What is the chemical formula for water?

• A. HO2
• B. H2O2
• C. H2O
• D. OH2

Answer: (C)

Chemical formulas show how many atoms of each element are in a molecule. Water is made up of two hydrogen atoms for every one oxygen atom, so its formula reflects that 2:1 ratio. Writing this as H2O places two hydrogens next to one oxygen, which is the standard way to denote the composition of water. Other formulas don’t match this composition: for example, a formula with two oxygens and one hydrogen isn’t water, and a variant that rearranges the symbols in an unconventional way isn’t the accepted notation for this molecule. So the proper representation of water is H2O.