Heat Exercise Solutions | KalviGavani

How to Use This Page

Solve every exercise question on your own first, then compare your answers below. Focus on understanding why each answer is correct — that builds exam confidence.

Key Concepts (Quick Recap)

I. Choose the Best Answer (MCQs)

Q1. Heat is a form of .............. (a) electrical energy (b) gravitational energy (c) thermal energy (d) None of these

Q2. If you apply some heat energy to a substance, which of the following can take place in it? (a) Expansion (b) Increase in temperature (c) Change of state (d) All the above

Q3. Which of the following substances will absorb more heat energy? (a) Solid (b) Liquid (c) Gas (d) All the above

Q4. If you apply equal amount of heat to a solid, liquid and gas individually, which of the following will have more expansion? (a) Solid (b) Liquid (c) Gas (d) All of them

Q5. The process of converting a liquid into a solid is called .............. (a) sublimation (b) condensation (c) freezing (d) deposition

Q6. Conduction is the heat transfer which takes place in a .............. (a) solid (b) liquid (c) gas (d) All of them

Q7. Which of the following has the highest specific heat capacity? (a) Iron (b) Water (c) Copper (d) Sand

Q8. One calorie equals .............. (a) 2.4 J (b) 4.2 J (c) 42 J (d) 0.42 J

Q9. The silvered walls in a thermos flask serve to .............. (a) prevent conduction (b) prevent convection (c) reflect radiated heat back (d) prevent evaporation

Q10. During the change of state from solid to liquid, the temperature .............. (a) increases continuously (b) decreases continuously (c) remains constant (d) first increases then decreases

II. Fill in the Blanks

1. A calorimeter is a device used to measure the heat capacity of water. 2. Specific heat capacity is defined as the amount of heat required to raise the temperature of 1 kg of a substance by 1 degree C. 3. A thermostat is a device which maintains temperature of an object constant. 4. The process of converting a substance from gas to solid is called deposition. 5. If you apply heat energy, the temperature of a system will increase. 6. If the temperature of a liquid in a container is decreased, then the inter-atomic distance will decrease. 7. The transfer of heat energy through electromagnetic waves is called radiation. 8. Latent heat is the heat energy absorbed or released during a change of state at constant temperature. 9. The vacuum between the double walls of a thermos flask prevents heat transfer by conduction and convection.

III. True or False (with corrections for false statements)

IV. Match the Following

V. Assertion and Reason

Q1. Assertion: Radiation is a form of heat transfer which takes place even in vacuum. Reason: The thermal energy is transferred from one part of a substance to another part without the actual movement of the atoms or molecules.

Q2. Assertion: A system can be converted from one state to another state. Reason: It takes place when the temperature of the system is constant.

VI. Answer Briefly

Q1. What are the applications of conduction in our daily life?

Conduction plays an important role in many everyday situations. We cook food in metal vessels because metals are good conductors that transfer heat efficiently from the flame to the food. When we iron clothes, heat is transferred from the hot iron plate to the cloth through conduction. Handles of cooking utensils are made of plastic or wood because these are poor conductors (insulators) that prevent heat from reaching our hands. In cold regions, the temperature inside an igloo stays warm because snow is a poor conductor of heat and traps warmth inside.

Q2. What are the effects of heat?

When heat energy is applied to a substance, three major effects can be observed. First, the substance may expand — solids, liquids, and gases all increase in volume when heated. Second, the temperature of the substance increases as the average kinetic energy of its molecules rises. Third, if sufficient heat is supplied at the right conditions, the substance can undergo a change of state, such as solid melting into liquid or liquid vaporising into gas.

Q3. Name three types of heat transfer.

The three types of heat transfer are conduction, convection, and radiation. Conduction occurs in solids where heat is transferred without the actual movement of molecules. Convection occurs in liquids and gases where heat is carried by the actual movement of molecules from one place to another. Radiation is the transfer of heat through electromagnetic waves and does not require any medium, which is why it can travel through vacuum.

Q4. What is conduction?

Conduction is the process of heat transfer in solids from a region of higher temperature to a region of lower temperature, without any actual movement of atoms or molecules. In this process, the molecules at the hotter end vibrate more vigorously and transfer energy to neighbouring molecules through collisions. This chain of energy transfer continues until thermal equilibrium is reached. Conduction is most effective in metals because they contain free electrons that help carry thermal energy quickly.

Q5. Write a note on convection.

The form of heat transfer from places of high temperature to places of low temperature by the actual movement of molecules is called convection. Convection takes place in liquids and gases because their molecules are free to move. When water is heated in a vessel, the molecules at the bottom get heated first, become lighter, and rise upward, while the cooler, denser molecules at the top sink downward — this cyclic movement creates convection currents. Convection is responsible for sea breezes, wind patterns, and the circulation of warm air in a heated room.

Q6. Define specific heat capacity.

Specific heat capacity of a substance is defined as the amount of heat energy required to raise the temperature of 1 kilogram of the substance by 1 degree C or 1 K. It is denoted by the symbol C and its SI unit is J/(kg K). Different substances have different specific heat capacities; for example, water has a high specific heat capacity of 4186 J/(kg K), which means it takes a lot of energy to heat water, and it also cools down slowly.

Q7. Define one calorie.

One calorie is defined as the amount of heat energy required to raise the temperature of 1 gram of water through 1 degree C. It is a smaller unit of heat energy. The relationship between calorie and joule is 1 calorie = 4.2 J. In everyday nutrition, the term "Calorie" (with a capital C) actually refers to a kilocalorie, which equals 1000 calories and is used to measure the energy content of food.

VII. Answer in Detail

Q1. With the help of a neat diagram, explain the working of a calorimeter.

A calorimeter is a device used to measure the amount of heat gained or lost by a substance during a physical or chemical process. It works on the principle of heat exchange — the heat lost by a hot body equals the heat gained by a cold body, assuming no heat escapes to the surroundings. Construction: - The calorimeter consists of a metallic vessel, usually made of copper or aluminium, because these metals are good conductors of heat and ensure uniform temperature distribution. - The metallic vessel is placed inside an insulating jacket made of wood or plastic to prevent heat loss to the environment. - A thermometer is inserted through one hole in the lid to measure the temperature of the contents accurately. - A stirrer is inserted through another hole to ensure uniform mixing and even temperature throughout the liquid. - A heating element or coil is placed inside the vessel to supply known amounts of heat energy. Working: - A known mass of the liquid whose heat capacity is to be measured is poured into the calorimeter vessel. - The initial temperature of the liquid is recorded using the thermometer. - Current is passed through the heating element for a known time, supplying a known quantity of heat energy. - The stirrer is used continuously to ensure that the heat is distributed evenly throughout the liquid. - The final temperature of the liquid is noted after the heating is stopped. - Using the formula Q = m x C x delta-T, the specific heat capacity of the liquid can be calculated. Precautions: - The calorimeter should be well insulated to minimise heat exchange with the environment. - The stirrer must be used continuously for uniform temperature distribution. - The thermometer should not touch the heating element or the walls of the vessel.

Q2. Write a note on thermostat.

A thermostat is a device that maintains the temperature of a place or an object at a constant, pre-set value. The word "thermostat" is derived from two Greek words — "thermo" meaning heat and "static" meaning staying the same. It is one of the most widely used temperature-control devices in everyday life. Principle: - A thermostat works by switching an appliance or circuit on or off when a particular temperature is reached. When the temperature drops below the set value, the thermostat turns the heating system on. When the temperature rises above the set value, it turns the heating system off. - Some thermostats use a bimetallic strip made of two different metals bonded together. Since the two metals expand at different rates when heated, the strip bends and breaks or makes the electrical contact, thereby controlling the circuit. Applications: - Building heaters: Thermostats regulate room temperature by controlling the central heating system. - Air conditioners: They maintain a comfortable indoor temperature by switching the compressor on and off. - Water heaters: The thermostat ensures that water is heated to the desired temperature and then shuts off the heating element. - Ovens: Baking requires precise temperatures; the thermostat maintains the oven at the set temperature. - Refrigerators: The thermostat keeps the interior cold by controlling the compressor cycle. Importance: - Thermostats save energy by preventing overheating or overcooling. - They improve comfort by maintaining a stable temperature. - They protect equipment from damage due to excessive temperature fluctuations.

Q3. Explain the working of a thermos flask.

A thermos flask (also called a vacuum flask or Dewar flask) is designed to keep hot liquids hot and cold liquids cold for extended periods by minimising all three modes of heat transfer — conduction, convection, and radiation. Construction: - The flask has double walls made of glass or stainless steel. - The space between the two walls is evacuated to create a vacuum. - The inner surfaces of both walls are silvered (coated with a reflective layer of silver). - The mouth of the flask is sealed with an insulated stopper, usually made of cork or plastic. - The two walls meet only at the neck of the flask, and an insulated support at the bottom provides structural stability. How it minimises heat transfer: - Conduction: The vacuum between the two walls eliminates conduction almost entirely because there are no molecules to carry heat. Conduction can only occur at the points where the two walls meet (the neck and the bottom support), which is a very small area. - Convection: Since there is a vacuum between the walls, there is no fluid (liquid or gas) to carry heat by convection. The insulated stopper at the top prevents convection currents from escaping through the opening. - Radiation: The silvered inner walls act like mirrors. They reflect infrared radiation back towards the liquid, preventing heat from escaping by radiation. If the flask is keeping a cold liquid cold, the silvered walls reflect external radiation away from the liquid. Practical considerations: - The stopper should be kept closed to prevent heat exchange through the opening. - The flask should not be dropped because the glass walls can break and the vacuum will be lost. - Over time, even the best thermos flask will allow some heat transfer, so it cannot maintain temperature indefinitely.

VIII. Higher Order Thinking Questions (HOTS)

Q1. Why does the bottom of a lake not freeze in severe winter even when the surface is all frozen?

Lakes do not completely freeze because the ice (and eventually snow) on the surface acts as an insulator for the water below. Ice is a poor conductor of heat, so it prevents the cold air from further cooling the water beneath. Also, water at 4 degree C has maximum density and sinks to the bottom, so the bottom of the lake stays at about 4 degree C.

Q2. Which one of the following statements about thermal conductivity is correct? Give reason. (a) Steel > Wood > Water (b) Steel > Water > Wood (c) Water > Steel > Wood (d) Water > Wood > Steel

Reason: Steel has high thermal conductivity (50.2 W/mK), water has moderate conductivity (0.6 W/mK), and wood has low thermal conductivity (0.12 W/mK).

IX. Numerical Problems

Q1. An iron ball requires 1000 J of heat to raise its temperature by 20 degree C. Calculate the heat capacity of the ball.

Q2. The heat capacity of the vessel of mass 100 kg is 8000 J/ degree C. Find its specific heat capacity.

Common Mistakes to Avoid

• Confusing conduction and convection: Conduction happens in solids without molecular movement; convection happens in fluids with actual molecular movement.
• Saying radiation needs a medium: Radiation travels through vacuum via electromagnetic waves — it does not need any medium.
• Mixing up freezing and condensation: Freezing = liquid to solid; condensation = gas to liquid.
• Confusing heat and temperature: Heat is the total thermal energy (unit: joule); temperature measures the average kinetic energy of molecules (unit: kelvin).
• Forgetting that temperature stays constant during change of state: During melting or boiling, the temperature does not increase until the entire substance has changed state.
• Confusing specific heat capacity with heat capacity: ❌ "Specific heat capacity depends on the mass of the substance." → ✅ Specific heat capacity is an intrinsic property (per unit mass) and does not depend on how much substance is present. Heat capacity (Q/delta-T) does depend on mass.
• Mixing up calorie and joule: ❌ "1 calorie = 42 J" → ✅ 1 calorie = 4.2 J. A common error is to shift the decimal point. Remember: calorie is a small unit; it takes about 4.2 of them to equal one joule... actually, one calorie equals 4.2 joules, so a calorie is the larger unit for everyday food energy, while joule is the SI unit.
• Thinking all three heat transfer modes need a medium: ❌ "Conduction, convection, and radiation all require a medium to transfer heat." → ✅ Only conduction and convection require a medium. Radiation is the only mode that can travel through vacuum — this is how the Sun's energy reaches Earth through empty space.

Exam Checklist

• [ ] Can you define and distinguish conduction, convection, and radiation with examples?
• [ ] Can you define specific heat capacity and heat capacity with units?
• [ ] Can you explain the working of a calorimeter and thermos flask?
• [ ] Do you know the six changes of state (melting, freezing, vaporisation, condensation, sublimation, deposition)?
• [ ] Can you solve numerical problems on heat capacity and specific heat capacity?
• [ ] Can you explain why temperature remains constant during change of state?
• [ ] Do you know that 1 calorie = 4.2 J?
• [ ] Can you distinguish between heat and temperature with proper units?
• [ ] Can you explain the principle and applications of a thermostat?
• [ ] Can you describe how a thermos flask prevents all three modes of heat transfer?
• [ ] Can you define latent heat and explain its role during change of state?
• [ ] Can you explain why water at 4 degree C has maximum density and how this prevents lakes from freezing completely?