Question

The latent heat of vaporization of water is more than latent heat of fusion of ice, because:
A. On vaporization much larger increase in volume takes place
B. Increase in Kinetic energy is much larger on boiling
C. Kinetic energy decreases on boiling
D. Volume decreases when the ice melts

Answer 1

Hint: The gain or loss of heat in changing the state of the system without change in temperature is called latent heat. Gas molecules do not have any forces of attraction between their molecules. On the other hand, liquid and gas molecules have molecular forces of attraction.

Complete answer:
Heat required to change the state of one gram/ kilogram of a substance at constant temperature is called latent heat. Latent heat is of two types:
1. Latent heat of fusion for change from solid to liquid at the melting point.
2. Latent heat of vaporization for change of state from liquid to gaseous state at boiling point.
Latent heat is used for doing work in increasing the distance between molecules during the change of state. This means that the potential energy of the molecules will increase. Latent heat of fusion of ice is the change of state of ice to water. Ice is a solid structure and its molecules are closely packed to each other. The molecules of water are not that closely packed to each other. Molecules of water have lesser forces of attraction as compared to molecules of solid. There is not much potential energy required for the latent heat of fusion.
The Latent heat of vaporization is the conversion of water to gas. The molecules of liquid must be provided with a great amount of energy to be converted into gas molecules. This is because when the molecules change into gaseous state they leave the system and they don’t have any forces of attraction with other molecules. Therefore, the latent heat of vaporization of water is more than latent heat of fusion of ice.The latent heat of fusion of ice is 80 cal/g and that of vaporization of water is 536 cal/g.

Hence, option A is the correct answer.

Note:In Latent heat of fusion, when ice is converted to water, the molecules of water will remain in the same system and will also have molecular forces of attraction (these forces will be less in water as compared to ice). But in Latent heat of vaporization, when water gets converted into vapor, the molecules completely leave the system and therefore there are no molecular forces of attraction.