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HeatSolved Questions· Unit 9
Science & Technology · Chapter 09
35 solved questions

Chapter 9: Heat

Every question in this chapter, answered and explained — step-by-step solutions drawn live from the chapter library across 8 question sections.

Very Short Answer Questions8Short Answer Questions6Give Reason Questions6Differentiate Between3Long Answer Questions4Numerical Problems3Diagram-Based Questions2Application / Conceptual Questions3
01

Very Short Answer Questions

8
1SEE Boardqb-q0

Define thermal energy.

Ans.Thermal energy is the sum of the kinetic energy of all the molecules of a substance.
2SEE Boardqb-q1

What is temperature? Write its SI unit.

Ans.Temperature is an index of the average kinetic energy of the molecules of a substance. Its SI unit is kelvin (K).
3SEE Boardqb-q2

Define heat and write its SI unit.

Ans.Heat is the amount of thermal energy transmitted from one object to another due to a temperature difference. Its SI unit is joule (J).
4SEE Boardqb-q3

What is thermal equilibrium?

Ans.Thermal equilibrium is the state in which two bodies in contact reach the same temperature, and no further heat flows between them.
5SEE Boardqb-q4

What is absolute zero temperature?

Ans.Absolute zero is the theoretically lowest possible temperature, at which the thermal energy of a substance becomes zero. It is equal to 0 K, or -273.15°C.
6SEE Boardqb-q5

At what temperature does water have its maximum density?

Ans.Water has its maximum density (and minimum volume) at 4°C.
7SEE Boardqb-q6

Define specific heat capacity and write its SI unit.

Ans.Specific heat capacity is the amount of heat required to change the temperature of 1 kg of a substance by 1°C. Its SI unit is joule per kilogram per degree Celsius (J/kg°C).
8SEE Boardqb-q7

What are the lower and upper fixed points on the Celsius scale?

Ans.The lower fixed point is 0°C (temperature of melting ice) and the upper fixed point is 100°C (temperature of steam above boiling water at one atmospheric pressure).
02

Short Answer Questions

6
1SEE Boardqb-q8

Differentiate between thermal energy and heat.

Ans.Thermal energy is the sum of kinetic energy of all molecules in a substance and can be possessed by an object. Heat is only the amount of thermal energy transferred from one object to another due to a temperature difference, and it is not stored in an object.
2SEE Boardqb-q9

Differentiate between heat and temperature.

Ans.Heat is the total thermal energy transferred between objects, measured in joules, and depends on the mass of the substance. Temperature is a measure of the average kinetic energy of molecules, measured in kelvin, and does not depend on mass.
3SEE Boardqb-q10

What is the heat equation? Write the formula.

Ans.The heat equation relates the heat absorbed or released by a substance to its mass, specific heat capacity, and temperature change: Q = ms(T2 - T1), where Q is heat, m is mass, s is specific heat capacity, and (T2 - T1) is the temperature change.
4SEE Boardqb-q11

Write any two applications of specific heat capacity.

Ans.
  1. (i) Water's high specific heat capacity makes it an effective coolant in car radiators and thermal power stations.
  2. (ii) A hot water bag uses water's high specific heat capacity to release heat slowly, providing soothing relief for muscular pain over a long period.
5SEE Boardqb-q12

Describe the condition in which water can be boiled at a temperature less than 100°C.

Ans.The boiling point of water depends on atmospheric pressure. At lower atmospheric pressure, such as at high altitudes (like in the Himalayan mountains), water boils at a temperature lower than 100°C because less energy is needed for the vapour pressure to equal the reduced atmospheric pressure.
6SEE Boardqb-q13

Write the types of thermometers used in daily life and mention their working principles.

Ans.
  1. (i) Liquid-in-glass thermometer — works on the principle of thermal expansion of mercury inside a capillary tube.
  2. (ii) Digital thermometer — works using a heat-sensitive thermistor whose electrical resistance changes with temperature.
  3. (iii) Radiation (infrared) thermometer — works by detecting the intensity of infrared radiation emitted by a body, without contact.
03

Give Reason Questions

6
1SEE Boardqb-q14

An iron bar heats up when it is hammered continuously for some time. Why?

Ans.Hammering does mechanical work on the iron bar, which is converted into thermal energy. This increases the kinetic energy and vibration of the iron's molecules, raising its temperature, so the bar heats up.
2SEE Boardqb-q15

Tea in an open teacup stops cooling after some time. Why?

Ans.The tea stops cooling further once it reaches the same temperature as the surrounding air. At this point, thermal equilibrium is achieved, and there is no longer a temperature difference to drive further heat flow.
3SEE Boardqb-q16

Water pipes crack in cold places in winter. Why?

Ans.As the temperature drops in winter, water inside the pipe cools and eventually freezes into ice. Due to the anomalous expansion of water, ice occupies a greater volume than the same mass of water. This expansion generates very high pressure on the pipe walls, causing the pipe to crack.
4SEE Boardqb-q17

Water is kept in a car's radiator to cool the engine. Why?

Ans.Water has a very high specific heat capacity, so it can absorb a large amount of heat from the hot engine without its own temperature rising too much. This makes it very effective at removing excess heat and cooling the engine.
5SEE Boardqb-q18

A hot water bag is used to give hot compression to parts of the body. Why?

Ans.Water's high specific heat capacity allows it to store a large amount of heat. When placed in a bag against the skin, this stored heat is released slowly and steadily over a long period, providing soothing warmth that helps relieve muscular pain.
6SEE Boardqb-q19

There is no significant difference in temperature between the daytime and nighttime in coastal areas. Why?

Ans.In coastal areas, the specific heat capacity of seawater is much higher than that of land. During the day, sea breeze from the cooler sea moderates the land's high temperature, and at night, land breeze from the relatively warmer sea moderates the land's low temperature. This continuous moderating effect keeps the day-night temperature difference small.
04

Differentiate Between

3
1SEE Boardqb-q20

Differentiate between thermal energy and heat.

Thermal EnergyHeat
Sum of kinetic energy of all molecules of a substance.Thermal energy transferred from one object to another due to temperature difference.
Depends on mass and average molecular speed.Depends on the temperature difference between two objects.
Can be possessed/stored by an object.Is not stored; describes energy in transit only.
2SEE Boardqb-q21

Differentiate between heat and temperature.

HeatTemperature
Total energy transferred, measured in joules (J).Measure of average kinetic energy, measured in kelvin (K).
Depends on mass of the substance.Independent of the mass of the substance.
Measured with a calorimeter.Measured with a thermometer.
3SEE Boardqb-q22

Differentiate between the lower fixed point and the upper fixed point of a thermometer.

Lower Fixed PointUpper Fixed Point
Temperature of pure melting ice.Temperature of steam above water boiling at one atmospheric pressure.
0°C on the Celsius scale.100°C on the Celsius scale.
273 K on the Kelvin scale.373 K on the Kelvin scale.
05

Long Answer Questions

4
1SEE Boardqb-q23

In the perception of a certain man, a bucket of lukewarm water contains more thermal energy than a large tank of cold water. Correct this understanding based on the definitions of thermal energy and temperature.

Ans.This understanding is incorrect if the tank of cold water is large enough. Thermal energy is the SUM of the kinetic energy of all the molecules in a substance, and it depends on both the mass of the substance and the average kinetic energy (temperature) of its molecules. While the lukewarm water in the bucket may have molecules with higher average kinetic energy (higher temperature) than the cold water, if the tank of cold water has a very large mass — that is, a much greater number of molecules — the total (summed) kinetic energy of all its molecules can be greater than that of the smaller amount of lukewarm water. Therefore, the large tank of cold water can actually contain more thermal energy than the small bucket of lukewarm water, even though its temperature is lower.
2SEE Boardqb-q24

Touching a cup of hot tea feels hot but touching an ice cube feels cool. Explain based on the motion of molecules.

Ans.The molecules in hot tea have very high kinetic energy and move rapidly. When our hand touches the hot tea, heat flows from the tea (higher temperature) into our hand (lower temperature), and our hand's molecules gain energy, so we feel hot. In contrast, the molecules in an ice cube have very low kinetic energy and move slowly. When our hand touches the ice, heat flows from our hand (higher temperature) into the ice (lower temperature), so our hand loses thermal energy and we feel cool.
3SEE Boardqb-q25

If the lid of a glass bottle does not open, how may it be opened using your knowledge of the effects of heat? Explain based on the kinetic energy of molecules.

Ans.The metal lid can be heated slightly, for example by running hot water over it or holding it near a warm surface. Heating increases the kinetic energy of the metal's molecules, making them vibrate faster. This weakens the force of attraction between them, causing the molecules to move farther apart, so the metal lid expands. Since metal typically expands more than glass for the same amount of heating, the lid becomes slightly larger than the neck of the glass bottle, loosening its grip and making it easier to twist open.
4SEE Boardqb-q26

Study the relationship between the volume and temperature of water shown in a graph where volume decreases from 0°C to 4°C and increases from 4°C to 10°C. Answer: (i) What is this special property of water called? (ii) What change in volume occurs between 0°C and 10°C? (iii) How does this differ from other liquids? (iv) Explain its importance in daily life.

Ans.(i): This special property is called the anomalous expansion of water.

(ii): As water is heated from 0°C to 4°C, its volume decreases, reaching a minimum at 4°C. From 4°C to 10°C, its volume increases again as heating continues.

06

Numerical Problems

3
1SEE Boardqb-q27

Calculate the amount of heat required to raise the temperature of 500 g of water from 15°C to 85°C.

Ans.Given: mass m = 500 g = 0.5 kg, T1 = 15°C, T2 = 85°C, specific heat capacity of water s = 4200 J/kg°C

Required: Heat Q

2SEE Boardqb-q28

The specific heat capacity of iron is 460 J/kg°C. Calculate the heat released by an iron sphere of mass 5 kg while cooling from 430°C to 30°C.

Ans.Given: mass m = 5 kg, s = 460 J/kg°C, T1 = 430°C, T2 = 30°C

Required: Heat released Q

3SEE Boardqb-q29

A bucket contains 16 kg of water at 25°C. Calculate the temperature of the mixture formed when 4 kg of water at 80°C is mixed with it. (Heat lost to the surroundings is neglected.)

Ans.Given: mass of cold water m1 = 16 kg, T1 = 25°C; mass of hot water m2 = 4 kg, T2 = 80°C

Required: Final temperature T of the mixture

07

Diagram-Based Questions

2
1SEE Boardqb-q30

Draw and label a diagram showing sea breeze during the day and land breeze at night.

Ans.Draw two panels. In the daytime panel, show the sun above the land, with warm air rising from the land and an arrow labelled 'cool sea breeze' flowing from the sea toward the land. In the nighttime panel, show the moon, with warm air rising from the relatively warmer sea and an arrow labelled 'cool land breeze' flowing from the land toward the sea.
2SEE Boardqb-q31

Draw a model graph to show the relationship between the density of water and temperature, based on the fact that water has maximum density at 4°C.

Ans.Draw a graph with temperature (°C) on the x-axis and density (g/cm^3) on the y-axis. The curve should start relatively low near 0°C, rise to a peak (maximum density) exactly at 4°C, and then decrease steadily as temperature increases beyond 4°C.
08

Application / Conceptual Questions

3
1SEE Boardqb-q32

Additional SEE Practice — Q1. Once in winter, while drinking water from a steel jug on the table, Samir felt the water to be warmer towards the bottom. Justify his experience based on scientific facts.

Ans.This can happen because of how heat is transferred through the steel jug and the surrounding air. If the room air near the tabletop is slightly warmer than the air higher up, or if the table surface itself retains some warmth, then the bottom of the steel jug (a good conductor of heat) would gain more heat from the table/warmer surrounding air, warming the water near the bottom slightly more than the water near the top, which is more exposed to the room's cooler air above. Since steel conducts heat well, this uneven heat transfer through the jug walls can create a noticeable temperature difference between the bottom and the top of the water.
2SEE Boardqb-q33

Additional SEE Practice — Q2. Of two ice cubes of identical shape, one is kept in an aluminum box and the other in a wooden box. Which ice cube melts faster? Explain in terms of the melting process.

Ans.The ice cube in the aluminum box will melt faster. Aluminum is a good conductor of heat, so it quickly transfers heat from the warmer surroundings into the ice, speeding up melting. Wood is a poor conductor of heat (a good insulator), so it transfers heat into the ice much more slowly, keeping the ice cube in the wooden box colder for longer and delaying its melting.
3SEE Boardqb-q34

Additional SEE Practice — Q3. What are the differences between the process of freezing ghee and honey in terms of their volume and density?

Ans.Like most substances, ghee and honey contract (decrease in volume and increase in density) as they cool and solidify, unlike water's anomalous behaviour. Ghee solidifies at a relatively higher temperature and becomes visibly firmer with a noticeable decrease in volume. Honey, being a very thick and viscous liquid with a lower freezing point, thickens and eventually crystallizes at low temperatures, also contracting slightly, though its exact volume and density change depends on its sugar and water content.
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