“Co-precipitation is a process in which normally soluble compounds are carried out of solution by a precipitate.” There are four types of co-precipitation as surface adsorption, mixed- crystal formation, occlusion and mechanical entrapment. Surface adsorption takes place for precipitates with larger surface areas. Specially coagulated colloids contaminate by this method. In mixed- crystal formation, one of the ions in the crystal lattice is replaced by another ion. Surface adsorption and mixed- crystal formation are equilibrium processes, whereas the other two are kinetic phenomena. When a crystal is growing rapidly, contaminant can trap inside the growing crystal and this is known as occlusion. Mechanical entrapment is the mechanism where some amount of solution is trapped inside the crystals. This happens when two growing crystals are close together, so that they grow together.