CBSE Class 10 Science Acids, Bases and Salts Notes Set 09

1. Acids

An acid is a substance that releases hydrogen ions (H⁺) when dissolved in water. In water the free H⁺ attaches to a water molecule and exists as the hydronium ion, H₃O⁺. Acids taste sour and turn blue litmus red.

Examples of ionisation: HCl → H⁺ + Cl⁻ and HNO₃ → H⁺ + NO₃⁻.

Acids come from two sources. Natural (organic) acids include citric acid in lemon and orange, lactic acid in sour milk, formic acid in ant and bee stings, and acetic acid in vinegar. Mineral acids, made in industry, include hydrochloric, sulphuric, nitric and phosphoric acid.

Strong acids such as HCl, HNO₃ and H₂SO₄ ionise almost completely in water, whereas weak acids such as acetic and formic acid ionise only slightly.

2. Bases and alkalis

A base is a substance that gives hydroxide ions (OH⁻) in water, for example NaOH → Na⁺ + OH⁻. Bases taste bitter, feel soapy and turn red litmus blue. A base that dissolves in water is called an alkali, so NaOH and KOH are alkalis while Mg(OH)₂ is a base but not an alkali. All alkalis are bases, but not all bases are alkalis.

Strong bases (NaOH, KOH) ionise completely; weak bases (Mg(OH)₂, NH₄OH) ionise only partly. Acids and bases both conduct electricity in solution because they produce ions.

3. Indicators

An indicator shows by a change in colour or smell whether a solution is acidic or basic. Natural indicators include litmus, red cabbage juice and turmeric. Olfactory indicators such as onion, vanilla and clove oil lose their smell in a basic solution. Synthetic indicators include phenolphthalein and methyl orange.

Colour changes: 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 turns red-brown in a base.

4. Chemical properties

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

2. Acid + metal carbonate or hydrogencarbonate → salt + water + carbon dioxide. CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. The CO₂ turns lime water milky.

3. Acid + base → salt + water (neutralisation). NaOH + HCl → NaCl + H₂O.

4. Acid + metal oxide → salt + water. CuO + 2HCl → CuCl₂ + H₂O. Metal oxides are basic in nature.

5. Base + non-metal oxide → salt + water. Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Non-metal oxides are acidic in nature.

6. Some metals such as zinc and aluminium react with NaOH to release hydrogen: Zn + 2NaOH → Na₂ZnO₂ + H₂.

Dilution: always add acid slowly to water, stirring, never water to acid, because the process releases a large amount of heat that can make the mixture spurt out.

5. The pH scale

pH measures the concentration of hydrogen ions in a solution on a scale from 0 to 14. A pH of 7 is neutral, below 7 is acidic and above 7 is basic. Each unit change in pH means a tenfold change in H⁺ concentration. Universal indicator or pH paper gives a colour that matches a pH value.

Everyday uses: the stomach works at about pH 1.5 to 3, tooth enamel starts to corrode below pH 5.5, soil pH affects crop growth, and bee stings (acid) are eased by mild bases such as baking soda. Antacids such as milk of magnesia neutralise excess stomach acid.

6. Salts and their families

A salt is formed when an acid reacts with a base. Salts of strong acid and strong base (NaCl) are neutral, pH about 7. Salts of strong acid and weak base (NH₄Cl) are acidic. Salts of strong base and weak acid (Na₂CO₃) are basic.

Common salt (NaCl) is the raw material for the chlor-alkali process. When electricity is passed through brine, 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Chlorine is given off at the anode and hydrogen at the cathode, with NaOH left in solution.

Bleaching powder, CaOCl₂, is made by passing chlorine over dry slaked lime. It is used to bleach cotton and linen, disinfect drinking water and as an oxidising agent.

Baking soda, NaHCO₃, is mildly basic. It is used in antacids and to make cakes soft and spongy, because heating gives CO₂: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.

Washing soda, Na₂CO₃·10H₂O, is obtained by recrystallising sodium carbonate. It is used for cleaning and to soften hard water.

Plaster of Paris, CaSO₄·½H₂O, is made by heating gypsum (CaSO₄·2H₂O) at 373 K. Mixed with water it sets into hard gypsum, so it is used for casts, toys and smoothing walls. Water of crystallisation is the fixed number of water molecules attached to one formula unit of a salt, as in blue copper sulphate crystals, CuSO₄·5H₂O.

Worked examples

Example 1. Write the balanced equation for the reaction of sodium hydrogencarbonate with dilute hydrochloric acid. Solution: NaHCO₃ + HCl → NaCl + H₂O + CO₂. The effervescence is carbon dioxide.

Example 2. A solution has [H⁺] = 10⁻⁴ mol/L. Find its pH and nature. Solution: pH = −log(10⁻⁴) = 4. Since 4 is below 7, the solution is acidic.

Example 3. A student finds that a solution turns phenolphthalein pink. What can be said about its pH? Solution: phenolphthalein is pink only in a base, so the solution is basic and its pH is above 7 (about 8.3 or more).

Quick Check Questions