Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water. The concentrated acid (about 37 %, density 1.19 g/cm³) is a colourless, sharp-smelling liquid that fumes in moist air. It shows the general properties of acids: it turns litmus red, reacts with metals before hydrogen (Zn, Fe, Mg) giving a salt and hydrogen (Zn + 2HCl → ZnCl₂ + H₂), reacts with basic oxides (CaO + 2HCl → CaCl₂ + H₂O) and bases giving a salt and water, and releases carbon dioxide from carbonates (CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂). The qualitative test for the chloride ion is silver nitrate, which gives a white curdy precipitate of AgCl: Ag⁺ + Cl⁻ → AgCl↓. The precipitate dissolves neither in water nor in nitric acid. Most chlorides dissolve well in water (AgCl and PbCl₂ hardly do). Table salt NaCl is used in food, food preservation and the chemical industry, and KCl is a potassium fertilizer. Hydrochloric acid is always handled by the teacher or an adult, with goggles and gloves.
“Reaction cards”: on 6 cards write HCl with Zn, CaO, NaOH, CaCO₃, Cu and AgNO₃. Predict which react and where a gas or precipitate forms (copper stands after hydrogen — no reaction). On paper only; they are tested in class by the teacher.