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Samacheer Class 9 Science - Heat

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22 Questions

Complete Grade 9 Science questions and answers for Heat. Click any question to expand the answer.

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📋 Sections in this chapter
I. Choose the Correct AnswerII. Fill in the BlanksIII. Assertion and Reason Type QuestionsIV. Answer BrieflyV. Answer in DetailVI. Numerical ProblemsIntext ActivitiesActivity – 1Activity – 2Activity – 3Activity – 4
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1I. Choose the Correct Answer5 questions
Q.1Calorie is the unit of▾
✓ Answer
  • (a) heat
  • (b) work
  • (c) temperature
  • (d) food

Answer:
(a) heat

Q.2SI unit of temperature is▾
✓ Answer
  • (a) Fahrenheit
  • (b) joule
  • (c) Celsius
  • (d) kelvin

Answer:
(d) kelvin

Q.3Two cylindrical rods of same material and length have cross-sectional area ratio 2:1. Which conducts heat faster?▾
✓ Answer
  • (a) Both rods
  • (b) Rod-2
  • (c) Rod-1
  • (d) None

Answer:
(c) Rod-1

Greater cross-sectional area allows faster heat conduction.
Q.4In which mode of heat transfer do molecules pass energy without moving from their positions?▾
✓ Answer
  • (a) Radiation
  • (b) Conduction
  • (c) Convection
  • (d) Both B and C

Answer:
(b) Conduction

Correction made:
Original answer “Radiation” was incorrect.
Q.5Device that minimizes heat loss by conduction, convection and radiation is▾
✓ Answer
  • (a) solar cell
  • (b) solar cooker
  • (c) thermometer
  • (d) thermos flask

Answer:
(d) thermos flask

2II. Fill in the Blanks0 questions
  • The fastest mode of heat transfer is radiation.
  • During daytime, air blows from sea to land.
  • Liquids and gases are generally poor conductors of heat.
  • The fixed temperature at which solid changes into liquid is called melting point.
3III. Assertion and Reason Type Questions3 questions
Q.1Question 1▾
✓ Answer

Assertion:
Food cooks faster in vessels with copper bottom.

Reason:
Copper is a very good conductor of heat.

Answer:
(a) Both are true and reason correctly explains assertion.

Q.2Question 2▾
✓ Answer

Assertion:
Maximum sunlight reaches Earth during noon.

Reason:
Heat from Sun reaches Earth by radiation.

Answer:
(b) Both are true but reason is not correct explanation.

At noon, sunlight falls more directly on Earth.
Q.3Question 3▾
✓ Answer

Assertion:
When water reaches (100^\circ C), temperature remains constant until all water changes into steam.

Reason:
Boiling point of water is (10^\circ C).

Answer:
(c) Assertion is true but reason is false.

Correction:
Boiling point of water is (100^\circ C), not (10^\circ C).
4IV. Answer Briefly8 questions
Q.1Define Conduction▾
✓ Answer

Answer:

Conduction is transfer of heat through a substance without actual movement of molecules.

It mainly occurs in solids.

Q.2Why is ice kept in double-walled containers?▾
✓ Answer

Answer:

Double walls contain insulating material or vacuum that reduces heat transfer by:

  • conduction
  • convection
  • radiation

Hence ice melts slowly.

Q.3Why does water remain cool in an earthen pot?▾
✓ Answer

Answer:

Water seeps through tiny pores and evaporates.

Evaporation absorbs heat from water, cooling it.

Q.4Differentiate Convection and Radiation▾
✓ Answer
ConvectionRadiation
Heat transfer by movement of fluid moleculesHeat transfer without material medium
Occurs in liquids and gasesOccurs in vacuum also
Requires matterDoes not require matter
Example: sea breezeExample: heat from Sun
Q.5Why do people wear white clothes in summer?▾
✓ Answer

Answer:

White colour reflects most heat radiation and absorbs very little heat.

Hence it keeps body cool.

Q.6What is Specific Heat Capacity?▾
✓ Answer

Answer:

Specific heat capacity is amount of heat required to raise temperature of 1 kg of substance by (1^\circ C) or (1K).

$$Q = mc\Delta T$$
Q.7Define Thermal Capacity▾
✓ Answer

Answer:

Thermal capacity is amount of heat required to raise temperature of an entire body by (1^\circ C).

$$\text{Thermal Capacity} = mc$$
Correction made:
Original definition incorrectly described specific heat capacity.
Q.8Define Specific Latent Heat Capacity▾
✓ Answer

Answer:

Specific latent heat is amount of heat required to change state of unit mass of substance without temperature change.

5V. Answer in Detail3 questions
Q.1Explain Convection in Daily Life▾
✓ Answer

Hot Air Balloon

Heated air becomes lighter and rises, lifting balloon upward.

Sea Breeze

Daytime:

  • Land heats faster
  • Hot air rises above land
  • Cool sea air moves toward land

This is called:

Sea Breeze

Land Breeze

Night:

  • Sea remains warmer
  • Warm air above sea rises
  • Cool land air moves toward sea

This is called:

Land Breeze

Chimneys

Hot gases rise upward due to convection.

Hence chimneys are built tall.

Q.2Changes of State in Water▾
✓ Answer

Melting

$$\text{Solid} \rightarrow \text{Liquid}$$

Ice melts into water.

Freezing

$$\text{Liquid} \rightarrow \text{Solid}$$

Water freezes into ice.

Vaporization

$$\text{Liquid} \rightarrow \text{Gas}$$

Water changes into steam.

Condensation

$$\text{Gas} \rightarrow \text{Liquid}$$

Steam changes into water.

Sublimation

$$\text{Solid} \rightarrow \text{Gas}$$

Without becoming liquid.

Q.3Water as Poor Conductor of Heat▾
✓ Answer

Experiment 1

Observation:

Top water boils while ice below remains unmelted.

Conclusion:

Water conducts heat poorly.

Experiment 2

Observation:

Heating from below melts ice quickly.

Reason:

Heat transfer occurs through convection currents.

6VI. Numerical Problems3 questions
Q.1Heat Required to Raise Temperature of Water▾
✓ Answer

Given:

$$m = 25g$$
$$c = 4.18J/g^\circ C$$
$$\Delta T = 100^\circ C$$

Using:

$$Q = mc\Delta T$$
$$Q = 25 \times 4.18 \times 100$$
$$Q = 10450J$$

In Calories

$$1cal = 4.18J$$
$$\frac{10450}{4.18}$$
$$= 2500cal$$

(approximately)

Q.2Final Temperature of Water Mixture▾
✓ Answer

Given:

$$m_1 = 100g$$
$$T_1 = 90^\circ C$$
$$m_2 = 600g$$
$$T_2 = 20^\circ C$$

Using heat balance:

$$m_1(T_1-T_f)=m_2(T_f-T_2)$$
$$100(90-T_f)=600(T_f-20)$$

Solving:

$$T_f = 30^\circ C$$
Q.3Heat Needed to Convert Ice into Water at (20^\circ C)▾
✓ Answer

Given:

$$m = 2kg$$
$$L = 334000J/kg$$
$$c = 4200J/kgK$$

Heat to Melt Ice

$$Q_1 = mL$$
$$= 2 \times 334000$$
$$= 668000J$$

Heat to Raise Temperature

$$Q_2 = mc\Delta T$$
$$= 2 \times 4200 \times 20$$
$$= 168000J$$

Total Heat

$$Q = Q_1 + Q_2$$
$$= 668000 + 168000$$
$$= 836000J$$
7Intext Activities0 questions
8Activity – 10 questions

Heat Transfer Between Water and Ice

Observation:

Ice melts in water.

Conclusion:

Heat flows from hotter body to colder body.

9Activity – 20 questions

Comparing Conductivity of Metals

Observation:

Wax on copper rod melts first.

Order of conductivity:

$$\text{Copper} > \text{Aluminium} > \text{Brass} > \text{Iron}$$
10Activity – 30 questions

Convection in Water

Observation:

Purple streaks rise upward.

Conclusion:

Heated water rises upward due to convection.

11Activity – 40 questions

Latent Heat of Water

Observation:

  • Ice remains at (0^\circ C) while melting.
  • Water remains at (100^\circ C) while boiling.

Conclusion:

Heat supplied is used for change of state, not temperature rise.

This heat is called:

Latent Heat

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