Water
Chemical tests show water is present: anhydrous copper(II) sulfate turns from white to blue; anhydrous cobalt(II) chloride from blue to pink. The physical test for pure water: it boils at exactly and melts at . Natural water contains useful substances (oxygen, minerals) and pollutants (sewage, nitrates, phosphates, metal compounds, plastics). Tap water is treated by sedimentation, filtration, chlorination (and sometimes carbon to remove tastes).
≈ 30 minFertilisers
Plants need nitrogen (N) for proteins and growth, phosphorus (P) for roots, and potassium (K) for flowers and fruit. NPK fertilisers supply these as soluble salts such as ammonium nitrate, ammonium phosphate and potassium chloride. Ammonia from the Haber process is neutralised by acids to make them. Fertilisers boost crop yields, but run-off causes eutrophication.
≈ 25 minAir quality and climate
Clean dry air is about 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide. Pollutants include carbon monoxide (incomplete combustion; toxic), sulfur dioxide (burning sulfur in fossil fuels; acid rain), oxides of nitrogen (car engines; acid rain, smog), particulates (lung damage). Carbon dioxide and methane are greenhouse gases: they absorb infrared radiation, causing global warming and climate change. Catalytic converters, flue-gas desulfurisation and renewable energy reduce emissions.
≈ 40 min