CBSE Class 10 Science Acids Bases and Salts Notes Set 08

Acids and Bases

Acids taste sour (lemon juice, vinegar, curd). An acid releases H⁺ ions in water, e.g. HCl → H⁺ + Cl⁻. In water, H⁺ exists as H₃O⁺ (hydronium ion). Natural acids (organic acids) include citric acid in citrus fruit, lactic acid in sour milk and formic acid in ant stings. Mineral acids such as hydrochloric, sulphuric, nitric and phosphoric acid come from minerals.

Bases taste bitter and feel soapy. A base releases OH⁻ ions in water, e.g. NaOH → Na⁺ + OH⁻. A base soluble in water is an alkali. Salts form when an acid and a base react.

Indicators

Indicators are dyes that show whether a solution is acidic or basic. Natural indicators: litmus, red cabbage juice, turmeric. Olfactory indicators change smell, e.g. vanilla smell vanishes in a base. Synthetic indicators: phenolphthalein, methyl orange.

Chemical Properties of Acids

With metals: acid + metal → salt + hydrogen gas. Zn + H₂SO₄ → ZnSO₄ + H₂; Mg + 2HCl → MgCl₂ + H₂.

With metal oxides (basic): acid + metal oxide → salt + water. CuO + 2HCl → CuCl₂ + H₂O.

With metal carbonates and hydrogencarbonates: they give salt, water and CO₂. CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂; NaHCO₃ + HCl → NaCl + H₂O + CO₂.

Acids that ionise almost completely are strong (HCl, HNO₃, H₂SO₄); those that ionise slightly are weak (acetic acid, formic acid).

Chemical Properties of Bases

With metals: zinc, tin and aluminium react with strong bases such as NaOH to give hydrogen. Zn + 2NaOH → Na₂ZnO₂ + H₂.

With non-metal oxides (acidic): base + non-metal oxide → salt + water. Ca(OH)₂ + CO₂ → CaCO₃ + H₂O.

Bases that ionise completely are strong (NaOH, KOH); those that ionise slightly are weak (Mg(OH)₂, NH₄OH).

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