Silica sol silica particles form the solid skeleton of the gel, and water is filled in it as a dispersion medium. There are four forms of water in the gel: (1) Water directly adsorbed on silica particles by hydrogen bonding, in the form of hydroxyl groups; (2) Polarized water layer adsorbed around the particles by hydrophilic groups, in the form of hydrated hydroxyl groups; (3) Water wrapped in the micropores of silica colloidal network structure, in the form of free water, but its movement is hindered, similar to "dead water"; (4) Water in the gaps of the gel, in the form of free water. Among them, (1) and (2) account for a very small proportion of the total water in the gel; (3) has a considerable amount of water, but its movement (evaporation) is limited; (4) has the largest water content, which exists in the capillaries of the gel and can move freely.