What is vapor pressure?
Water always evaporates no matter what the temperature or atmospheric pressure is. Even in the coldest of winter, snow and ice can evaporate by a process called sublimation. They can turn straight into vapor (the gaseous stage) without melting.
The vapor that escapes into the air whether by evaporation or sublimation have sufficient energy to break away from the molecular bond of the water. The vapor molecules are in constant motion exerting a force on the container or the air around. This force is termed vapor pressure.
Vapor pressure is the force the gas exerts in a closed container or space.
Vapor pressure is also defined as the equilibrium pressure where the pressure exerting downwards is equal to the pressure exerting upwards between the gas and liquid phase.
In order for a liquid to boil the vapor pressure of the liquid must equal that of the atmospheric pressure