Question

Which of the following processes requires the most energy input ?
Given below is the properties of the water.

Property	Value
Specific Heat	1.0 cal/g.${}^o$C
Latent Heat of Fusion	80 cal/g
Latent Heat of Vaporization	540 cal/g

• A. Melting 100 g of ice at 0${}^o$C
• B. Freezing 100 g of water at 0${}^o$C
• C. Boiling 100 g of water at 100${}^o$C
• D. Condensing 100 g of steam at 100${}^o$C
• E. Heating 100 g of water from 0C to 100${}^o$C

Answer 1

Answer: (C)

Boiling 100 g of water at 100${}^o$C

For melting of ice : Heat required $Q=m{L}_f=100\times 80=8000$ cal

For freezing of water : Heat released $Q=m{L}_f=100\times 80=8000$ cal

For boiling of water : Heat required $Q=m{L}_v=100\times 540=54000$ cal

For condensing the steam : Heat released $Q=m{L}_v=100\times 540=54000$ cal

For heating of water from $0^{o}C$ to ${100}^{o}C$ :

Heat required $Q=mS(T_2-T_1)=100\times 1\times (100-0)=10000$ cal

Thus heat input is the most for boiling 100 g water at ${100}^{o}C$