Evaporation of water in an open vessel is a process which takes place by itself, by absorption of heat from the surroundings, because the gaseous water molecules are more random than the liquid water molecules. H2O(I)→H2O(g);ΔH=+40.8kJmol−1 In other words, the process is spontaneous because it is accompanied by increase of entropy, which is further a measure of randomness or disorder of the system.