Evaporation and condensation
The escape of molecules from a liquid surface is evaporation, and the change of vapour into liquid is condensation. In evaporation the fastest molecules leave, so the liquid cools; evaporation is faster when the temperature is higher, the surface larger and there is wind. In a closed vessel the rates of evaporation and condensation become equal, a dynamic equilibrium is set up, and the vapour is called saturated; unsaturated vapour has a pressure smaller than the saturated one. Boiling is the formation of vapour bubbles throughout the whole volume of the liquid; it begins when the saturated vapour pressure equals the external pressure, so a rise in pressure raises the boiling point (pressure cooker) and in mountains it is lower (at 5 km altitude water boils at about 84 °C). The heat needed to turn 1 kg of liquid at its boiling point into vapour is the specific heat of vaporisation r; Q = rm, and for water r = 2.3·10⁶ J/kg.
Observation: pour equal amounts of water into a wide saucer and a narrow glass and mark the water level over two days. In which did the water decrease more? Work with boiling water and steam only with an adult: steam burns; never open a pressure cooker lid yourself.