FAQ's - What is the physics of state change in water ?

In water like all liquids, when moving from one state to another, the energy levels change in order to jump from phase to another. In the case for this paper the change from water as a liquid to a gas is the most pertinent. The molecules within the liquid water have an average kinetic energy and this will dictate the temperature of the fluid.  Those molecules that have a high kinetic value have the potential of escaping the liquid and therefore escape as a vapour. This reduces the average kinetic value of the remaining molecules and hence reducing the temperature.

The effects of evaporation on a wall can be considerable due to the high specific heat capacity of water. In order to achieve the break of the molecular bonding by vibration, for every gram of water evaporated, 2500J is extracted (E Grinzato, 1999) (Nave, 2005)- which is the heat of vaporisation for water.  This has a considerable effect on the substrate. Fick’s Law shows the flux of evaporating water (Ф). The law states that the movement of energy from a high concentration to an area of low concentration is proportional to the function of diffusion coefficient and distance in a single dimension

Ф = -D ΔC


(Ljungberg, 1994)

D is the diffusion coefficient

C is the Molar concentration of water

z  is the area that the migration takes place


This evaporative effect with its high heat of vapourisation of water is a major driver for the identification of moisture in buildings.