CBSE Class 10 · Science · board exam 2027
Chapter 2: Acids, Bases and Salts — notes
The chapter’s 10 parts in the book’s order — the first idea of each and the points Medha Ma’am writes on the board — and the summary at the end. The full lesson and a check question for each part are in the app.
1. Indicators: telling acids from bases
First, one simple point.
- • Acids: sour in taste
- • Bases: bitter in taste, soapy to touch
- • Indicator: a substance that changes colour or smell to show acidic or basic nature
- • Acid: blue litmus ⇒ red
- • Base: red litmus ⇒ blue
- • Natural indicators: litmus (from lichen), turmeric
- • Turmeric + base (soap) ⇒ reddish-brown
- • | Indicator | In acid | In base |
- • | Blue litmus | red | stays blue |
- • | Red litmus | stays red | blue |
- • | Methyl orange | red | yellow |
- • | Phenolphthalein | colourless | pink |
- • Olfactory indicators: they change their smell
- • Examples: onion, vanilla essence, clove oil
2. Acids and bases with metals
The figure, this is the activity.
- • Activity: zinc granules + dilute sulphuric acid
- • Test: burning candle ⇒ pop sound ⇒ hydrogen gas
- • Word equation: zinc + sulphuric acid → zinc sulphate + hydrogen
- • | Element | Left | Right |
- • | Zn | 1 | 1 |
- • | H | 2 | 2 |
- • | S | 1 | 1 |
- • | O | 4 | 4 |
- • Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
- • Acid + Metal → Salt + Hydrogen gas
- • NaOH + Zn → Na₂ZnO₂ + H₂ (unbalanced)
- • | Element | Left | Right |
- • | Na | 1 ⇒ 2 | 2 |
- • | O | 1 ⇒ 2 | 2 |
- • | H | 1 ⇒ 2 | 2 |
- • | Zn | 1 | 1 |
- • 2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g)
3. Metal carbonates and hydrogencarbonates with acids
The figure.
- • Activity: sodium carbonate + dilute HCl
- • Gas turns lime water milky ⇒ CO₂
- • Metal carbonate or hydrogencarbonate + Acid → Salt + CO₂ + Water
- • Na₂CO₃ + HCl → NaCl + H₂O + CO₂ (unbalanced)
- • | Element | Left | Right |
- • | Na | 2 | 1 ⇒ 2 |
- • | Cl | 1 ⇒ 2 | 2 |
- • | H | 2 | 2 |
- • | C | 1 | 1 |
- • | O | 3 | 1 + 2 = 3 |
- • Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
- • NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)
- • Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
- • CaCO₃: white precipitate, so milky
- • CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)
- • Ca(HCO₃)₂: soluble, so the solution clears
- • Limestone, chalk, marble: all CaCO₃
4. Neutralisation, and metallic and non-metallic oxides
The figure.
- • Activity: NaOH + phenolphthalein ⇒ pink
- • Dilute HCl drop by drop ⇒ colourless
- • Neutralisation: Acid + Base → Salt + Water
- • | Element | Left | Right |
- • | Na | 1 | 1 |
- • | O | 1 | 1 |
- • | H | 1 + 1 = 2 | 2 |
- • | Cl | 1 | 1 |
- • NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
- • CuO + HCl → CuCl₂ + H₂O (unbalanced)
- • | Cl | 1 ⇒ 2 | 2 |
- • | H | 1 ⇒ 2 | 2 |
- • CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)
- • Metal oxide + Acid → Salt + Water ⇒ metallic oxides are basic
- • Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
- • Base + Non-metallic oxide → Salt + Water ⇒ non-metallic oxides are acidic
5. Acids and bases in water: H⁺ and OH⁻ ions
The figure.
- • Activity: the bulb glows in an acid solution
- • Glucose, alcohol: the bulb does not glow
- • Acids give H⁺(aq) ions in water ⇒ they conduct
- • Dry HCl gas: dry blue litmus stays blue
- • Moist blue litmus: turns red
- • HCl + H₂O → H₃O⁺ + Cl⁻
- • | H | 1 + 2 = 3 | 3 |
- • | Charge | 0 | +1 − 1 = 0 |
- • H⁺ + H₂O → H₃O⁺
- • NaOH(s) →[water] Na⁺(aq) + OH⁻(aq)
- • Alkali: a base that dissolves in water
- • All alkalis are bases, but all bases are not alkalis
- • Add acid to water, never water to acid
- • Reason: highly exothermic, the mixture can splash and burn
- • Dilution: more water ⇒ fewer ions per unit volume
6. The pH scale: how strong is an acid or a base
For this we use a universal indicator.
- • Universal indicator: a mixture of indicators, a different colour at each concentration
- • pH scale: 0 to 14
- • pH 7: neutral
- • pH below 7: acidic
- • pH above 7: basic
- • More H⁺ ⇒ lower pH ⇒ more acidic
- • More OH⁻ ⇒ higher pH ⇒ more basic
- • Gastric juice ≈ 1.2, lemon juice ≈ 2.2
- • Pure water 7, milk of magnesia ≈ 10, NaOH ≈ 14
- • Strong acids: more H⁺, e.g. HCl, H₂SO₄, HNO₃
- • Weak acids: fewer H⁺, e.g. CH₃COOH, H₂CO₃
- • Strong bases: more OH⁻, e.g. NaOH, KOH
- • Weak bases: fewer OH⁻, e.g. NH₄OH
7. Importance of pH in everyday life
First, our body works within a pH range of seven point zero to seven point eight.
- • ① Body: pH 7.0 to 7.8
- • ② Acid rain: pH below 5.6 ⇒ hard for aquatic life
- • ③ Soil: a specific pH for healthy growth
- • ④ Stomach: HCl helps digestion
- • Indigestion ⇒ antacid, milk of magnesia Mg(OH)₂
- • ⑤ Teeth: mouth pH below 5.5 ⇒ tooth decay
- • Prevention: clean after eating, basic toothpaste
- • ⑥ Bee sting: acid ⇒ apply baking soda
- • Nettle: methanoic acid ⇒ dock leaf
- • | Source | Acid |
- • | Vinegar | Acetic acid |
- • | Orange, lemon | Citric acid |
- • | Tamarind | Tartaric acid |
- • | Tomato | Oxalic acid |
- • | Sour milk (curd) | Lactic acid |
- • | Ant sting, nettle sting | Methanoic acid |
8. Salts: families, pH and common salt
Salts are formed by neutralisation.
- • Family of salts: the same cation or the same anion
- • Na⁺ family: NaCl, Na₂SO₄
- • Cl⁻ family: NaCl, KCl
- • | Acid | Base | Salt | pH |
- • | strong | strong | neutral | 7 |
- • | strong | weak | acidic | less than 7 |
- • | weak | strong | basic | more than 7 |
- • NaCl: HCl (strong) + NaOH (strong) ⇒ neutral
- • NH₄Cl: HCl (strong) + NH₄OH (weak) ⇒ acidic
- • Na₂CO₃: H₂CO₃ (weak) + NaOH (strong) ⇒ basic
- • Common salt NaCl: from sea water; rock salt (brown)
- • Chlor-alkali process: electricity through brine
- • Anode: Cl₂, cathode: H₂, near cathode: NaOH
- • NaCl + H₂O → NaOH + Cl₂ + H₂ (unbalanced)
- • | Cl | 1 ⇒ 2 | 2 |
- • | Na | 2 | 1 ⇒ 2 |
- • | H | 2 ⇒ 4 | 2 + 2 = 4 |
- • | O | 2 | 2 |
- • 2NaCl(aq) + 2H₂O(l) →[electricity] 2NaOH(aq) + Cl₂(g) + H₂(g)
- • H₂: fuels, margarine
- • Cl₂: water treatment, PVC, bleaching powder
- • NaOH: soaps, detergents, paper, artificial fibres
9. Chemicals from common salt: bleaching powder, baking soda, washing soda
The figure, how many chemicals come from one salt.
- • From NaCl: bleaching powder, baking soda, washing soda
- • Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
- • | O | 2 | 1 + 1 = 2 |
- • | Cl | 2 | 2 |
- • Bleaching powder: bleaching cloth and wood pulp, oxidising agent
- • Making drinking water free from germs
- • NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
- • | H | 2 + 3 = 5 | 4 + 1 = 5 |
- • | N | 1 | 1 |
- • NaHCO₃ → Na₂CO₃ + H₂O + CO₂ (unbalanced)
- • | Na | 1 ⇒ 2 | 2 |
- • | O | 6 | 3 + 1 + 2 = 6 |
- • 2NaHCO₃ →[heat] Na₂CO₃ + H₂O + CO₂
- • Baking powder: baking soda + tartaric acid
- • NaHCO₃ + H⁺ → CO₂ + H₂O + Na⁺
- • Uses: baking powder, antacid, soda-acid fire extinguisher
- • Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O
- • Washing soda: a basic salt
- • Washing soda: glass, soap, paper industries; borax
- • Domestic cleaning; removing permanent hardness of water
10. Water of crystallisation and Plaster of Paris
The figure.
- • Activity: heating copper sulphate crystals
- • Blue ⇒ white, water droplets in the tube
- • Water of crystallisation: the fixed number of water molecules in one formula unit of a salt
- • CuSO₄·5H₂O →[heat] CuSO₄ + 5H₂O
- • | Salt | Formula | Water molecules |
- • | Copper sulphate | CuSO₄·5H₂O | 5 |
- • | Washing soda | Na₂CO₃·10H₂O | 10 |
- • | Gypsum | CaSO₄·2H₂O | 2 |
- • | Ferrous sulphate | FeSO₄·7H₂O | 7 |
- • CaSO₄·2H₂O →[373 K] CaSO₄·½H₂O + 1½H₂O
- • Gypsum ⇒ Plaster of Paris (calcium sulphate hemihydrate)
- • | H | 4 | 1 + 3 = 4 |
- • | O | 4 + 2 = 6 | 4 + ½ + 1½ = 6 |
- • CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
- • Keep away from moisture: it would set into hard gypsum
- • Uses: fractured bones, toys, decoration, smooth surfaces
Summary
Done! We covered a lot today, so let us revise it together once. Indicators told us acid or base. Acids turn blue litmus red, bases turn red litmus blue. Then we balanced the reactions of acids with metals, carbonates and bases, and saw that metallic oxides are basic and non-metallic oxides are acidic. Then we saw that acids give hydrogen ions in water and bases give hydroxide ions, and that acid is always added to water. On the pH scale, seven is neutral, lower is acidic, higher is basic. At the end, the pH of salts, the chlor-alkali process, bleaching powder, baking soda, washing soda, water of crystallisation and Plaster of Paris. Write all of today's equations once in your notebook, balancing them without looking, and learn the table of natural acids. Your homework is on your screen. If anything was not clear, press the Ask button and ask me. You did very good work today!
AskMedha · Medha Ma’am — CBSE Class 10 Science, Chapter 2. Written from the textbook, checked twice.
Questions parents and students ask
- How do I get the "Acids, Bases and Salts" notes as a PDF?
- Press "Save as PDF" — the phone’s print sheet opens; choose "Save as PDF" there. The PDF holds the notes only.
- Where are these notes written from?
- From the chapter’s own textbook text, part by part, in the book’s own terms; a second, independent model then checked every part against the book. The full lesson, a check question for each part and the practice are in the AskMedha app.
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