On nonequilibrium liquid-gas coexistence

Condensation and evaporation are ubiquitous phenomena in nature. When we have a liquid partially filling a closed container, evaporation starts to happen, causing vapour formation in the remaining space. As the gaseous molecules bounce around, some of them hit the liquid surface and condense back into their original phase. This process evolves until there are as many molecules condensing as there are evaporating, that is, when the system reaches equilibrium. In this paper we introduce a very simple model addressed to undergraduate students and teachers that provides the temporal behaviour of this process. Using the results we propose an experimental technique to measure condensation coefficients.