Air is a:

**The reasoning** Air is a **mixture** because it contains multiple substances (nitrogen ~78%, oxygen ~21%, carbon dioxide, water vapor, argon, etc.) that are *physically combined* but not chemically bonded together. The key principle: **Mixtures** have components that keep their individual properties and can be separated by physical means (like fractional distillation). Each gas in air behaves independently — the oxygen doesn't chemically react with the nitrogen just sitting there. Compare this to a **compound** like water (H₂O), where hydrogen and oxygen are *chemically bonded* in fixed ratios and you can't just filter them apart. **Why the wrong options tempt you** - **Element (B)**: You might think "air is one thing" — but elements are pure substances made of one type of atom (like gold or oxygen alone). - **Compound (C)**: Tricky! Air *contains* compounds (CO₂, H₂O vapor), but air itself isn't bonded in fixed ratios. - **Pure substance (D)**: Air's composition varies by location (humid vs. dry air, polluted vs. clean) — that variable composition screams "mixture." **Quick takeaway** If you can separate it without breaking chemical bonds, it's a mixture — air is just gases chilling together, not married by chemistry.