Typical precursors are metal alkoxides and metal chlorides, which undergo hydrolysis

(Hydrolysis is a chemical reaction or process in which a chemical compound is broken down by reaction with water) and polycondensation reactions.

(A chemical reaction in which two or more molecules combine upon the separation of water or some other simple substance) to form a colloid, a system composed of solid particles (size ranging from 1 nm to 1 μm) dispersed in a solvent.