Goyal Brothers Solutions for ICSE Class 9 Physics Chapter 6 Heat And Energy

Exercise

Question 1. Define heat energy (a) on conventional basis (b) on the basis of kinetic theory of matter.

(a) Heat energy : It is a form of energy which causes in us the sensation of hotness or coldness.
(b) Heat energy : On the basis of kinetic theory of matter, heat energy is defined as the sum total of kinetic and potential energies of all the molecules of a given substance.

Answer:
(a) Heat energy is the type of energy that makes us feel if something is hot or cold.
(b) According to science, heat is the total energy (both moving and stored) of all the tiny particles that make up an object.
In simple words: Heat is the energy of vibrating molecules. The more the tiny particles inside a bottle move, the "hotter" the bottle becomes.

📝 Teacher's Note: Use the "dancing crowd" analogy: a crowd where everyone is jumping around represents a hot substance, while a crowd standing still is a cold one.

🎯 Exam Tip: Always distinguish between "sum total" for heat and "average" for temperature to get full marks.

Question 2. Define temperature (a) on conventional basis (b) on the basis of kinetic theory of matter.

(a) Temperature : The degree of hotness coldness of a body is called temperature. OR Temperature is the effect of heat energy which determines the thermal state of a body.
(b) Temperature : On the basis of kinetic theory of matter, temperature is the average kinetic energy of all the molecules of a substance.

Answer:
(a) Temperature is a measurement that tells us how hot or cold something is.
(b) Scientifically, temperature measures the average speed of the moving particles in a substance.
In simple words: If heat is the total energy of the "whole team" of particles, temperature is the energy of just one "average player."

📝 Teacher's Note: Explain that two buckets of water can have the same temperature, but the larger bucket has more heat because it has more total molecules.

🎯 Exam Tip: The keyword for the kinetic definition is "average kinetic energy." Ensure this phrase is included.

Question 3. State three differences between heat and temperature.

Answer:

In simple words: Heat is the energy itself, while temperature is like a "level" that tells us which way the energy will travel.

📝 Teacher's Note: Compare this to water: Heat is the amount of water in a tank, and temperature is the height of the water. Water always flows from a higher height to a lower one, regardless of the total amount.

🎯 Exam Tip: When asked for units, specify that Joule is for energy (heat) and Kelvin is for thermal state (temperature).

Question 4. Define calorie. How many joules make one calorie?

Answer: One Calorie : One calorie is the amount of heat energy required to raise the temperature of one gram of water through \( 1^\circ\text{C} \).
There are \( 4.2\text{ J} \) in one calorie.
\( 1\text{ Cal} = 4.2\text{ J} \)
In simple words: A calorie is just another way to measure energy, specifically based on how much it warms up water. It takes about 4.2 Joules to equal 1 Calorie.

📝 Teacher's Note: Most food labels use Kilocalories (kcal), which is 1,000 of these scientific calories. It's a great way to connect physics to nutrition.

🎯 Exam Tip: Remember the exact conversion: \( 1\text{ cal} = 4.186\text{ J} \), but for most exams, \( 4.2\text{ J} \) is acceptable.

Question 5. A body absorbs 1680 J of heat energy. How many calories of heat is absorbed by the body?

Answer: Heat energy absorbed by body \( = Q = 1680\text{ J} \)
We know \( 1\text{ cal} = 4.2\text{ J} \)
\( \implies Q = \frac{1680}{4.2}\text{ Cal} \)
\( Q = 400\text{ Cal} \)
In simple words: To change Joules to Calories, you divide the number by 4.2.

📝 Teacher's Note: Practice decimal division with students, as many make mistakes when dividing by 4.2. Multiplying both numbers by 10 first (\( 16800 / 42 \)) makes it easier.

🎯 Exam Tip: Always show the formula \( 1\text{ cal} = 4.2\text{ J} \) before you start the calculation to get step marks.

Question 6. A body radiates out 300 calories of heat energy. How much heat energy is radiated out by the body in joules?

Answer: Heat energy radiated by the body \( = Q = 300\text{ Cal} \)
As \( 1\text{ cal} = 4.2\text{ J} \)
\( Q = 300 \times 4.2\text{ J} \)
\( Q = 1260\text{ J} \)
In simple words: To change Calories to Joules, you multiply the number by 4.2.

📝 Teacher's Note: This is the inverse of the previous problem. Ensure students understand when to multiply and when to divide.

🎯 Exam Tip: Joule is the larger number in the conversion (\( \text{J} > \text{cal} \) is wrong, actually \( 1\text{ cal} = 4.2\text{ J} \), so Joules will always have a larger numerical value for the same amount of heat).

Question 7. What do you understand by the anomalous expansion of water?

Answer: Anomalous expansion : A liquid which does not behave like a normal liquid on heating or cooling is said to have anomalous expansion. On heating or cooling, water does not behave like a normal liquid. Volume of water increases if we heat or cool it, provided initially the water is at \( 4^\circ\text{C} \).
In simple words: Most things shrink when they get cold. But water is weird—between \( 4^\circ\text{C} \) and \( 0^\circ\text{C} \), it actually expands and gets bigger as it gets colder!

📝 Teacher's Note: Use the "weird water" story. It's a lifesaver for nature because it allows ice to float on top of lakes instead of freezing from the bottom up.

🎯 Exam Tip: Mention the specific temperature range \( 0^\circ\text{C} \) to \( 4^\circ\text{C} \) to define "anomalous" behavior correctly.

Question 8. Draw a graph between volume and temperature, when \( 5\text{ cm}^3 \) of ice at \( -10^\circ\text{C} \) is heated to form water at \( +10^\circ\text{C} \).

Answer: Mass \( 5\text{ cm}^3 \) of water is \( 5\text{ g} \). Volume of water is minimum at \( 4^\circ\text{C} \).
In simple words: The graph shows the volume going down as it reaches \( 4^\circ\text{C} \) (where water is most compact) and then going back up as it gets hotter.

📝 Teacher's Note: The "V" shape or dip at \( 4^\circ\text{C} \) is the most critical part of this graph. It shows that water is at its smallest volume at that specific point.

🎯 Exam Tip: Label the X-axis as Temperature and the Y-axis as Volume. Mark the point \( 4^\circ\text{C} \) as the "Minimum Volume."

Question 9. How do fishes survive in frozen lakes?

Answer: Fishes survive in pond even when the atmospheric temperature is below \( 0^\circ\text{C} \). It is because of anomalous expansion of water. When the temperature falls below \( 0^\circ\text{C} \) water changes into ice and rises to the top and ice being poor conductor of heat helps to keep water below it at \( 4^\circ\text{C} \) and fish can live in pond water.
In simple words: Since ice floats and acts like a warm blanket, the water at the very bottom of a frozen lake stays liquid and relatively warm at \( 4^\circ\text{C} \), letting fish swim safely.

📝 Teacher's Note: Point out that if water contracted like other liquids, lakes would freeze solid from the bottom up, killing all aquatic life.

🎯 Exam Tip: Use the term "poor conductor of heat" for ice and "anomalous expansion" to explain the temperature layering.

Question 10. Explain, why are soft drink bottles not completely filled?

Answer: On lowering the temperature below \( 4^\circ\text{C} \), water expands due to its anomalous behaviour. So, if soft drink bottles are completely filled, then they may burst and cause accident. As a result, soft drink bottles are not completely filled.
In simple words: When a drink gets really cold or freezes, the liquid inside needs extra "growing room" because it expands. If there's no space, the bottle will pop!

📝 Teacher's Note: Relate this to putting a water bottle in the freezer. Students can see the plastic bulge or the ice push out the top.

🎯 Exam Tip: The reason is specifically "expansion below \( 4^\circ\text{C} \)."

Question 11. Explain, why do water pipes burst during severe winter?

Answer: In severe winter, water pipe lines often burst because water freeze at sub-zero temperature and in doing so expands. Since there is no space within the pipes for expanding ice, it exerts very large pressure and bursts open the steel pipes.
In simple words: When water turns into ice inside a pipe, it gets bigger and pushes against the walls with massive force until the pipe cracks.

📝 Teacher's Note: This is a powerful demonstration of the force of expansion. Not even steel can always hold back the pressure of freezing water.

🎯 Exam Tip: The keyword "exerts very large pressure" is essential to explain *why* the pipe actually breaks.

Question 12. Explain, why does a glass bottle completely filled with water and tightly capped burst when placed in a freezer?

Answer: A glass bottle completely filled with water and closed tightly is likely to burst in the freezer of refrigerator because water changes into ice and its volume increases. Since water when cooled from \( 4^\circ\text{C} \) to \( 0^\circ\text{C} \) expands. Thus, exerts large pressure and glass bottle bursts.
In simple words: Just like the water pipes, a glass bottle has no "give." When the water inside expands as it freezes, the glass simply shatters under the pressure.

📝 Teacher's Note: This is a common household mistake. It serves as a great practical reminder of the anomalous expansion of water.

🎯 Exam Tip: Contrast the "increase in volume" with the "fixed volume of the container."

Question 13. Explain, why are the taps left dripping in sub-zero temperature during winter?

Answer: Water freezes at sub-zero temperature and in doing so expands (anomalous behaviour of water). So taps are left dripping in sub-zero temperature during winter so as to provide sufficient space for expansion of freezing water.
In simple words: Letting a tap drip keeps the water moving so it doesn't freeze easily, and if it does start to freeze, it gives the ice an "exit" so it doesn't build up pressure and break the tap.

📝 Teacher's Note: Flowing water is harder to freeze than standing water because of the constant movement and friction.

🎯 Exam Tip: The goal of dripping is to "provide space" and prevent "pressure build-up."

Question 14. Explain, why are the exposed water pipes lagged with straw during severe winter?

Answer: Water freezes at sub-zero temperature and in doing so expands (anomalous behaviour of water). So exposed water pipes during severe winter are lagged with straw so as to provide sufficient space for expansion of freezing water.
In simple words: Straw is a good insulator, which means it acts like a coat for the pipes. It keeps the heat in so the water stays above freezing even when it's icy outside.

📝 Teacher's Note: Clarify that straw/lagging doesn't "make heat," it just slows down the heat escaping to the cold air.

🎯 Exam Tip: Focus on straw being a "poor conductor" or "insulator" of heat.

Question 15. Explain why do vegetables and fruits get damaged during severe frost?

Answer: Vegetables and fruits get damaged during severe frost because the water present in them on freezing expands and burst open the cell walls of cells.
In simple words: Plants are full of water. When that water freezes into ice, it expands and rips apart the tiny delicate cells inside the fruit, making them mushy and ruined.

📝 Teacher's Note: This is why "frozen then thawed" strawberries look mushy—the cell walls were destroyed by ice crystals.

🎯 Exam Tip: The key phrase is "burst open the cell walls."

Question 16. At what temperature the pure water has (a) maximum density (b) minimum volume?

Answer: At \( 4^\circ\text{C} \), pure water has maximum density and minimum volume.
In simple words: At \( 4^\circ\text{C} \), water molecules are packed together as tightly as they can possibly be.

📝 Teacher's Note: This is the "magic number" for water. It's the point where its behavior flips from normal to anomalous.

🎯 Exam Tip: \( 4^\circ\text{C} \) is the answer for both. Don't let the different wording of (a) and (b) confuse you.

Question 17. A deep pond of water has its top layer frozen. What will be the likely temperature of water layer
1. just in contact with ice
2. at the bottom of pond?

Answer:
(i) Temperature of water layer just in contact with ice is \( 0^\circ\text{C} \).
(ii) At the bottom of pond is \( 4^\circ\text{C} \).
In simple words: The layer touching the ice is freezing cold (\( 0^\circ\text{C} \)), but the heavy, dense water sinks to the bottom and stays at a safer \( 4^\circ\text{C} \).

📝 Teacher's Note: This temperature gradient is essential for the survival of bottom-dwelling pond creatures.

🎯 Exam Tip: The coldest water (\( 0^\circ\text{C} \)) is at the top because it's less dense than the water at \( 4^\circ\text{C} \).

Question 18. What is greenhouse effect?

Answer: The process by which absorption and emission of infra-red radiation by the atmospheric gases warm up the surface of a planet and its lower atmosphere is called green-house effect.
In simple words: It's like a car parked in the sun: the glass lets light in, but traps the heat inside. Earth's gases act like that glass.

📝 Teacher's Note: Emphasize that the greenhouse effect is actually natural and necessary; without it, Earth would be too cold for life. The problem is when it gets too strong.

🎯 Exam Tip: Mention "infra-red radiation" and "atmospheric gases" for a scientific definition.

Question 19. Which gas is chiefly responsible for global warming?

Answer: Carbon dioxide gas is chiefly responsible for global warming.
In simple words: Burning things like coal and oil releases CO2, which is the main gas trapping too much heat in our air.

📝 Teacher's Note: While methane and water vapor are also greenhouse gases, Carbon Dioxide (\( \text{CO}_2 \)) is the most significant one caused by human activity.

🎯 Exam Tip: If the question asks for the "chief" or "main" gas, always write Carbon Dioxide.

Question 20. How can global warming can be reduced?

Answer: With increasing economic growth and population, energy consumption is increasing and hence this leads to global warming. To minimise global warming following three measures should be taken.
(i) Internal combustion engines in vehicles should be replaced by electric battery vehicles to reduce carbon dioxide emission.
(ii) Reforestation and sustainable use of land should be encouraged. Forest habitats should be maintained.
(iii) Controlling population, through family planing, welfare reform and the empowerment of women. This will help in reducing population, then consumption for energy and hence will reduce global warming.
In simple words: We can stop global warming by switching to electric cars, planting more trees to soak up CO2, and being smarter about how many resources we use.

📝 Teacher's Note: Connect these solutions to everyday actions, like turning off lights or walking instead of driving.

🎯 Exam Tip: "Reforestation" and "Reduction of \( \text{CO}_2 \) emissions" are the two most important points for this question.

Question 21. State four effects of global warming.

Answer: The effects of global warming on the life on earth is as below :
(i) The variable changes in the climate in different parts of the world has created difficulty and forced the people and animals to migrate from one place to the other.
(ii) If has affected the blooming season of the different plants.
(iii) The climatic changes has shown the immediate effect on simple organism and plants.
(iv) It has affected the world’s ecology.
(v) It has increased the heat stroke deaths.
In simple words: It makes the weather unpredictable, confuses plants on when to grow, forces animals to move house, and makes heatwaves much more dangerous for humans.

📝 Teacher's Note: Discuss current news events like melting glaciers or extreme heatwaves to make these points feel real for students.

🎯 Exam Tip: "Climate change" and "migration of species" are very strong points to include in your list.