Latent Heat of Fusion and Vaporisation MCQs

Q1. A condenser removes 45200 J of heat as steam at 100 degC turns to water at 100 degC. Using the latent heat of vaporisation 2260 J/g, what mass of steam condensed?

• A.47460 g
• B.102152000 g
• C.0.05 g
• D.20 g

Answer: D. 20 g

From Q = m x L, mass m = Q / L. Here Q = 45200 J and the latent heat of vaporisation L = 2260 J/g, so m = 45200 / 2260 = 20 g.

Q2. Taking the latent heat of vaporisation as 2260 J/g, if a 1000 W immersion rod sends all its energy into boiling away 50 g of water already at 100 degC, how long does this take?

• A.0.0088 s
• B.113000000 s
• C.113 s
• D.0.05 s

Answer: C. 113 s

Heat needed Q = m L = 50 x 2260 = 113000 J. A heater of power 1000 W delivers 1000 J each second, so time = Q / P = 113000 / 1000 = 113 s.

Q3. Dew forms on grass overnight when water vapour in the air turns into tiny liquid drops. During this overnight change, is heat absorbed or released, and what is the process called?

• A.Heat is absorbed; the process is evaporation
• B.Heat is absorbed; the process is sublimation
• C.Heat is released; the process is fusion
• D.Heat is released; the process is condensation

Answer: D. Heat is released; the process is condensation

Water vapour changing into liquid droplets is condensation, the reverse of evaporation. As the gas particles come together into a liquid, the stored latent heat is given out, so condensation releases heat.