A precipitate forms . If the amount of precipitate is very small, you can sometimes detect fine particles of precipitate suspended in the solution by shining a very bright light through the liquid. If you can see the beam of light in the solution, fine suspended particles are present. For reactions that occur in a melt, don't confuse precipitation with simple freezing. A color change occurs .
Every compound absorbs a characteristic set of colors of light.
This absorption spectrum is a chemical fingerprint for detecting the presence of that compound. When the compound is altered in a chemical reaction, the fingerprint will change- and so the color of the reacting mixture may change, as the reaction progresses. It is possible to have a color change without a chemical change, however, because a compound's absorption spectrum isn't the only thing that affects its color. For example, heating zinc oxide changes it from white to yellow but no real chemical change occurs. The color change is caused by holes and other defects that are created in the zinc oxide lattice as the compound is heated.