Multiple choice

Which one of the following substances is made up of only one type of atoms?

  1. Water

  2. Milk

  3. Hydrogen

  4. Air

Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Hydrogen gas (H2) consists solely of hydrogen atoms bonded together in pairs. The other options are all mixtures or compounds: water (H2O) contains hydrogen and oxygen, milk is a complex mixture of compounds, and air is a mixture of nitrogen, oxygen, and other gases.