- Air is made of nitrogen, oxygen, carbon dioxide, water vapor, and argon
- If two substances together form a mixture then those substances can be separated from one another by physical changes
- Hence, air is a mixture

So, air is a mixture
Which of the following substances can be separated into several elements?
- Air is made of nitrogen, oxygen, carbon dioxide, water vapor, and argon 
- If two substances together form a mixture then those substances can be separated from one another by physical changes