Properties of sodium and potassium and their most important compounds
Sodium and potassium are soft, silvery, light metals that oxidise quickly in air, so they are stored under kerosene. They colour a flame — Na yellow, K violet — which is a way of detecting the ions. They are made industrially by electrolysing molten salts: 2NaCl → 2Na + Cl₂. Sodium burns in oxygen to give the peroxide: 2Na + O₂ → Na₂O₂ (4Na + O₂ → 2Na₂O, the oxide, with little oxygen); with chlorine, 2Na + Cl₂ → 2NaCl; with sulfur, 2Na + S → Na₂S; with hydrogen, 2Na + H₂ → 2NaH. With water the reaction is violent: 2Na + 2H₂O → 2NaOH + H₂↑ (even stronger for potassium, whose hydrogen ignites), so only the teacher performs this with a tiny piece behind a safety screen. NaOH, caustic soda, is a strong alkali that is very soluble in water and burns skin and eyes; industrially it is made by electrolysing NaCl solution: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂, and it is used for soap, paper and fibres. Na₂O₂ absorbs carbon dioxide and releases oxygen: 2Na₂O₂ + 2CO₂ → 2Na₂CO₃ + O₂ (in submarines). Important salts: NaCl, Na₂SO₄·10H₂O (Glauber's salt), NaNO₃, KCl, KNO₃ and K₂CO₃.
Chain game: teams write the equations for the chains Na → NaOH → Na₂SO₄ → NaCl and K → KOH → K₂CO₃ → KNO₃. No experiments: only the teacher works with sodium, potassium and alkalis (goggles, gloves, safety screen).