A carbonate is a salt in chemistry: a substance made when an acid meets a base, and this one has carbon and oxygen joined in a set way. The map is tidy once you hear it: carbonic acid gives carbonates, the way sulfuric acid gives sulfates — that -ate ending is the ‘salt of that -ic acid’ ending. You meet carbonates in very ordinary things, though people rarely use the chemistry name for them. Limestone, chalk, marble, and eggshells are mostly calcium carbonate. Washing soda is sodium carbonate. Baking soda is a close relative (a bicarbonate). In a lab or on a mineral-water label you will see the word itself; in daily talk someone will just say chalk or limestone. Though actually, carbonate also works as a verb: to carbonate a drink is to push carbon dioxide into it so it fizzes. Same carbon-and-oxygen idea, different job. The noun is the salt; the verb is the ‘add the gas’ action. Almost nobody says carbonate about a rock in the street — that use lives in science class, geology, and ingredient lists.