One of the first applications of refrigeration technology was in that most fundamental of human activities: eating food. Thermodynamics became a serious branch of physics when scientists and engineers needed to understand the behaviour of steam engines. Thermodynamics deals with the mutual transformations between heat and work. The first law of thermodynamics expresses the conservation of energy for heat and thermodynamic processes. The second law of thermodynamics defines the direction of heat transfer from hot bodies to cold bodies and determines the maximum efficiency of heat engines through the Carnot theorem. The third law of thermodynamics also referred to as Nernst-Simon theorem. Vapour pressure is the pressure exerted by a vapour in thermodynamic equilibrium with its condensed phase in a closed system at a temperature T. The latent heat of vapourisation of a substance is the quantity of heat required to convert a given amount of that substance from a liquid to vapour phase at constant temperature.