Answer the following questions

HEAT

TEXT BOOK EXERCISES

IV. Answer briefly:

1. Define conduction.

Answer: The process of transfer of heat in solids from a region of higher temperature to a region of lower temperature without the actual movement of molecules is called conduction.

2. Ice is kept in a double-walled container. Why?

Answer: An ice-box is made of double wall and the space between the walls is filled with some non-conducting materials to provide heat insulation, so that the loss of heat can be minimized. Hence ice is kept in a double-walled container.

3. How does the water kept in an earthen pot remain cool?

Answer: As the water seeps out of the earthern pot, it gets evaporated and takes away heat from the vessel. The water in the pot gets cooled.

4. Differentiate convection and radiation.

Answer:

Convection

• The process of transfer of heat in which the heated molecules of a liquid (or gas) themselves move to carry heat from the hot to the cold end is called convection.

• Ex: Land and sea breeze

• Convection need matter to be present.

Radiation

• The process of transfer of heat in which a material medium is not necessary and heat is directly transferred from the hot body to the cold body is called radiation.

• Ex: Transfer of heat energy from the sun.

• Radiation can occur even in vacuum.

5. Why do people prefer wearing white clothes during summer?

Answer: White clothes absorb the least heat from the sun and hence keep us comfortable in summer. On the other hand, dark coloured clothes absorb more heat from the sun and keep us warm in winter.

6. What is specific heat capacity?

Answer: Specific heat capacity of a substance is defined as the amount of heat required to raise the temperature of 1 kg of the substance by 1°C or 1 K.

7. Define thermal capacity.

Answer: Thermal capacity is the heat required to raise the temperature of a unit mass of the body by 1°C.

8. Define specific latent heat capacity.

Answer: Specific latent heat is the amount of heat energy absorbed or liberated by unit mass of a substance during change of state without causing any change in temperature.

V. Answer in detail:

1. Explain convection in daily life.

Answer: Convection is the flow of heat through a fluid from places of higher temperature to places of lower temperature by movement of the fluid itself.

Hot air balloons:

Air molecules at the bottom of the balloon get heated by a heat source and rise. As the warm air rises, cold air is pushed downward and it is also heated. When the hot air is trapped inside the balloon, it rises.

Breeze:

During day time, the air in contact with the land becomes hot and rises. Now the cool air over the surface of the sea replaces it. It is called sea breeze. During night time, air above the sea is warmer. As the warmer air over the surface of the sea rises, cooler air above the land moves towards the sea. It is called land breeze.

Chimneys:

Tall chimneys are kept in kitchen and industrial furnaces. As the hot gases and smoke are lighter, they rise up in the atmosphere.

2. What are the changes of state in water? Explain

Answer:

(i) The process of changing of substance from one physical state to another at a definite temperature is defined as change of state.

(ii) For example, water molecules are in liquid state at normal temperature.

(iii) When water is heated to 100°C, it becomes steam which is a gaseous state of matter. On reducing the temperature of the steam it becomes water again.

(iv) If we reduce the temperature further to 0°C, it becomes ice which is a solid state of water. Ice on heating, becomes water again.

(v) Thus, water changes its state when there is a change in temperature.

(vi) The process in which a solid is converted to liquid by absorbing heat is called melting or fusion.

(vii) The process in which a liquid is converted to solid by releasing heat is called freezing.

(viii)The process in which a liquid is converted to vapor by absorbing heat is called boiling or vaporization.

(ix) The process in which a vapour is converted to liquid by releasing heat is called condensation.

(x) The process in which a solid is converted to gaseous state is called sublimation.

3. How can you experimentally prove water is a bad conductor of heat? How is it possible to heat water easily while cooking?

Answer:

(a) Half fill a test tube with cold water. Wrap a piece of ice in wire gauze and drop it in the tube.

(i) It will sink to the bottom. 

(ii) Now heat the top end of the test tube.

(iii) The water soon begins to boil at the top but the ice below has still not fully melted.

This activity shows that water is a bad conductor of heat. It does not easily conduct heat from the top to the bottom of the test tube.

(b) It is possible to heat water easily while cooking:

(i) Fill a test tube with cold water.

(ii) Drop an ice-cube in this water.

(iii) Now heat the water from below.

(iv)You will find that the ice melts quickly.

This shows that though water is a bad conductor of heat, heat easily flows upwards in it. This method of transfer of heat is called convection. This method is involved in cooking.

VI. Numerical Problems.

1. What is the heat in joules required to raise the temperature of 25 grams of water from 0°C to 100°C? What is the heat in Calories?

(Specific heat of water = 4.18 J/g°C)

Given: Mass of water m = 25g

Initial temperature T₁ = 0°C

Final temperature T₂ = 100°C

Change in temperature ΔT = (T₂ – T₁)

 = (100−0)° C                 ΔT = 100°C

Specific heat of water C = 4.18 J/g°C

Solution:

Heat required H (in joules) = m × c × ΔT

 = 25 × 4.18 × 100

 = 25×418

 = 10450 J

Heat required in calories = 1 calorie = 4.18 J

 10450 J = 2497.60 calories

2. What could be the final temperature of a mixture of 100 g of water at 90°C and 600 g of water at 20°C .

Given:

 Mass of water m₁ = 100g = 0.1 kg

Specific heat capacity of water c = 4186 J

 Temperature = 90°C

 Mass of water m₂= 600 g = 0.6 kg

 Temperature = 20°C

Solution:

Heat lost by hot water = Heat gained by cold water

m₁ × c × θ₁ = m₂ × c × θ₂

0.1×4186 × (90−T_F) = 0.6×4186 × (T_F−20)

0.1× (90−T_F) = (T_F−20) × 0.6

9 − 0.1T_F = 0.6T_F – 12

0.7 T_F = 21

T_F = 30°C

Final temperature of a mixture = 30°C

3. How much heat energy is required to change 2 kg of ice at 0°C into water at 20°C? (Specific latent heat of fusion of water = 3,34,000J/kg, Specific heat capacity of water = 4200JKg⁻¹K⁻¹).

Given:

Mass of ice m = 2kg

Specific latent heat of fusion of water = L = 3, 34,000 J/Kg

Change in temperature ΔT = (T₂ – T₁)

 = (20 – 0)^o C

 ΔT = 20^oC

 Specific heat capacity of water C = 4200 J Kg^-1 K^-1

 Heat Energy required = m × c × ΔT + m × L

 = 2 × 4,200 × 20 + 2 × 3,34,000

 = 1,68,000 + 6,68,000

 Heat energy required = 8,36,000 J

Intext Activities

ACTIVITY - 1

Take a glass of water and put some ice cubes into it. Observe it for some time. What happens? The ice cubes melt and disappear. Why did it happen? It is because heat energy in the water is transferred to the ice.

Aim:

To demonstrate transfer of heat.

Material required:

A glass of water, ice cubes.

Procedure:

Take a glass of water and put some ice cubes into it. Observe it for some time. What happens? 

Observation:

The ice cubes melt and disappear. It is because heat energy in the water is transferred to the ice.

Conclusion:

Heat transfer takes place when heat energy flows from the object of higher temperature to an object with lower temperature.

ACTIVITY - 2

Take metal rods of copper, aluminum, brass and iron. Fix a match stick to one end of each rod using a little melted wax. When the temperature of the far ends reach the melting point of wax, the matches drop off. It is observed that the match stick on the copper rod would fall first, showing copper as the best conductor followed by aluminum, brass and iron.

Aim:

To compare the conducting powers of various metals .

Materials required:

Metal roads of copper, aluminium, brass and iron, match stick, melted wax.

Procedure:

Fix a match stick to one end of each rod using the little melted wax. When the temperature of the far ends reach the melting point of wax, the matches drop. Observe what happens?

Observation:

The match stick on the copper rod would fall first, showing copper as the best conductor followed by aluminum, brass and then iron.

Conclusion:

Metals are good conductors of heat. Copper is the best conductor of heat.

ACTIVITY-3

Drop a few crystals of potassium permanganate down to the bottom of a beaker containing water.When the beaker is heated just below the crystals, by a small flame, purple streaks of water rise upwards and fan outwards.

Aim:

To demonstrate transfer of heat through convection in liquids .

Materials required:

Crystals of potassium permanganate, beaker containing water .

Procedure:

Drop a few crystals of potassium permanganate down to the bottom of a beaker containing water, heat it by a small flame.

Observation:

When the beaker is heated, just below the crystals purple streaks of water rise upwards and fan outward.

Conclusion:

Water molecules at the bottom of the beaker receive heat energy and move upward and replace the molecules at the top.

This activity shows that the flow of heat through a fluid from places of higher temperature to places of lower temperature by movement of the fluid itself.

ACTIVITY - 4

Take some crushed ice cubes in a beaker and note down the temperature using thermometer. It will be 0°C. Now heat the ice in the beaker. You can observe that ice is melting to form water. Record the temperature at regular intervals and it will remain at 0°C until whole ice is converted to liquid. Now heat the beaker again and record the temperature. You can notice that the temperature will rise up to 100°C and it will retain the same even after continuous heating until the whole mass of water in the beaker is vaporized.

Aim:

To understand latent heat of water

Materials Required:

Crushed ice cubes, beaker and thermomerter.

Procedure:

Take some crushed ice cubes in a beaker and note down the temperature using thermometer. It will be 0°C. Now heat the ice in the beaker, (i) Observe and record the temperature at regular intervals. Heat the beaker again and record the temperature.

Observation:

(i) Ice is melting to form water.

(ii) Water will remain at 0°C until whole ice is converted to liquid.

(iii) On further heating, we can observe that the temperature will rise up to 100°C and the temperature will be at 100°C even after continuous heating until the whole mass of water in the beaker is vapourized.

Conclusion:

In this activity, the temperature is constant at 0°C until entire ice is converted into liquid and again constant at 100°C until all the water is converted into vapour. It is because, when a substance changes from one state to another, a considerable amount of heat energy is absorbed or liberated. This energy is called latent heat.