CG Board Class 10 Science Chapter 3 Solutions: Heat and Temperature (ऊष्मा और ताप) (2026-27)

1. Hot tea or milk kept at room temperature gradually cools down.
2. Cold water taken out of a refrigerator warms up to room temperature when kept outside.
3. Cold iron rods or water are heated on a stove or burner for cooking and washing.

No, hot and cold are not two separate categories or divisions. They are relative values on the same measurement scale. An object is called hot or cold only in comparison to another object having a lower or higher degree of hotness.

Yes, the level of colored water rises in the refill. This happens because the warm hands heat the air trapped inside the bottle. The heated air expands and exerts pressure on the water, forcing it to rise up into the refill.

No, the temperature does not get divided or doubled. Temperature is an intensive property that represents the average thermal state (degree of hotness) of a substance, not the total heat quantity. Dividing or combining equal portions at temperature T leaves the temperature of the resulting portions at T.

1. Hot object: When we touch an object at a higher temperature than our body, heat flows from the object into our hand, making it feel hot.
2. Cold object: When we touch an object at a lower temperature than our body, heat flows from our hand into the object, making it feel cold.

No, heat transfer does not always require a material medium. Heat can travel through vacuum by the process of Radiation (such as heat reaching Earth from the Sun).

(i) Can you tell from which rod did the awl-pin fall first and why?**

(ii) What happens when you heat the rod from the middle? From which direction do the awl-pins fall first? What can you say about the direction of heat transfer?

(i) ?

(i) The awl-pin falls first from the aluminium rod because aluminium is a good conductor of heat compared to glass and wood.

(ii) When heated from the middle, heat flows outwards in both directions towards the colder ends. The pins closer to the heated center melt their wax and fall first, showing that heat flows from higher temperature to lower temperature regions.

(i) Can you see convection currents?**

(ii) Can you explain why heat transfer cannot take place by the process of convection in solid substances?

(i) ?

(i) Yes, by adding a few drops of ink or potassium permanganate crystals to water while heating, rising warm currents and falling cold currents become clearly visible.

(ii) Convection requires the actual movement of particles/molecules from one place to another. In solids, molecules are tightly bound in fixed positions and cannot move freely, so convection cannot occur in solids.

1. Summers: Light-colored clothes absorb very little heat and reflect most of the solar radiation, keeping our body cool.
2. Winters: Dark-colored clothes are good absorbers of heat; they absorb maximum solar radiation and keep our body warm.

(i) Which utensil has water with the highest temperature and which one has the lowest temperature water after keeping in sunlight?**

(ii) If all three utensils are kept in a cold place, the temperature of which utensil reduces the fastest?

(i) ?

(i) Water in the black-painted utensil reaches the highest temperature because black surfaces are good absorbers of heat. Water in the shiny steel utensil has the lowest temperature because shiny surfaces reflect heat.

(ii) The temperature of the black-painted utensil reduces the fastest because good absorbers of heat are also good emitters of heat.

If all substances had the same specific heat capacity, every material of equal mass would heat up and cool down at the exact same rate. Land and water would heat up equally, preventing sea breezes and land breezes. Utensils, water cooling systems, and climate regulations would lose their functional uniqueness.

Water has a very high specific heat capacity and high latent heat of vaporization. When poured on fire, it absorbs a tremendous amount of heat energy from the burning fuel, rapidly dropping its temperature below its ignition point. The formed steam also cuts off oxygen supply.

Given:**

• Heat supplied (Q) = 487.5 J
• Mass (m) = 25 g
• Change in temperature (Δ T) = 75°C – 25°C = 50°C
• Formula:

Q = m · S · Δ T ⇒ S = (Q)/(m · Δ T)

• Calculation:

S = (487.5)/(25 × 50) = (487.5)/(1250) = 0.39 J/g°C

• Result: The specific heat of copper is 0.39 J/g°C.

1. Wires: Electric/telephone wires are tied loosely so that during cold winter days, when metals contract due to thermal contraction, the wires do not snap or break.
2. Concrete Blocks: Gaps are left between concrete road blocks to allow space for thermal expansion during hot summer days, preventing the road blocks from cracking or bending.

No, the heated head of the nail can no longer pass through the loop. Heating causes thermal expansion in the metal, increasing the dimensions (volume/diameter) of the nail’s head.

On applying heat, the glass flask absorbs heat first and expands, causing the liquid level to drop initially from A to B (expansion of container). Soon after, heat reaches the liquid, which expands much more than glass, causing the level to rise from B to C (real expansion).

The air inside the bottle gets heated by warm hands and expands at constant pressure, exerting an upward push on the coin to escape. This represents volume expansion of gas at constant pressure (γ_p).

Liquids and gases do not have a fixed shape or length of their own; they take the shape of their container. Therefore, linear or superficial expansion cannot occur, and only volumetric expansion is possible.

(i) Ratio of linear (α), area (β) and volume (γ) expansion in solids is:**

• (a) 1:1:1
• (b) 1:2:3
• (c) 1:2:1
• (d) 3:2:1

(b) 1:2:3

Explanation: In solids, superficial expansion coefficient β = 2α and volumetric expansion coefficient γ = 3α. Hence, α : β : γ = 1 : 2 : 3.

(i) Heat is not matter instead it is ………………………. .**

Energy (form of energy)

(ii) ……………………… results in heat transfer.

Temperature difference

(iii) Heat transfer is possible in …………………….. due to conduction.

Solids

(iv) ………………………… is not required for heat transfer due to radiation.

Medium (Material medium)

(v) Heat provided to change the physical state of matter results in change in temperature, this statement is ……………………………………. .

False / No change in temperature occurs (Latent heat)

Heat energy naturally flows from the object at a higher temperature to the object at a lower temperature due to the temperature difference. This transfer continues until thermal equilibrium is established, at which point both objects reach the exact same temperature.

The amount of heat energy absorbed or released by a unit mass of a substance to change its physical state (solid to liquid or liquid to gas) without any change in its temperature is called latent heat.

• Formula: Q = m · L ⇒ L = Q/m (Unit: J/kg or cal/g).

When a gas-filled balloon is brought close to a fire, the air inside absorbs thermal energy and expands rapidly according to the laws of gaseous thermal expansion. The increased internal pressure exceeds the elastic limit of the rubber wall, causing the balloon to burst.

(i) 14°F into Celsius**

(ii) 100°C into Fahrenheit

(iii) 12 K into Celsius

(i) 14°F

(iii) 12 K

Answer & Step-by-Step Working:

(i) Convert 14°F into Celsius:

• Formula:

(C – 0)/(100) = (F – 32)/(180) ⇒ C = 5/9(F – 32)

• Calculation:

C = 5/9(14 – 32) = 5/9(-18) = 5 × (-2) = -10°C

• Result: -10°C

(ii) Convert 100°C into Fahrenheit:

• Formula:

F = 9/5C + 32

• Calculation:

F = 9/5(100) + 32 = (9 × 20) + 32 = 180 + 32 = 212°F

• Result: 212°F

(iii) Convert 12 K into Celsius:

• Formula:

K = C + 273 ⇒ C = K – 273

• Calculation:

C = 12 – 273 = -261°C

• Result: -261°C

(ii) 100°C :

(iii) 12 K :

Water has a exceptionally high specific heat capacity (1 cal/g°C or 4186 J/kg°C).

1. Coolant in Engines: Used in automobile radiators to absorb excess heat without a sharp rise in temperature.
2. Hot Water Bags: Used for pain-relief fomentation because water retains heat for a long time.
3. Climate Moderation: Land and sea breezes moderate temperatures in coastal regions.

Given:**

• Mass of cold water (m₁) = 1 kg, Temperature (t₁) = 40°C
• Mass of hot water (m₂) = 1 kg, Temperature (t₂) = 60°C
• Specific heat of water (S₁ = S₂ = S)
• Formula for Equivalent Temperature (t):

t = (m₁ t₁ + m₂ t₂)/(m₁ + m₂)

• Calculation:

t = ((1 × 40) + (1 × 60))/(1 + 1) = (40 + 60)/(2) = 100/2 = 50°C

• Result: The final temperature of the mixture is 50°C.

Given:**

• Mass (m) = 500 g
• Temperature rise (Δ T) = 40°C
• Specific heat capacity (S) = 0.09 J/g°C
• Formula:

Q = m · S · Δ T

• Calculation:

Q = 500 × 0.09 × 40 = 500 × 3.6 = 1800 Joules

• Result: The required heat energy is 1800 J.

Given:**

• Initial length (L) = 100 cm
• Temperature change (Δ T) = 50°C – 30°C = 20°C
• Linear expansion coefficient (α) = 26 × 10^-6/°C
• Formula:

Δ L = α · L · Δ T

• Calculation:

Δ L = 26 × 10^-6 × 100 × 20

Δ L = 26 × 10^-6 × 2000 = 52000 × 10^-6 = 0.052 cm

• Result: The increase in length is 0.052 cm (or 0.52 mm).

Specific heat capacity of a substance is defined as the amount of heat energy required to raise the temperature of a unit mass (1 kg or 1 g) of that substance by 1°C (or 1 K).

• Formula: S = (Q)/(m · Δ T) (SI Unit: J/kg°C or J/kg · K).

There are three modes of heat transfer

1. Conduction: Heat transfer in solids from one particle to the next without actual movement of the particles.
2. Convection: Heat transfer in liquids and gases through the actual bodily movement of heated fluid particles.
3. Radiation: Heat transfer in straight lines through electromagnetic waves without requiring any material medium (can travel through vacuum).

1. Rise in Temperature: Water gets hot when heated on a stove.
2. Thermal Expansion: Expansion of mercury in thermometers; gaps left between railway tracks and concrete roads.
3. Change of State: Ice melting into water; water evaporating into steam upon heating.