Sulfuric acid
Sulfuric acid H₂SO₄ (Mᵣ = 98) is a colourless, odourless, oily, heavy liquid; 96 % acid has density 1.84 g/cm³. When it dissolves in water a great deal of heat is released, so the acid is always poured into water, little by little, with stirring, never the reverse. Dilute acid shows the general properties: it reacts with metals before hydrogen (Zn + H₂SO₄ → ZnSO₄ + H₂), oxides, bases and carbonates. Concentrated acid is a strong oxidizing agent: Cu + 2H₂SO₄ → CuSO₄ + SO₂ + 2H₂O (on heating); it does not attack gold and platinum and passivates iron in the cold. It also takes up water, charring sugar, paper and wood, and burns the skin. The reagent for the sulfate ion SO₄²⁻ is barium chloride: Ba²⁺ + SO₄²⁻ → BaSO₄↓ (a white precipitate insoluble in acid). Important sulfates: gypsum CaSO₄·2H₂O, which on heating gives alabaster CaSO₄·½H₂O (building, medicine), copper vitriol CuSO₄·5H₂O and iron vitriol FeSO₄·7H₂O (agriculture; harmful, handle with care), Glauber’s salt Na₂SO₄·10H₂O. Only the teacher works with concentrated acid: goggles, gloves, apron; if it splashes, rinse with plenty of water and tell an adult at once.
“Rule poster”: on an A4 sheet show the rule “water first, then acid” in a drawing (and why not the reverse) and draw 5 safety signs (goggles, gloves, apron, ventilation, first aid). No real acid is used; nothing is done with sulfuric acid at home (battery acid is dangerous too).