Air & water

Dry air is about 78% nitrogen, 21% oxygen, with around 1% argon and 0.04% carbon dioxide. The gases can be separated by fractional distillation of liquefied air.

Greenhouse gases such as ${\text{CO}}_2$ (from combustion) and ${\text{CH}}_4$ (from agriculture and decay) trap heat and contribute to climate change. Pollutants include $\text{CO}$ (toxic, from incomplete combustion), ${\text{SO}}_2$ and ${\text{NO}}_x$ (which cause acid rain and respiratory problems).

Chlorine is added to drinking water to kill microorganisms, and fluoride is sometimes added to help prevent tooth decay.

• Quoting the wrong proportions (air is roughly 78% N₂ and 21% O₂).
• Confusing the roles of chlorine (disinfection) and fluoride (teeth).