Ideal Solutions or Mixtures

From a molecular perspective, an ideal solution is one in which the intermolecular interactions between unlike molecules are, on average, the same as those between like molecules. This is the case for ideal gas mixtures, where the molecules are far apart and do not interact with each other. However, for condensed phases like liquids or solids, the molecules are close together and interact with each other. In an ideal solution, the molecules of different species are so similar to each other that replacing molecules of one species with molecules of another species does not change the spatial structure or the intermolecular interaction energy in the solution.

The thermodynamic behavior of ideal solutions can be described using the equation ΔG_mix = RT(n_B In x_B + n_C In x_C), where ΔG_mix is the change in Gibbs free energy of the solution, R is the gas constant, T is the temperature, n_B and n_C are the number of moles of components B and C in the solution, and x_B and x_C are the mole fractions of B and C in the solution.

However, the key thermodynamic properties are the chemical potentials (μ) in the solution, so the ΔG_mix equation can be modified as μ_i = μ_i* (T, P) + RT ln x_i, where μ_i* is the chemical potential of a pure substance i at the same temperature T and pressure p of the solution.

Thermodynamically, an ideal solution is defined as a solution where the chemical potential of every component obeys the above equation for all solution compositions and a range of temperature and pressure.