There are different ways to express activity coefficients, including in terms of concentration, mole fraction, or partial pressure. Here, we focus on activity coefficients expressed in terms of mole fraction (χ) henceforth, but it is worth remembering the connections to concentration and other forms. The reference ideal state also plays an important part in the definition of activity coefficients. Here, we define activity coefficients with reference to an ideal solution in the sense of Raoult's law, where, for each component in a mixture, γ i → 1 as χ i → 1. In this definition, the pure liquid is considered an ideal solution because all the interactions between its components have the same magnitude. With this reference state (called the Lewis-Randall standard state), the activity coefficient is 1 for the pure solution.