This definition of free energy is useful for gas-phase reactions or in physics when modeling the behavior of isolated systems kept at a constant volume. For example, if a researcher wanted to perform a combustion reaction in a bomb calorimeter, the volume is kept constant throughout the course of a reaction. Therefore, the heat of the reaction is a direct measure of the free energy change, "q" = Δ"U". In solution chemistry, on the other hand, most chemical reactions are kept at constant pressure. Under this condition, the heat "q" of the reaction is equal to the enthalpy change Δ"H" of the system. Under constant pressure and temperature, the free energy in a reaction is known as Gibbs free energy G.