Acids Bases and Salts Class 10 Notes (2026-27) — CBSE
Class 10 Science Chapter 2 notes: indicators, reactions of acids and bases, the pH scale, and important salts like baking soda, washing soda and plaster of Paris.
Acids, Bases and Salts — Class 10 Science Notes
Chapter Snapshot
This chapter explains how acids and bases are identified (indicators), how they react with metals, carbonates, and each other, why they behave the way they do (H⁺ and OH⁻ ions in water), and how the pH scale measures acidity. It ends with important salts — common salt and the useful chemicals made from it: sodium hydroxide, bleaching powder, baking soda, washing soda, and plaster of Paris.
Board relevance: almost always gives an equation/reaction question, a pH-based reasoning question, and a "preparation and uses" question on one of the named salts. Learn the formulas exactly.
Key Concepts & Definitions
Acids — sour-tasting substances that turn blue litmus red and produce H⁺ ions in water. Bases — bitter, soapy substances that turn red litmus blue and produce OH⁻ ions in water. A base that dissolves in water is an alkali (e.g. NaOH, KOH).
Indicators — substances that show a different colour in acids and bases:
Indicator In acid In base
Litmus Red Blue
Methyl orange Red/pink Yellow
Phenolphthalein Colourless Pink
Olfactory indicators — substances whose smell changes in acidic/basic media (e.g. onion, vanilla, clove).
Why acids need water: acids produce H⁺ ions only in water; the H⁺ ion does not exist alone but as the hydronium ion (H₃O⁺). Dissolving an acid in water is highly exothermic, so acid is always added slowly to water (never water to acid, which can splash dangerously).
Strong vs weak: a strong acid (HCl, H₂SO₄, HNO₃) ionises completely; a weak acid (CH₃COOH, carbonic acid) ionises only partly. Same for strong bases (NaOH, KOH) and weak bases (NH₄OH).
Chemical Properties
Reactions you must know (with a salt always forming):
Reaction type General equation Example
Acid + metal acid + metal → salt + H₂↑ 2HCl + Zn → ZnCl₂ + H₂↑
Acid + metal carbonate → salt + water + CO₂↑ 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂
Acid + metal hydrogencarbonate → salt + water + CO₂↑ HCl + NaHCO₃ → NaCl + H₂O + CO₂
Acid + base (neutralisation) → salt + water HCl + NaOH → NaCl + H₂O
Acid + metal oxide → salt + water 2HCl + CuO → CuCl₂ + H₂O
Base + non-metal oxide → salt + water 2NaOH + CO₂ → Na₂CO₃ + H₂O
Base + metal → salt + H₂↑ 2NaOH + Zn → Na₂ZnO₂ + H₂↑
- The H₂ gas test: burns with a "pop" sound when a lighted matchstick is brought near.
- The CO₂ test: turns limewater [Ca(OH)₂] milky due to CaCO₃; excess CO₂ dissolves it again.
- Metal oxides are basic; non-metal oxides are acidic. (Some metal oxides like Al₂O₃, ZnO are amphoteric — react with both acids and bases.)
The pH Scale
pH = "power of hydrogen"; a scale from 0 to 14 measuring the H⁺ ion concentration.
- pH < 7 → acidic (lower = stronger acid, more H⁺ ions).
- pH = 7 → neutral (pure water).
- pH 7 → basic/alkaline (higher = stronger base).
A universal indicator shows the full range of colours for different pH values.
Importance of pH in everyday life:
- Tooth decay starts when mouth pH falls below 5.5; bacteria make acid from sugar; basic toothpaste neutralises it.
- Living organisms survive in a narrow pH range (blood ≈ 7.4).
- Soil pH affects crops; farmers add lime (basic) to acidic soil.
- Digestion: the stomach uses HCl; excess causes acidity, treated with antacids (mild bases like milk of magnesia, Mg(OH)₂).
- Self-defence: a bee/ant sting is acidic (methanoic acid) — rubbing baking soda (basic) relieves it; a wasp sting is basic.
- Acid rain (pH < 5.6) harms plants, aquatic life, and buildings.
Salts and Their Uses
A salt is formed from the positive ion of a base and the negative ion of an acid. Salts of a strong acid + strong base are neutral (pH 7); strong acid + weak base are acidic; weak acid + strong base are basic.
Common salt (NaCl) is the raw material for many chemicals. The chlor-alkali process passes electricity through brine (concentrated NaCl solution):
2NaCl + 2H₂O → 2NaOH + Cl₂↑ + H₂↑
(Cl₂ at anode, H₂ at cathode, NaOH in solution.)
Salt Formula Made from / how Uses
Sodium hydroxide NaOH Chlor-alkali process Soap, paper, degreasing
Bleaching powder CaOCl₂ Cl₂ + dry slaked lime Ca(OH)₂ Bleaching, disinfecting water
Baking soda NaHCO₃ NaCl + H₂O + CO₂ + NH₃ Antacid, baking, fire extinguishers
Washing soda Na₂CO₃·10H₂O Heating baking soda then recrystallising Cleaning, softening hard water, glass
Plaster of Paris CaSO₄·½H₂O Heating gypsum (CaSO₄·2H₂O) to 373 K Casts for fractures, moulds
Water of crystallisation — a fixed number of water molecules in one formula unit of a crystalline salt (e.g. CuSO₄·5H₂O blue; on heating it turns white, and water added again restores blue). Washing soda has 10 (Na₂CO₃·10H₂O).
Baking soda in cooking: on heating, 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂; the CO₂ makes cakes rise. Plaster of Paris must be stored dry — it absorbs moisture and sets: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (gypsum).
Important Question Patterns
1. Complete/balance the reaction (2–3 marks): acid + metal / carbonate / base; write products and the gas test.
2. pH reasoning (2–3 marks): arrange solutions by pH; explain tooth decay, antacids, soil treatment, or acid rain.
3. Salts preparation & uses (3 marks): name the salt from a description; give its formula, preparation, and two uses (baking soda, washing soda, bleaching powder, plaster of Paris are favourites).
4. Water of crystallisation (2 marks): CuSO₄·5H₂O colour change on heating and rehydration; formula of washing soda.
5. Distinguish/identify (2 marks): identify a gas by its test (H₂ pop, CO₂ limewater); strong vs weak acid.
⚡ Quick Revision
- Acids: H⁺ (as H₃O⁺) in water, blue litmus → red. Bases: OH⁻ in water, red litmus → blue; soluble base = alkali.
- Indicators: litmus (red/blue), methyl orange (red/yellow), phenolphthalein (colourless/pink).
- Always add acid to water (dilution is exothermic).
- Reactions: acid + metal → salt + H₂↑; acid + carbonate → salt + water + CO₂↑; acid + base → salt + water (neutralisation).
- Gas tests: H₂ = pop; CO₂ = limewater turns milky.
- pH: 0–14; <7 acidic, 7 neutral, 7 basic. Tooth decay below 5.5; antacids = mild bases.
- Chlor-alkali: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂.
- Formulas: bleaching powder CaOCl₂, baking soda NaHCO₃, washing soda Na₂CO₃·10H₂O, plaster of Paris CaSO₄·½H₂O, gypsum CaSO₄·2H₂O.
- Blue CuSO₄·5H₂O → white on heating (loses water of crystallisation).
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