When two liquids are mixed together, their constituents may react chemically, forming new products whose chemical properties are different from those of the original constituents of the mixtures.
Then, some of the products formed may be insoluble in the mix. In this case is when the precipitate (solid that cannot stand in the solution) will get formed.
The formation of a precipitate is a clear signal that a chemical reaction has taken place between some constituents (elements or compounds) of the mix.
One example of this situation is the combination of a solution of baking soda, NaHCO₃(aq), and a solution of calcium chloride, CaCl₂ (aq).
Those two solutions are clear, and when combined will form a solid (precipitate) which is calcium carbonate, CaCO₃ (s). The calcium carbonate precipitates because it is not soluble in the mix.
