The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the pressure of the surrounding atmosphere. When the pressure of the atmosphere is increased, the boiling point of a substance will also increase.

This phenomenon can be explained by the relationship between the vapor pressure of a liquid and its boiling point. Vapor pressure is the pressure exerted by the vapor of a liquid when it is in equilibrium with its liquid phase. As the temperature of a liquid increases, the vapor pressure of the liquid also increases.

When the vapor pressure of a liquid equals the pressure of the surrounding atmosphere, the liquid will boil. If the pressure of the atmosphere is increased, the vapor pressure of the liquid must also increase in order to reach equilibrium. This means that the temperature of the liquid must also increase in order for the liquid to boil.

Therefore, the boiling point of a substance will increase with pressure because the vapor pressure of the liquid must also increase in order to reach equilibrium with the increased pressure of the surrounding atmosphere.

This relationship between pressure and boiling point is known as the Clausius-Clapeyron equation. The Clausius-Clapeyron equation states that the change in the vapor pressure of a substance is directly proportional to the change in its boiling point. This means that as the pressure of a substance increases, the boiling point will also increase.