What is a compound?
A compound is a substance made of two or more different elements chemically bonded together in a fixed ratio. Water, table salt and carbon dioxide are common compounds.
Water is made of two hydrogen atoms bonded to one oxygen atom, written as H2O. Table salt, sodium chloride, contains sodium and chlorine in equal amounts. A compound has its own properties, often very different from its elements. Sodium is a reactive metal and chlorine is a poisonous gas, but together they form a safe, everyday seasoning.
Atoms in compounds are held together by chemical bonds. In ionic bonds, atoms transfer electrons, as in salt. In covalent bonds, atoms share electrons, as in water and sugar. Compounds can be separated into their elements only through chemical reactions, not by physical methods like filtering.
Compounds and mixtures
A mixture, such as salt water or air, contains substances that are not chemically bonded and can be separated physically. In a compound, the elements are always in the same proportion. Water from a river, an ocean or a glass always has the same H2O ratio.
Did You Know?
There are millions of known chemical compounds, and organic compounds, which contain carbon, make up the vast majority of them.