Salts

• Salt — ionic compound from acid + base (H replaced by metal/ammonium).
• Common salts — NaCl, Na₂CO₃, NaHCO₃, CaSO₄, CuSO₄, etc.
• pH of salt solutions — strong acid + strong base → neutral; others may be acidic/basic.
• Bleaching powder, washing soda, baking soda, Plaster of Paris — preparation and uses.
• Water of crystallisation — fixed water in crystal lattice.

1. NaCl — table salt; from HCl + NaOH; neutral pH.
2. NaHCO₃ (baking soda) — mild base; antacid; CO₂ in baking: NaHCO₃ + acid → CO₂.
3. Na₂CO₃·10H₂O (washing soda) — cleaning; softens hard water.
4. Ca(OCl)Cl (bleaching powder) — from chlorine on slaked lime; disinfectant, bleaching.
5. CaSO₄·½H₂O (Plaster of Paris) — from gypsum heating; casts, smooth surfaces.
6. CuSO₄·5H₂O (blue vitriol) — water of crystallisation; turns white anhydrous on heating.
7. Acidic salt — e.g. NH₄Cl (from strong acid weak base).
8. Basic salt — e.g. Na₂CO₃ (from strong base weak acid).
9. pH test — universal indicator on salt solution predicts nature.
10. NCERT preparations — know reactants for baking soda, washing soda, bleaching powder.

Why is aqueous Na₂CO₃ solution basic?

Step 1 — Na₂CO₃ from NaOH (strong base) + H₂CO₃ (weak acid).
Step 2 — CO₃²⁻ hydrolyses: CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻.
Step 3 — OH⁻ increases → pH > 7 (basic).
Step 4 — Test with red litmus → turns blue.
Conclusion: salt of strong base + weak acid gives basic solution.

• Assuming all salts neutral (Na₂CO₃ and NH₄Cl are exceptions).
• Misconception: covalent compounds like sugar are salts.
• Confusing bleaching powder with baking soda.
• Forgetting water of crystallisation in CuSO₄·5H₂O.
• Mixing Plaster of Paris with too much water (speeds setting — exothermic).

• Name salt formed from H₂SO₄ and NaOH.
• Give one use each of baking soda and bleaching powder.
• What is water of crystallisation? Example?
• Would NH₄Cl solution be acidic or basic? Why?

Open the Practice tab for graded questions on Salts.