The sol-gel transformation of silica in aqueous solution was studied by viscosity measurements. The first two steps of this process are nucleation and growth of particles. These early stages can be described theoretically by the theory of nucleation and condensation in a closed isochoric system. For this purpose, the Helmholtz free energy surface of sol formation was calculated as a function of particle size and sol particle concentration. The sol formation proceeds along the bottom of the valley of this free energy surface. A rate equation for the process can be derived. Good agreement between experimental results and the theoretical model was generally found. Experiments were carried out at different temperatures and pH values. The activation energy of the condensation of silica was determined to be 4.7 kJ/mol and the order of the reaction of the hydroxyl ion concentration was found to be 0.7.