Enthalpy (/ ˈ ɛ n θ əl p i / ) is the sum of a thermodynamic system 's internal energy and the product of its pressure and volume. It is a state function in thermodynamics used in many measurements in chemical, biological, and physical systems at a constant external pressure, which is conveniently provided by the large ambient atmosphere. The pressure–volume term expresses the work W that was done against constant external pressure P e x t to establish the system's physical dimensions from V s y s t e m , i n i t i a l = 0 to some final volume V s y s t e m , f i n a l (as W = P e x t Δ Δ V ), i.e. to make room for it by displacing its surroundings. The pressure-volume term is very small for solids and liquids at common conditions, and fairly small for gases. Therefore, enthalpy is a stand-in for energy in chemical systems; bond, lattice, solvation, and other chemical "energies" are actually enthalpy differences. As a state function, enthalpy depends only on the final configuration of internal energy, pressure, and volume, not on the path taken to achieve it.