A metal, or a base that is insoluble in water, is added a little at a time to the acid until all of the acid has reacted. The mixture is then filtered to remove the excess solid, leaving a solution of the salt. The solid salt is made when water is evaporated from the solution so that it crystallizes.
Ammonia solution is an alkali that does not contain a metal. It forms ammonium salts, such as ammonium nitrate, which are used as fertilisers.
Insoluble salts can be made by mixing solutions of soluble salts that contain the ions needed. E.g., we can make lead iodide by mixing solutions of lead nitrate and potassium iodide. The lead iodide forms a precipitate that can be filtered and dried.
Some pollutants can be removed from water as precipitates by adding ions that react with them to form insoluble salts.
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