A nitrate is a salt of nitric acid. Nitrate salts contain the nitrate ion (NO3-). Some nitrates, notably saltpetre (potassium nitrate, KNO3) and Chile saltpetre (sodium nitrate, NaNO3) are important naturally occurring chemicals; others are made by the action of nitric acid on the metal or its salts. Nitrates are used in medicine, as food preservers, fertilizers, and explosives, and as a source of nitric acid. Esters of nitric acid (RONO2) are also called nitrates.