Question

give reasons for the following 1.evaporation takes place only at the surface 2.when a gas is cooled to a certain point , it condenses 3.the melting point and freezing point of a substance are the same

Answer 1

Dear Student
1.
During evaporation the molecules in the liquid state escape into vapour phase, which requires total cut off the molecule from the attractive forces of other molecules in the liquid phase. This is possible only for the molecules present at the surface thus evaporation occurs only at the surface of the liquid.

2. When a gas is cooled to a certain point, the energy of gas decreases and hence the forces of attraction between molecules prevent molecules from moving apart, and the gas condenses to either a liquid or a solid.

3.
Freezing occurs at the same temperature as melting, hence, the melting point and freezing point of a substance are the same temperature. The melting/freezing point of a substance is defined as the temperature above which, the substance is liquid and below which, it is solid.


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