When solutions of two ionic compounds are combined and a solid forms, the process is called precipitation.
A precipitate is a solid that forms out of solution. A common example is that of the mixing of two clear solutions: (1) silver nitrate (AgNO₃) and (2) sodium chloride (NaCl).
The reaction is: AgNO₃ + NaCl --------------> AgCl - NaN0₃
The precipitate forms because the solid (AgCl) is insoluble in water. That is true for all precipitates - the solids are insoluble in aqueous solutions.
To learn more, see links below:
wps.prenhall.com - Precipitation Reactions
www.chemicool.com - Definition of Precipitate
www.iun.edu - Precipitation Reactions.
To learn about predicting precipitation reactions, refer to this Boundless open textbook.