precipitation
/ prih-sip-ih-TAY-shun /
Think of stirring too much sugar into iced tea: at some point it stops dissolving and settles as solid at the bottom. Precipitation in chemistry is the same surprise, but engineered on purpose — you mix two clear solutions and a cloudy solid suddenly appears.
Precipitation is the formation of an insoluble solid, called the precipitate, out of a solution. It happens when ions that were happily dissolved meet a partner with which they form a compound too poorly soluble to stay in the water, so it drops out of solution.
In gravimetric analysis this is the key step: the analyte is deliberately precipitated so it can be separated and weighed. Whether the result is good depends on getting large, pure crystals rather than a fine, dirty sludge, which is why so much technique surrounds this single moment.
Adding clear silver nitrate solution to clear sodium chloride solution instantly produces a milky white cloud of solid silver chloride — a textbook precipitation.
Two clear solutions meet and an insoluble solid drops out.
A good gravimetric precipitate must be both very insoluble (so almost none stays dissolved) and easy to filter (large crystals). These two goals can pull in opposite directions, so conditions are chosen carefully.