What is the chemical formula for water?

• A. CO2
• B. H2O
• C. NaCl
• D. H2SO4

Answer: (B)

Understanding what a chemical formula expresses about a substance’s composition. Water is made up of two hydrogen atoms bonded to one oxygen atom, so its formula is H2O. The subscript 2 tells you there are two hydrogen atoms for every one oxygen atom in each molecule. This simple ratio captures the molecule’s makeup and leads to water’s distinctive properties, such as polarity and the ability to form hydrogen bonds, which explain why water is such a versatile solvent. Other common substances have different formulas—for example, a molecule with one carbon and two oxygens is carbon dioxide, a compound with sodium and chlorine is table salt, and a molecule with sulfur, oxygen, and four hydrogens is sulfuric acid.