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Vapor pressure is a fundamental concept when understanding the boiling point of a liquid. It is the pressure exerted by a vapor in equilibrium with its liquid or solid form. At any temperature, a liquid will have a certain number of molecules escaping into the vapor phase, creating a pressure collectively.
The higher the temperature, the more molecules have the energy to escape, leading to higher vapor pressure. This pressure is crucial because it tells us at which point the liquid will boil—when its vapor pressure equals the external pressure.
In simple terms, vapor pressure increases with temperature and dictates the conditions required for boiling.

External pressure is essentially the pressure exerted on a liquid by its surroundings, most commonly due to the atmosphere. This atmospheric pressure varies depending on altitude and weather conditions.
When we discuss boiling points, we regard how this external pressure influences when a liquid boils. A higher external pressure means that the vapor pressure has to be higher for the liquid to reach its boiling point. In contrast, lower external pressure, like those at higher elevations, allows the liquid to boil at a lower temperature.

• High external pressure = Higher boiling point
• Low external pressure = Lower boiling point

Therefore, understanding external pressure helps us comprehend how the environment affects boiling.

The relationship between pressure and boiling point is crucial in many scientific and everyday contexts. Essentially, the boiling point of a liquid changes with changes in external pressure.
When external pressure increases, more energy is needed to raise the liquid's vapor pressure to equal the external pressure. This means the boiling point is higher because the liquid needs to reach a higher temperature to boil.
Conversely, reducing the external pressure results in a lower boiling point because the liquid can reach its vapor pressure sooner.

• Higher external pressure = Needs more heat = Higher boiling point
• Lower external pressure = Needs less heat = Lower boiling point

Thus, the effect of pressure on boiling is a straightforward yet powerful concept that explains why water boils at different temperatures based on the environment.