Change of State and Latent Heat - UNSOLVED PRACTICE SET

Q7. Define latent heat of fusion and latent heat of vaporisation. Write their SI units.

Q8. Why does the temperature remain constant during a change of state even though heat is being supplied?

Q9. Calculate the heat required to convert 10 g of ice at 0°C to steam at 100°C. (Lf = 80 cal/g, Lv = 540 cal/g, specific heat of water = 1 cal/g·°C)

Q10. In your school, a student notices that steam at 100°C causes more severe burns than boiling water at 100°C. Explain why.

Q11. What is regelation? Explain with an example.

Q12. Why does evaporation cause cooling? Explain using the concept of latent heat.

Q13. Explain the phenomenon of change of state. Discuss:

(i) The heating curve of water from ice to steam

(ii) Why temperature remains constant during phase changes

(iii) The molecular interpretation of latent heat

(iv) The difference between evaporation and boiling

(v) Factors affecting the rate of evaporation

Q14. A 20 g piece of ice at −10°C is dropped into a calorimeter containing 100 g of water at 40°C. The calorimeter has water equivalent 10 g.

(a) Calculate the heat required to bring the ice to 0°C. (Specific heat of ice = 2100 J/kg·K)

(b) Calculate the heat required to melt the ice.

(c) Calculate the heat available from water and calorimeter cooling to 0°C.

(d) Determine the final state of the mixture and calculate the final temperature.

(e) Verify your answer by checking heat lost equals heat gained.

Q15. Analyse the following practical situations:

(i) Pressure cookers cook food faster

(ii) Ice skating is possible due to regelation

(iii) Refrigerators work using latent heat

For each case, explain:

(a) The physics involved

(b) How latent heat plays a role

(c) Practical implications and design considerations

Q16. A 500 W electric heater is used to melt 2 kg of ice at 0°C.

(a) Calculate the time required to melt all the ice. (Lf = 3.36 × 10⁵ J/kg)

(b) If the heater continues for twice this time, calculate the final temperature of the water.

(c) Calculate the total time to convert all the water to steam at 100°C. (Lv = 2.26 × 10⁶ J/kg)

(d) Calculate the cost of electricity at ₹8 per kWh for the entire process (ice to steam).

(e) Discuss why the actual time in practice would be longer than calculated.

Q17. In a school experiment, students determine the latent heat of fusion of ice using the method of mixtures:

Mass of water in calorimeter = 200 g

Mass of calorimeter = 100 g (specific heat = 400 J/kg·K)

Initial temperature of water and calorimeter = 30°C

Mass of ice added = 40 g at 0°C

Final temperature = 10°C

(a) Calculate the latent heat of fusion of ice.

(b) Calculate the percentage error if the accepted value is 3.36 × 10⁵ J/kg.

(c) Why should the ice be dry and at 0°C?

(d) What would happen if too much ice were added?

(e) Suggest two improvements to reduce experimental error.

Q18. A metal ball of mass 100 g at 200°C is dropped into an ice cube of mass 50 g at 0°C.

(a) Calculate the maximum mass of ice that can melt. (Specific heat of metal = 500 J/kg·K)

(b) If the ball is made of copper (specific heat = 400 J/kg·K), recalculate the mass of ice melted.

(c) What is the final temperature if not all ice melts?

(d) Calculate the final temperature if the initial mass of ice were 20 g.

(e) Discuss why ice is used in cold storage rather than just cold water.