Vapour pressure of water

The correct answer is option 'C', vapour pressure of water.

When a liquid is heated, its molecules gain kinetic energy and start moving faster. As a result, some of the molecules at the surface gain enough energy to overcome the attractive forces between them and escape into the gas phase. This process is called evaporation.

The pressure exerted by the vapour molecules above the liquid surface is known as vapour pressure. Vapour pressure is a measure of the tendency of molecules to escape from the liquid phase and enter the gas phase.

Explanation:

1. Evaporation and vapour pressure:
- Evaporation is a surface phenomenon that occurs at any temperature, but it increases with temperature.
- As the temperature of a liquid increases, the average kinetic energy of its molecules increases, leading to an increase in the number of molecules with enough energy to evaporate.
- This increases the rate of evaporation and consequently the vapour pressure of the liquid.

2. Boiling point and vapour pressure:
- Boiling is a phase change process in which a liquid changes into a gas throughout its volume.
- When the temperature of a liquid reaches its boiling point, the vapour pressure becomes equal to the atmospheric pressure.
- At this point, the liquid starts boiling, and bubbles of vapour form throughout the liquid.
- The boiling point of a substance is the temperature at which its vapour pressure equals the atmospheric pressure.

3. Freezing and vapour pressure:
- Freezing is a phase change process in which a liquid changes into a solid.
- As the temperature of a liquid decreases, the kinetic energy of its molecules decreases, and the rate of evaporation also decreases.
- At the freezing point, the vapour pressure of a liquid becomes zero because the molecules do not have enough energy to evaporate.
- The freezing point of a substance is the temperature at which its vapour pressure becomes zero.

4. Melting point and vapour pressure:
- Melting is a phase change process in which a solid changes into a liquid.
- The melting point of a substance is the temperature at which it changes from a solid to a liquid.
- The vapour pressure of a solid is generally negligible compared to that of a liquid, as the solid molecules are tightly packed and have limited mobility.

In summary, the pressure exerted by the water molecules at a given temperature remains constant during the process of transformation from liquid to vapour. This constant pressure is known as the vapour pressure of water.