CBSE Class 10 Science Acids, Bases and Salts Notes Set 10

1. Acids and bases

Acids taste sour, turn blue litmus red and give H⁺ ions in water. Common examples are HCl, H₂SO₄, HNO₃ (mineral acids) and citric, acetic and lactic acids (organic acids). Bases taste bitter, feel soapy, turn red litmus blue and give OH⁻ ions in water. A base that dissolves in water is called an alkali, for example NaOH and KOH.

Indicators show whether a solution is acidic or basic by changing colour. Litmus is red in acid and blue in base. Phenolphthalein is colourless in acid and pink in base. Methyl orange is red in acid and yellow in base. Turmeric, red cabbage juice and China rose petals are natural indicators. Olfactory indicators such as onion and vanilla lose their smell in a base.

2. Chemical properties

1. Acid + metal → salt + hydrogen gas. Example: Zn + H₂SO₄ → ZnSO₄ + H₂. The gas burns with a pop sound.

2. Base + metal → salt + hydrogen. Example: 2NaOH + Zn → Na₂ZnO₂ + H₂.

3. Acid + metal carbonate or hydrogencarbonate → salt + water + carbon dioxide. Example: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂.

4. Acid + base → salt + water. This is neutralisation. Example: NaOH + HCl → NaCl + H₂O.

5. Metal oxides are basic and non-metal oxides are acidic. Acid + metal oxide gives salt and water, and base + non-metal oxide gives salt and water: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O.

All acids contain hydrogen, but not all hydrogen-containing compounds are acids (glucose, alcohol). Dilution of an acid or base is exothermic, so always add the acid to water slowly, never water to acid.

3. Strength and the pH scale

A strong acid such as HCl ionises almost completely, while a weak acid such as ethanoic acid ionises only partly. The pH scale, from 0 to 14, measures the concentration of hydrogen ions. pH 7 is neutral, below 7 is acidic and above 7 is basic. It is checked using pH paper or universal indicator. The lower the pH, the stronger the acid.

pH in daily life: our body works in a pH range of about 7.0 to 7.8. Acid rain with pH below 5.6 harms aquatic life. Tooth decay begins when the mouth pH falls below 5.5, so basic toothpaste helps. Antacids such as milk of magnesia neutralise excess stomach acid. Bee and nettle stings contain acid and are soothed with a mild base like baking soda.

4. Salts and important compounds

A salt is formed when an acid reacts with a base. Salts of strong acid and strong base are neutral (pH 7), of strong acid and weak base are acidic, and of strong base and weak acid are basic. Common salt, NaCl, is the raw material for the chlor-alkali process: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂ (electrolysis of brine).

1. Bleaching powder, CaOCl₂: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. Used for bleaching and disinfecting drinking water.

2. Baking soda, NaHCO₃: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃. On heating, 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Used as an antacid and in baking powder (baking soda + tartaric acid).

3. Washing soda, Na₂CO₃.10H₂O: obtained by recrystallising sodium carbonate. Used in glass, soap and paper industries and to soften hard water.

4. Plaster of Paris: CaSO₄.2H₂O → CaSO₄.½H₂O + 1½H₂O at 373 K. Used for setting fractured bones, statues and toys. It must be stored in a moisture-proof container.

Water of crystallisation is the fixed number of water molecules in one formula unit of a salt. Blue copper sulphate crystals, CuSO₄.5H₂O, turn white on heating and regain blue colour when water is added.

Worked examples

Example 1. A solution turns red litmus blue and has a pH of 12. What is its nature?

It turns red litmus blue, so it is basic, and pH 12 means a fairly strong base. Such a solution would feel soapy and would turn phenolphthalein pink.

Example 2. Name the gas X formed when dilute HCl is added to sodium carbonate, and how is it tested?

X is carbon dioxide. When passed through lime water, Ca(OH)₂ + CO₂ → CaCO₃ + H₂O, the lime water turns milky. Passing excess gas turns it clear again.

Example 3. Why does dry HCl gas not change the colour of dry blue litmus paper but HCl solution does?

Dry HCl has no H⁺ ions because it does not ionise without water. In water, HCl produces H₃O⁺ ions, which are responsible for acidic behaviour, so the litmus turns red.

Quick Check Questions

Answer these five questions to test your understanding of the chapter.