bottom" of the volume curve, where water is most compact.

🎯 Exam Tip: Always associate \( 4^\circ \text{C} \) with "minimum volume" and "maximum density" of water.

Question 12. Fill in the blanks with the correct terms.
Answer:

• Maximum
• Increases; decreases
• Coefficient; expansion
• Anomalous expansion of water
• Calorie
• Kelvin(K)

In simple words: These are the key words used to describe temperature, expansion, and heat energy.

📝 Teacher's Note: Use these terms in a vocabulary drill. Ask students what each term relates to—e.g., Kelvin is for temperature, Calorie is for heat energy.

🎯 Exam Tip: Memorize "Kelvin" as the SI unit of temperature and "Calorie" as a unit of heat energy.

Question 13. How can you remove a stopper stuck in the neck of a glass bottle?
Answer: We should heat the neck of the bottle because due to heating the neck will expand and loosen the stopper stuck in the neck. In this way, we can easily remove the stopper.
In simple words: Heating the glass neck makes the opening slightly bigger, which lets the stuck stopper pop right out.

📝 Teacher's Note: This is a practical example of thermal expansion. Be careful not to heat the stopper itself, or it will expand too!

🎯 Exam Tip: Specify that you heat the *neck* of the bottle specifically to make it expand away from the stopper.

Question 14. Explain the difference between apparent and real expansion of a liquid.
Answer: When a liquid is heated in a flask. Due to heat, the flask expands along with the liquid, thus providing more space for the liquid. Hence the level of the liquid falls. Thus this dropped level shows the apparent expansion of the liquid. The actual expansion of the liquid, when heated, is the real expansion.
In simple words: Apparent expansion is what we *see* (the liquid level rising), but real expansion is the total growth, including the extra room created when the container also expands.

📝 Teacher's Note: Use the formula: Real Expansion = Apparent Expansion + Expansion of the Vessel. It's like measuring a child's height while they are standing in a small hole.

🎯 Exam Tip: Remember that real expansion is always greater than apparent expansion because the container also expands.

Question 15. Will mercury and alcohol have the same volume increase when heated over the same temperature range?
Answer: No, the both liquids i.e. mercury and alcohol will not have same volume on heating as they have different coefficient of expansions, thus they expand differently on heating at same temperature range. Alcohol expands more than mercury.
In simple words: Different materials react to heat in their own way. Alcohol is more "active" and grows more than mercury when they get hot.

📝 Teacher's Note: This is why different materials are chosen for different types of thermometers. Alcohol thermometers are more sensitive but mercury can handle higher temperatures.

🎯 Exam Tip: The keyword is "different coefficient of expansions." Different substances never expand by the exact same amount.

Question 16. Describe an experiment to show the expansion of liquids on heating.
Answer: Fill a round bottom flask with a coloured liquid up to the brim. Fit it with a one holed cork and pass a narrow glass tube through it. Mark the level of the liquid in the glass tube. Place the flask in a water bath and heat the bath. You will see that the level of liquid of water rises in the glass tube. This is due to the expansion of the liquid on heating.
In simple words: When we heat a flask full of colored water, the water has to go somewhere as it grows, so it climbs up the thin tube.

📝 Teacher's Note: Use colored water (like food coloring) to make it much easier for students to see the level change in the tube.

🎯 Exam Tip: Mention that the level first falls slightly (as the glass expands) and then rises significantly (as the liquid expands).

Question 17. Describe an experiment to show that gases expand on heating.
Answer: Take a round bottom flask which is filled with air and closed with a one holed rubber cork. A narrow capillary tube is passed through the cork and the flask is supported on a tripod in an inverted position as shown in the figure, so that the end of the capillary tube is under water. Now, heat the flask gently with a spirit lamp. You will observe bubbles coming out from the capillary tube. This shows that gases expand on heating.
In simple words: When you heat the air inside a flask, it tries to get out because it's growing. Since the tube is underwater, we see the air escape as bubbles.

📝 Teacher's Note: This is a very dramatic experiment. It shows that gases expand much more easily and visibly than solids or liquids.

🎯 Exam Tip: The appearance of "bubbles" is the key observation that proves the air inside is expanding and escaping.

Question 18. Why does a hot glass chimney often crack when a drop of water falls on it?
Answer: A hot glass chimney often crack when a drop of water falls on it because the glass of the chimney expands due to constant heating by the flame of the chimney but as a cooler water drop falls on the glass, the outer surface contracts more than the inner side of the glass. This unequal contraction between the two surfaces causes a strain and the glass cracks.
In simple words: The hot glass is stretched out. When cold water hits it, that one spot tries to shrink instantly. This tug-of-war between the hot and cold parts snaps the glass.

📝 Teacher's Note: This is the reverse of the boiling water in a cold glass bottle. In both cases, the "uneven" change in size is what causes the break.

🎯 Exam Tip: Use the term "unequal contraction" to describe the effect of the cold water drop on the hot surface.

Question 19. Can we fuse an iron wire into a glass rod? Why or why not?
Answer: No, we cannot fuse iron wire in glass rod because glass is bad conductor of heat and it does not allow heat to pass through it and iron wire would not be able to get sufficient heat to fuse.
In simple words: Because glass doesn't share heat well and expands at a different rate than iron, they won't stick together properly and will likely crack.

📝 Teacher's Note: For two materials to be fused, they must have very similar expansion coefficients (like platinum and certain glasses), otherwise they will pull apart as they cool.

🎯 Exam Tip: Highlight that "different rates of expansion/contraction" would cause the seal to break as it cools.

Question 20. How do you remove a tight glass stopper?
Answer: We should heat the neck of the bottle because due to heating the neck will expand and loosen the stopper stuck in the neck. In this way, we can easily remove the stopper from the bottle.
In simple words: Warming the bottle's neck makes the hole bigger, letting the stopper slide out.

📝 Teacher's Note: This is a repetition of Q13, emphasizing its importance as a practical application of expansion in solids.

🎯 Exam Tip: Always mention that heating the *neck* specifically causes it to expand *outward*.

Question 21. Name two substances that expand on heating.
Answer: Two substances that expand on heating are water and iron.
In simple words: Almost everything, from the water in your glass to the metal in a bridge, gets bigger when it's hot.

📝 Teacher's Note: Use this to show that expansion happens in both liquids (water) and solids (iron).

🎯 Exam Tip: Any common metal or liquid (except for the anomalous range of water) is a correct answer here.

Question 22. Name two substances that contract on heating.
Answer: Two substances that contract on heating are plastic and poly-ethene.
In simple words: Some materials like certain plastics actually shrivel up and get smaller when they are heated.

📝 Teacher's Note: This is an interesting exception. Polymers can sometimes coil up when heated, leading to contraction instead of expansion.

🎯 Exam Tip: These are "special cases." Most substances expand, so remember these as exceptions.

Question 23. Explain the ball and ring experiment.
Answer: Take a ball and ring apparatus as shown in figure. At room temperature, the ball can just pass through the ring. Now heat the ball over a flame and try to pass it through the ring. You will find that the ball when heated is not able to pass through the ring. The diameter of the ball increases when the ball is heated. This shows the expansion of solid on heating
In simple words: A cold ball fits through a hole perfectly. But if you heat the ball, it gets too "fat" to fit through the same hole.

📝 Teacher's Note: This is the classic demonstration for solid expansion. It's very clear and easy for students to visualize.

🎯 Exam Tip: The conclusion of this experiment is that "solids expand on heating." Make sure to state this clearly.

Question 24. Why do joints in metal pipes loosen in summer?
Answer: The joints in metal pipes loosen in summers because the joints get more space to expand due to the heat in summers as compared to the pipes therefore the joints become loose in summers.
In simple words: The connectors at the joints grow bigger in the summer heat, which can make the connection feel loose.

📝 Teacher's Note: Expansion must be accounted for in engineering. This is why bridges have "expansion joints" that look like metal teeth.

🎯 Exam Tip: The key reason is the "increase in space" due to thermal expansion in the summer heat.

Question 25. What factors does the increase in length of a rod depend on?
Answer: The increase in length of a rod depends on the following factors:

• material of the rod
• Original length of the rod
• Rise in temperature

In simple words: How much a rod grows depends on what it's made of, how long it was to start with, and how much hotter it gets.

📝 Teacher's Note: This leads directly to the formula for linear expansion: \( \Delta L = \alpha L_0 \Delta T \). Each bullet point corresponds to a part of that formula.

🎯 Exam Tip: If asked for three factors, these are the exact three you need to list for full marks.

Question 26. What does the statement "coefficient of linear expansion is \( 0.000016 \text{ per } ^\circ \text{C} \)" mean?
Answer: The given statement states that the change in length of the metal will be \( 0.000016\text{m} \) when its temperature changes by \( 1^\circ \text{C} \).
In simple words: For every degree hotter it gets, each meter of this metal grows by that tiny fraction of a meter.

📝 Teacher's Note: Make sure to emphasize that this change is per unit length (e.g., per 1 meter). It's a very small number, but it adds up!

🎯 Exam Tip: Always include the original length (1 unit) and the temperature change (\( 1^\circ \text{C} \)) in your explanation.

Question 27. What happens when you heat a copper washer?
Answer: On heating the copper washer:

• Its internal diameter will increase as the copper washer expands outwards
• Volume will increase as the size of the metal expands on heating
• Mass will remain the same as the mass does not change with change in temperature
• Density will decrease as the volume of the washer increases on heating and density is inversely proportional to volume
• External diameter will increase because the copper washer expands outwards

In simple words: Everything about the washer gets bigger (inside and outside), but it still weighs the same, so it actually becomes a little "lighter" for its size (less dense).

📝 Teacher's Note: A common mistake is thinking the inner hole gets smaller. Tell students to imagine the hole as a "plug" of metal that also expands—so the hole must get larger.

🎯 Exam Tip: Remember: Mass NEVER changes during thermal expansion, but volume increases and density decreases.

Question 28. Describe the flask experiment to show apparent vs. real expansion.
Answer: Fill a round bottom flask with a coloured liquid up to the brim. Fit it with a one holed cork and pass a narrow glass tube through it. Mark the level of the liquid in the glass tube as A. Place the flask in a water bath and heat the bath. You will see that the level of liquid of water starts falling to level B and then it rises to level C. In this, AC is the apparent expansion and is due to expansion of the flask due to heating which provides more space for the liquid. Hence the level of liquid drops. As soon as the liquid also starts getting heated up, it expands and the level of liquid rises. BC is the real expansion. So the actual expansion of the water is sum of AC and AB.
In simple words: When you start heating, the container grows first and the water level drops (B). Then the water heats up and shoots way past where it started (C). Real expansion counts the whole journey from the very bottom (B) to the top (C).

📝 Teacher's Note: This is a key experiment. The drop from A to B is crucial proof that the container expands too. If the container didn't expand, the level would never drop.

🎯 Exam Tip: Be sure to define the segments: AB is the vessel's expansion, AC is apparent expansion, and BC is real expansion. Formula: \( \text{Real} = \text{Apparent} + \text{Vessel} \).

Question 29. At what temperature does water have its maximum density?
Answer: At \( 4^\circ \text{C} \), water has the maximum density due to anomalous expansion.
In simple words: Water is at its "heaviest" and most packed together at exactly \( 4^\circ \text{C} \).

📝 Teacher's Note: This simple fact is the basis for understanding how life survives in frozen ponds. Remind students that after \( 4^\circ \text{C} \), water expands as it cools toward \( 0^\circ \text{C} \).

🎯 Exam Tip: Just writing "\( 4^\circ \text{C} \)" is often enough for a one-mark question, but mention "anomalous expansion" for a complete answer.

Question 30. Use a diagram to show how gases expand.
Answer: It can be seen from the graph that the volume of the water decrease from \( 0^\circ \text{C} \) to \( 4^\circ \text{C} \) and the volume is minimum at \( 4^\circ \text{C} \). After \( 4^\circ \text{C} \) the volume increases with the increase in temperature.
In simple words: This setup shows that air inside a flask grows when heated, pushing itself out through the tube.

📝 Teacher's Note: This is a repeat of the gas expansion experiment. It emphasizes the importance of the visual observation (bubbles) in scientific proof.

🎯 Exam Tip: When drawing this, ensure the tube is clearly submerged in a container of water to show where the bubbles appear.