Vapor pressure is defined as the pressure of a vapor in equilibrium with its non-vapor phases. The equilibrium exists where the phase boundary is at the same temperature because it’s not possible to obtain or maintain a system when different parts are of different temperatures. When there are two phases present, an imaginary line is drawn between them. Above this line, there’s only one phase and below it, there’s only the other phase. In a closed system, like for example a glass beaker, such a line does not exist.

What is vapor pressure? – Definition

The vapor pressure of water is the pressure of the water molecules that have evaporated from a liquid. The higher the temperature, the more molecules are in a given volume, and thus, the higher their vapor pressure.

Water has two forms: solid (ice) and liquid. When you heat ice up it turns into water (liquid). When you cool down a liquid like water it turns into ice (solid). This means that while we’re not heating or cooling our glass jar filled with “snow” aka dry ice (solid CO2), under normal circumstances we’re heating or cooling said glass jar filled with “water” aka H20 vapors (liquid).

Vapor pressure of water

Vapor pressure is the equilibrium partial pressure of a vapor in equilibrium with its non-vapor phases. In the case of a pure substance, such as water, the vapor pressure depends on temperature only and is determined by the interplay of various types of forces due to different types of molecules present in the liquid or solid phase and their interactions with each other and their surroundings.

Vapor pressure is an indicative measure of a substance’s volatility at a given temperature; it does not measure volatility per se, as this term refers to the tendency to escape from high-pressure regions into lower-pressure ones (such as down through capillaries), rather than “escape” from one phase into another. For example: If you have ever taken off your coat when entering an air-conditioned room despite still wearing long gloves underneath, this is because your body heat causes evaporation from your skin which increases its water content; increasing its latent heat capacity thus maintaining a higher level of the body temperature than would otherwise occur without this process occurring (this process continues until all sweat on our skin has evaporated).

Factors influencing vapor pressure

There are several factors influencing vapor pressure:

  • Temperature: The higher the temperature, the more likely molecules will be able to escape from a liquid and form a gas. In other words, as the temperature goes up, vapor pressure increases.
  • Pressure: As atmosphere pressure increases (or the weight of air above your sample), it becomes harder for molecules to escape from their liquid state. This results in lower vapor pressures at higher altitudes where there is less oxygen in comparison with sea level; seawater has an average density of 1 g/cm3 compared with 0 g/cm3 on land.


What is the normal water vapor pressure?

At normal body temperature, 37°C (98.6°F), this vapor pressure is 47 mm Hg.

What is the vapor pressure of water at 30 C?

The vapor pressure of water at 30 degrees Celsius is 31.82 mmHg and the density is 0.9956 g/mL.

How is vapor pressure measured?

A simple measurement involves injecting a little of the liquid into a closed flask connected to a manometer.