Nitric acid
In nitric acid HNO₃ nitrogen is tetravalent with oxidation state +5. The pure acid is a colourless, sharp-smelling liquid that fumes in air (density 1.5 g/cm³). It is unstable: in light and heat 4HNO₃ → 4NO₂ + O₂ + 2H₂O, so it turns brown and is stored in dark bottles. In industry it is made in three steps: 4NH₃ + 5O₂ → 4NO + 6H₂O (over a catalyst), 2NO + O₂ → 2NO₂, 4NO₂ + 2H₂O + O₂ → 4HNO₃. As a strong monobasic acid it reacts with oxides, bases and carbonates (CuO + 2HNO₃ → Cu(NO₃)₂ + H₂O). With metals hydrogen is usually not released, because the oxidizing nitrogen of NO₃⁻ is reduced: with concentrated HNO₃ copper gives NO₂, with dilute acid NO, with very dilute acid active metals give NH₄NO₃. In the cold concentrated acid passivates Fe, Al and Cr. A mixture of 1 volume HNO₃ and 3 volumes HCl, aqua regia, dissolves gold and platinum. Nitrates dissolve well in water and decompose on heating: 2KNO₃ → 2KNO₂ + O₂; 2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂; 2AgNO₃ → 2Ag + 2NO₂ + O₂. Nitrates are used mainly as fertilizers, and also for explosives, dyes and medicines. Concentrated HNO₃ stains the skin yellow and burns it: only the teacher works with it, in goggles, gloves and a fume cupboard.
“Nitrate pathway”: draw the chain from ammonia to nitrate: NH₃ → NO → NO₂ → HNO₃ → Ca(NO₃)₂ → fertilizer. Write the equation over each arrow and show oxidation states. No acid or metal is used; the reactions are on paper only.