When atmospheric pressure is decrease, the boiling point of water will also decrease because boiling point is the temperature at which the vapour pressure of a liquid becomes equal to that of the atmosphere. If the atmospheric pressure decreases, then the vapour pressure of the liquid will also have to be decreased, for which the liquid has to be cooled down. Hence the boiling point of a liquid decreases.

Boiling point is the temperature at which a liquid changes its state from liquid to gas. The boiling point of a liquid depends on the pressure exerted on the surface of the liquid. When the pressure is reduced, the boiling point of the liquid decreases. This can be explained by the following factors:

1. Effect of pressure on the boiling point: The boiling point of a liquid is the temperature at which the vapor pressure of the liquid becomes equal to the atmospheric pressure. When the pressure is reduced, the vapor pressure required to reach the atmospheric pressure is also reduced. Thus, the boiling point of the liquid decreases.

2. Effect of intermolecular forces: Intermolecular forces are the forces of attraction between the molecules of a liquid. These forces are responsible for holding the molecules together in a liquid state. When the pressure is reduced, the intermolecular forces become weaker. This results in the easy escape of molecules from the surface of the liquid, leading to a decrease in the boiling point.

3. Effect of molecular weight: The molecular weight of a liquid also affects its boiling point. When the pressure is reduced, the lighter molecules of the liquid escape easily from the surface, while the heavier molecules require more energy to escape. This results in a decrease in the boiling point of the liquid.

In conclusion, as the pressure of air decreases, the boiling point of the liquid decreases due to the reduction in vapor pressure required to reach the atmospheric pressure, weakening of intermolecular forces, and escape of lighter molecules.

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