Humidity & Vapour Pressure — A/L Physics

Vapour Pressure The partial pressure exerted by water vapour molecules in a gas mixture. At any given temperature, water molecules evaporate from a liquid surface into the air above, creating a pressure contribution independent of other gases. This pressure increases with temperature because more molecules have sufficient kinetic energy to escape the liquid. For example, a sealed bottle of water at 30°C has a higher vapour pressure than one at 20°C.

Saturated Vapour Pressure (SVP) The maximum vapour pressure that can exist at a given temperature without condensation occurring. When the air above a liquid contains as many vapour molecules as it can hold at that temperature, the vapour is said to be saturated. Any further addition of vapour (or cooling) causes condensation. SVP depends only on temperature and the nature of the liquid—for water, it rises rapidly with temperature (e.g., SVP at 0°C ≈ 0.61 kPa, at 100°C ≈ 101.3 kPa).