The idea of free energy has been used in the discussion of physical conditions for the formation of colloidal particles. The formulas for critical concentration of impurity atoms and donor atoms and for critical values of crystal nuclei are deduced and discussed under the assumption that all the crystal nuclei have the same size. It is pointed out that there is a critical temperature for the transformation of colloidal particles into impurities of semiconductor or F-centres of alkali halides. The characteristics of surfaces of free energy and the sufficient condition of the formation of colloidal particles are also discussed in this paper.