The
enthalpy of mixing (also called
heat of mixing) is the
heat that is taken up or released upon mixing of two (non-reacting) chemical substances. When the enthalpy of mixing is positive, mixing is
endothermic while negative enthalpy of mixing signifies
exothermic mixing. In
ideal mixtures the enthalpy of mixing is null. In non-ideal mixtures the
thermodynamic activity of each component is different from its concentration by multiplying with the
activity coefficient.