A mixture of substances B, C, ... is called an ideal mixture when \(a_{\rm{B}}=x_{\rm{B}}\), \(a_{\rm{C}}=x_{\rm{C}}\), ... or \(f_{\rm{B}}=1\), \(f_{\rm{C}}=1\) where \(a_{\rm{B}}\) and \(f_{\rm{B}}\) are the relative activity and the activity coefficient, respectively, of a substance B in a liquid or solid mixture.