Which of the following is a pure substance?

• A. Water
• B. Air
• C. Rocks
• D. Oranges

Answer: (A)

Pure substances have fixed composition, meaning every sample contains the same types of particles in the same proportions and show uniform properties throughout. Water is a compound with the formula H2O, so a sample of pure water is made entirely of water molecules in a constant proportion. That definite, single composition is what defines it as a pure substance.

Air is a mixture of several gases (mostly nitrogen and oxygen) with variable proportions and the ability to be separated into components, so it’s not a pure substance. Rocks are mixtures of minerals, and oranges contain many substances such as water, sugars, acids, and fibers, also not a single pure substance.