Testing and purifying water
Two chemical tests show that water is present: anhydrous copper(II) sulfate turns from white to blue, and blue cobalt(II) chloride paper turns pink. To show water is pure, check its physical constants: pure water boils at exactly 100 °C and freezes at 0 °C (at normal pressure).
Key idea
Water treatment: filtration removes insoluble solids, sedimentation lets particles settle, and chlorination kills harmful bacteria.
Fertilisers
Plants need nitrogen (N), phosphorus (P) and potassium (K). NPK fertilisers supply all three. Nitrogen helps make proteins for healthy leaf growth. Ammonia is reacted with acids to make soluble fertiliser salts, e.g. ammonium nitrate (from nitric acid) and ammonium sulfate (from sulfuric acid).
Worked example
Find the % by mass of nitrogen in ammonium nitrate, NH4NO3 (Ar: N=14, H=1, O=16). Mr = 14 + 4 + 14 + 48 = 80. Mass of N = 2 × 14 = 28. %N = (28 ÷ 80) × 100 = 35%.
Remember
- White copper(II) sulfate + water → blue (test for water).
- Pure water: boils 100 °C, freezes 0 °C.
- Too much fertiliser washed into rivers causes eutrophication: algae grow, then oxygen falls and fish die.