Water is important for life, weather, and many processes. While it’s elemental for our existence, is water an element? Scientists classify water as a compound and not an element. Here’s how it works.
Why Water Is not an Element
An element is a substance composed of a single type of atom. Hydrogen (element symbol H) and oxygen (element symbol O) are examples of elements. You may have heard water called “H2O.” The name refers to the chemical formula of water, H2O. So, water is made from elements, but is not itself an element.
Before people knew about atoms and molecules, water was considered to be an element. Other “elements” included fire, earth, air, and sometimes metal, wood, or spirit. Some traditional practices still consider water to be an element, but this is different from the scientific definition.
Water as a Molecule and Compound
When two atoms form a chemical bond, they make a molecule. Hydrogen and oxygen are both elements, but they also normally exist as molecules consisting of two atoms (H2 and O2). Because hydrogen forms bonds with oxygen to make water, water is a molecule.
Water is also a compound. Sometimes people use the term “molecule” and “compound” interchangeably, but chemists distinguish between them. A molecule is made whenever two or more atoms form chemical bonds, while a compound occurs whenever two or more different atoms form chemical bonds. Under this narrow definition, oxygen and hydrogen are molecules but not compounds. Water is both a molecule and a compound.
Why Water Is not a Mixture
Another common question is whether water is a mixture. The thinking is that since water consists of a combination of hydrogen and oxygen atoms, it must be a mixture. However, scientists define a mixture to be a material produced when two substances are physically combined. If you release gas from hydrogen cylinder and oxygen cylinders into a space, you get a mixture of hydrogen and oxygen molecules. You don’t get water because the result is just a physical combination of hydrogen and oxygen. If you add enough energy to the mixture of hydrogen and oxygen, then a chemical reaction occurs (a chemical combination) that makes water. Water is a pure substance and not a mixture.
- IUPAC (1997). Compendium of Chemical Terminology (2nd ed.) (the “Gold Book”). Oxford: Blackwell Scientific Publications. ISBN 0-9678550-9-8. doi:10.1351/goldbook
- Whitten K.W., Gailey K. D. Davis R. E. (1992). General Chemistry (4th ed.). Philadelphia: Saunders College Publishing. ISBN 978-0-03-072373-5.