Class 10 Science Acid Bases and Salts Exam Notes Set 01

This is a compact exam revision page for Class 10 Science Chapter 2, Acids, Bases and Salts. It gathers the key definitions, indicator colours, general reactions, pH facts, the family of chemicals made from common salt, the important equations, memory aids and the usual mistakes made in board answers.

Key Definitions

1. Acid: a substance that gives H⁺ (H₃O⁺) ions in aqueous solution. Acids taste sour and turn blue litmus red. Alcohol and glucose contain hydrogen but do not give H⁺ ions, so they are not acids.
2. Base: a substance that reacts with an acid to form salt and water. Alkali: a base that dissolves in water and gives OH⁻ ions, for example NaOH and KOH. All alkalis are bases but not all bases are alkalis.
3. Indicator: a substance that shows a different colour (or smell) in acidic and basic media. Natural: litmus, turmeric, red cabbage, petunia, hydrangea. Synthetic: methyl orange, phenolphthalein. Olfactory: onion, vanilla essence.
4. Neutralisation: reaction of an acid with a base to form salt and water; H⁺(aq) + OH⁻(aq) → H₂O(l). It gives out heat.
5. pH: the negative logarithm (base 10) of the hydrogen ion concentration, pH = −log[H⁺]. The scale runs from 0 to 14; a higher H⁺ concentration means a lower pH.
6. Dilution: mixing an acid or base with water, which lowers the concentration of H₃O⁺ or OH⁻ ions per unit volume. It is highly exothermic, so always add the acid to water slowly with stirring.
7. Water of crystallisation: the fixed number of water molecules chemically combined with one formula unit of a salt in its crystals, for example CuSO₄·5H₂O. Hydrated salts contain it; anhydrous salts do not.
8. Salt: compound formed when an acid reacts with a base. Salts with the same positive or negative ion form a family, for example NaCl and Na₂SO₄ are sodium salts.

Indicator Colours at a Glance

Reactions of Acids and Bases

Ions, Strength and pH

1. Acids show acidic behaviour only in water, where they give H⁺ ions. Dry HCl gas does not change dry blue litmus; moist litmus turns red. Solutions of acids and bases conduct electricity because they contain ions; glucose and alcohol solutions do not.
2. Strong acids (HCl, H₂SO₄, HNO₃) ionise almost completely; weak acids (CH₃COOH, H₂CO₃, citric acid) ionise only partly. Strong bases: NaOH, KOH, Ca(OH)₂; weak bases: NH₄OH, Mg(OH)₂.
3. pH below 7 is acidic, 7 is neutral and above 7 is basic. Each fall of one pH unit means ten times more H⁺ ions. Dilution moves the pH of an acid up towards 7 and of a base down towards 7.

pH of Salts

Chemicals from Common Salt

Water of Crystallisation

1. Blue copper sulphate crystals, CuSO₄·5H₂O, turn white on heating because they lose the five water molecules: CuSO₄·5H₂O → CuSO₄ + 5H₂O. A few drops of water restore the blue colour.
2. Number of water molecules: copper sulphate 5, washing soda 10, gypsum 2, green vitriol FeSO₄·7H₂O 7, plaster of Paris half.
3. Sodium chloride, baking soda and sodium carbonate (soda ash) are anhydrous.

Important Equations to Practise

1. 2NaOH + H₂CO₃ → Na₂CO₃ + 2H₂O
2. ZnO + 2NaOH → Na₂ZnO₂ + H₂O (zinc oxide is amphoteric)
3. 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂↑
4. Mg(OH)₂ + 2HNO₃ → Mg(NO₃)₂ + 2H₂O
5. CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑ (egg shell, marble and limestone are calcium carbonate)
6. CaOCl₂ + CO₂ → CaCO₃ + Cl₂ (bleaching powder loses chlorine in air)
7. SO₃ + H₂O → H₂SO₄; Na₂O + H₂O → 2NaOH; CaO + H₂O → Ca(OH)₂
8. 2NaOH + CO₂ → Na₂CO₃ + H₂O
9. 2NaCl + H₂SO₄ (conc.) → Na₂SO₄ + 2HCl↑

Memory Aids

1. Litmus: Acid makes Blue litmus Red (A-B-R); base does the opposite to red litmus.
2. Low pH, high H⁺: the two numbers move in opposite directions.
3. Always Add Acid to water (A-A-W), never water to acid.
4. Hydrated salt water counts: copper sulphate 5, gypsum 2, washing soda 10.
5. Salt pH rule: the stronger parent decides the nature of the salt; if both parents are strong the salt is neutral.

Common Mistakes