Chapter 2: Acids, Bases and Salts

Science · CBSE Class 10 · NCERT Solutions

📖 Chapter 2 — Acids, Bases and Salts (CBSE Class 10 Science): Detailed notes — indicators, reactions, pH scale, salts and important chemicals.

Chapter 2 — Acids, Bases and Salts

🧪 1. Acids, Bases & Indicators

Acid: gives H⁺ (H₃O⁺) ions in water; sour; turns blue litmus red. Base: gives OH⁻ ions in water; bitter, soapy; turns red litmus blue.

Indicator	Acid	Base
Litmus	Red	Blue
Methyl orange	Red	Yellow
Phenolphthalein	Colourless	Pink

Olfactory indicators (onion, vanilla) change smell in acids/bases.

⚗️ 2. Chemical Properties

Acid + Metal → Salt + H₂↑ (Zn + H₂SO₄ → ZnSO₄ + H₂)
Acid + Metal carbonate/bicarbonate → Salt + H₂O + CO₂↑ (turns lime water milky)
Acid + Base → Salt + Water (neutralisation): HCl + NaOH → NaCl + H₂O
Acid + Metal oxide → Salt + Water (metal oxides are basic)
Base + Non-metal oxide → Salt + Water (non-metal oxides are acidic)

💧 3. Acids/Bases in Water

Acids ionise in water to give H⁺ (as H₃O⁺); bases give OH⁻. Dilution: adding water lowers ion concentration — always add acid to water (exothermic, safe), never water to acid.

📊 4. pH Scale (0–14)

pH measures H⁺ ion concentration. pH < 7 = acidic, pH = 7 = neutral, pH > 7 = basic. Lower pH → more acidic.

pH in daily life: ① Digestion (stomach HCl; antacid for acidity). ② Tooth decay (below pH 5.5). ③ Soil pH for plants. ④ Self-defence (bee/nettle sting acidic → baking soda). ⑤ Acid rain (pH < 5.6).

🧂 5. Salts & Important Chemicals

Chemical	Formula & Uses
Common salt	NaCl — food; raw material for NaOH, baking/washing soda, bleaching powder
NaOH (chlor-alkali)	Electrolysis of brine → NaOH + Cl₂ + H₂
Bleaching powder	CaOCl₂ — bleaching, disinfecting water
Baking soda	NaHCO₃ — baking, antacid, fire extinguisher
Washing soda	Na₂CO₃·10H₂O — cleaning, softening hard water
Plaster of Paris	CaSO₄·½H₂O — plaster casts, moulds (+ water → gypsum)

Water of crystallisation: fixed water molecules in a salt crystal (e.g., CuSO₄·5H₂O blue; Na₂CO₃·10H₂O).

🧠 Remember: Acid=H⁺ (red litmus? no — turns BLUE litmus red). pH<7 acid, >7 base. Acid+metal→H₂. Acid+carbonate→CO₂. Baking soda=NaHCO₃, washing soda=Na₂CO₃·10H₂O, PoP=CaSO₄·½H₂O.

⭐ Exam Tips: ① Indicators table. ② 5 reactions (metal/carbonate/base/oxides). ③ pH scale + daily life. ④ Chlor-alkali products. ⑤ Baking/washing soda, bleaching powder, PoP + water of crystallisation.

📘 Textbook Question Answers

Q1-4MCQs: pH turning red litmus blue; egg-shell gas; NaOH-HCl neutralisation; indigestion medicine.

Q1: (d) 10 (basic → turns red litmus blue). Q2: (b) HCl (gives CO₂ that turns lime water milky). Q3: (d) 16 mL (double the NaOH needs double HCl). Q4: (c) Antacid.

Q5Word + balanced equations for dilute acid + metals.

(a) Zinc + sulphuric acid → zinc sulphate + hydrogen: Zn + H₂SO₄ → ZnSO₄ + H₂↑
(b) Mg + 2HCl → MgCl₂ + H₂↑
(c) 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂↑
(d) Fe + 2HCl → FeCl₂ + H₂↑

Q6Alcohols/glucose contain H but aren't acids — prove by activity.

Pass electric current through the solution using a bulb circuit. Acids (HCl) conduct electricity (glow the bulb) because they give H⁺ ions; alcohol/glucose solutions do not conduct — showing they don't give H⁺ ions, so they aren't acids.

Q7-8Why distilled water doesn't conduct but rain water does; why dry HCl isn't acidic.

Q7: Distilled water has no ions; rain water has dissolved salts/CO₂ giving ions, so it conducts. Q8: Dry HCl gas has no H⁺ ions — it needs water to ionise; acidic behaviour requires H⁺ (H₃O⁺) ions in water.

Q9Five solutions pH 4,1,11,7,9 — classify and arrange.

(a) neutral = D (7) (b) strongly alkaline = C (11) (c) strongly acidic = B (1) (d) weakly acidic = A (4) (e) weakly alkaline = E (9). Increasing H⁺-ion concentration (decreasing pH): C < E < D < A < B.

Q10HCl vs acetic acid + Mg ribbon — which fizzes more?

Test tube A (HCl) — fizzing is more vigorous because HCl is a strong acid (fully ionised, more H⁺ ions) than acetic acid (weak, partly ionised).

Q11-12pH of milk turning to curd; adding baking soda to milk.

Q11: pH decreases (more acidic) because lactic acid is produced. Q12(a): To make it slightly alkaline so it doesn't turn sour quickly. (b): Because the alkaline milk takes longer for lactic acid to lower the pH enough to set curd.

Q13-15PoP storage; neutralisation; uses of washing/baking soda.

Q13: PoP absorbs moisture and sets into hard gypsum, so it must be stored moisture-proof. Q14: Neutralisation = acid + base → salt + water (HCl + NaOH → NaCl + H₂O; H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O). Q15: Washing soda: cleaning, softening hard water. Baking soda: baking (cakes), antacid/fire extinguisher.

Extra practice (4× the textbook) — MCQ, short answer, long answer & equations.

🎯 Extra MCQ

1Which is the strongest acid: pH 2, 5, 7, 10?

pH 2 (lowest pH = strongest acid).

2Gas evolved when baking soda reacts with vinegar?

Carbon dioxide (CO₂).

3Which salt gives a basic solution: Na₂CO₃ or NH₄Cl?

Na₂CO₃ (salt of strong base + weak acid → basic).

4The gas released at the cathode in chlor-alkali —

Hydrogen (H₂).

📝 Short Answer

5Why does an acid conduct electricity in water?

Because it ionises to give free H⁺ (and other) ions that carry the current.

6What happens to CuSO₄·5H₂O on heating?

It loses water of crystallisation and turns white (anhydrous CuSO₄); on adding water it turns blue again.

7Why is common salt important industrially?

It is the raw material for NaOH, washing soda, baking soda and bleaching powder (and chlorine, hydrogen).

8How is bleaching powder prepared?

By passing chlorine over dry slaked lime: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O.

9Why is baking soda used in fire extinguishers?

It releases CO₂ (with acid) which smothers the fire.

⚗️ Equations

10NaHCO₃ on heating →

2NaHCO₃ →Δ→ Na₂CO₃ + H₂O + CO₂.

11Reaction of NaOH with zinc.

Zn + 2NaOH → Na₂ZnO₂ + H₂ (sodium zincate).

12Neutralisation of H₂SO₄ with NaOH.

H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O.

📖 Long Answer

13Explain the importance of pH in everyday life (any four).

① Digestion — stomach acid; excess causes acidity, cured by antacids. ② Tooth decay — bacteria produce acid below pH 5.5; toothpaste (basic) neutralises it. ③ Soil pH — plants grow in a specific pH range. ④ Self-defence — bee/nettle stings are acidic, treated with baking soda; wasp stings are basic, treated with vinegar. ⑤ Acid rain harms plants, buildings and aquatic life.

14Describe the chlor-alkali process and its products' uses.

Electrolysis of brine (NaCl solution) gives NaOH (at cathode side), chlorine (at anode) and hydrogen. Uses: NaOH — soaps, paper; Cl₂ — water treatment, PVC, bleaching powder; H₂ — fuel, margarine.

15What is water of crystallisation? Give two examples with uses.

It is the fixed number of water molecules present in one formula unit of a salt. Examples: CuSO₄·5H₂O (blue vitriol, used as fungicide) and CaSO₄·½H₂O (Plaster of Paris, used for casts and moulds).