Simply CBSE

Acids, Bases and Salts — Class 10 Science

Quick revision notes for exam preparation.

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📌 Key Points

  • Acids produce H⁺ (H₃O⁺) ions in water; bases produce OH⁻ ions in water
  • Litmus: acid → red, base → blue. Phenolphthalein: acid → colourless, base → pink
  • Acid + Metal → Salt + H₂↑ (pop sound test). Acid + Metal carbonate → Salt + H₂O + CO₂↑ (lime water turns milky)
  • Acid + Base → Salt + Water (neutralisation, exothermic)
  • Metal oxides are basic; non-metal oxides are acidic
  • Dry HCl does not show acidic behaviour — water is essential to produce H⁺ ions
  • Always add acid to water (never water to acid) — dilution is highly exothermic
  • pH scale: 0–14. pH < 7 = acidic, pH 7 = neutral, pH > 7 = basic
  • Tooth enamel corrodes below pH 5.5. Bee sting = acidic (use baking soda). Wasp sting = basic (use vinegar)
  • Chlor-alkali process: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂ (electrolysis of brine)
  • Bleaching powder: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
  • Baking soda = NaHCO₃ (antacid, baking). Washing soda = Na₂CO₃·10H₂O (cleaning, softening hard water)
  • Plaster of Paris: CaSO₄·½H₂O (from gypsum CaSO₄·2H₂O at 373 K). Sets back to gypsum on adding water
  • CuSO₄·5H₂O (blue) → CuSO₄ (white) on heating. Blue returns on adding water
  • Strong acids (HCl, H₂SO₄, HNO₃) fully ionise. Weak acids (CH₃COOH, H₂CO₃) partially ionise

📘 Important Definitions

Acid
A substance that produces H⁺ (or H₃O⁺) ions when dissolved in water. Turns blue litmus red. Example: HCl, H₂SO₄.
Base
A substance that produces OH⁻ ions when dissolved in water. Turns red litmus blue. Example: NaOH, Ca(OH)₂.
Alkali
A base that is soluble in water. All alkalis are bases but not all bases are alkalis. Example: NaOH (alkali), Cu(OH)₂ (base, not alkali).
Salt
An ionic compound formed by the neutralisation of an acid and a base. Contains cation from base and anion from acid. Example: NaCl.
Neutralisation
Reaction between acid and base to form salt and water. Acid + Base → Salt + Water. It is exothermic.
pH Scale
A scale from 0 to 14 measuring the hydrogen ion concentration. pH < 7 = acidic, pH = 7 = neutral, pH > 7 = basic.
Water of Crystallisation
Fixed number of water molecules present in one formula unit of a crystalline salt. Example: CuSO₄·5H₂O has 5 water molecules.
Indicator
A substance that shows different colours in acidic and basic solutions. Examples: litmus, phenolphthalein, methyl orange.
Hydronium Ion
H₃O⁺ ion formed when H⁺ combines with water. It is the actual species responsible for acidic properties in solution.
Antacid
A mild base used to neutralise excess acid in the stomach. Examples: Mg(OH)₂ (milk of magnesia), NaHCO₃ (baking soda).

🔢 Formulas & Laws

Acid + Metal

2HCl + Zn → ZnCl₂ + H₂↑

H₂ burns with a pop sound

Acid + Metal carbonate

2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂↑

CO₂ turns lime water milky

Chlor-alkali process

2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂

Electrolysis of brine

Plaster of Paris preparation

CaSO₄·2H₂O →(373K) CaSO₄·½H₂O + 1½H₂O

Gypsum → POP

Setting of POP

CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O

POP + water → gypsum (hard)

⚠️ Common Mistakes

✗ Wrong: Saying 'acid turns litmus red'

✓ Correct: Be specific: acid turns BLUE litmus red. Red litmus shows no change in acid.

✗ Wrong: Confusing NaHCO₃ (baking soda) with Na₂CO₃ (washing soda)

✓ Correct: NaHCO₃ = baking soda (hydrogen carbonate). Na₂CO₃·10H₂O = washing soda (carbonate with 10H₂O).

✗ Wrong: Writing CaSO₄ as the formula of plaster of Paris

✓ Correct: POP is CaSO₄·½H₂O (hemihydrate). Gypsum is CaSO₄·2H₂O (dihydrate). Anhydrous CaSO₄ is different.

✗ Wrong: Adding water to concentrated acid during dilution

✓ Correct: Always add acid to water slowly. Adding water to acid causes violent spattering due to extreme heat.

✗ Wrong: Mixing up bee sting and wasp sting treatment

✓ Correct: Bee = acidic → apply baking soda (base). Wasp = basic → apply vinegar (acid). B→B, W→V.

📝 Exam Focus

These questions are frequently asked in CBSE exams:

Chemical properties of acids (5 reactions with equations)
5m★★★
pH scale and importance in everyday life
3m★★★
Preparation and uses of baking soda, washing soda, POP
5m★★★
Chlor-alkali process with products and uses
3m★★★
Why dry HCl does not change dry litmus
1m★★★
CuSO₄·5H₂O heating activity and water of crystallisation
3m★★
Bee sting vs wasp sting treatment (pH application)
2m★★
Difference between base and alkali with examples
2m★★

Diagram to practice: Practice drawing: pH scale with colours, chlor-alkali process setup, electrolysis of brine, reaction of acid with metal/carbonate apparatus

🎯 Last-Minute Recall

Close your eyes and try to recall: Key definitions, formulas, and 3 common mistakes. If you can recall 80% without looking, you're exam-ready!