A colloid solution precipitates by:

Coagulation is the process by which a colloid precipitates out of a solution. The precipitation is brought about by induced aggregation. For e.g., an iron (III) hydroxide sol can be made to aggregate by addition of an ionic solution. A positively charged particle of iron (III) hydroxide gathers a layer of anions around it. The thickness of this layer is determined by the charge on the anions. The greater the magnitude of the negative charge, the more compact the layer of charge.