UPSC Prelims 2001 Q120: Science & Technology – Chemistry
An aqueous solution of copper sulphate is acidic in nature because the salt undergoes
- (a) dialysis
- (b) electrolysis
- (c) hydrolysis
- (d) photolysis
Answer: (c) hydrolysis
Explanation
Why an aqueous solution of copper sulphate is acidic
| Process | Meaning |
|---|---|
| Hydrolysis | Reaction of a salt's ions with water — Cu²⁺ ions take OH⁻ from water, leaving H⁺ |
| Copper sulphate | Salt of a weak base (Cu(OH)₂) and a strong acid (H₂SO₄) |
| Electrolysis | Breaking down by electric current |
| Dialysis / photolysis | Separation through a membrane / breakdown by light |
Copper sulphate comes from a weak base and a strong acid. In water the Cu²⁺ ions undergo hydrolysis, producing H⁺ ions, so the solution is acidic (pH below 7).
Answer: (c) hydrolysis.
Why each option is right or wrong
- (a) Incorrect. Dialysis separates particles through a membrane.
- (b) Incorrect. Electrolysis needs an electric current.
- (c) CORRECT. Hydrolysis of Cu²⁺ ions makes the solution acidic.
- (d) Incorrect. Photolysis is breakdown by light.
Key point
Salt of strong acid + weak base = acidic (CuSO₄, NH₄Cl); strong base + weak acid = basic (Na₂CO₃); strong + strong = neutral (NaCl).
Remember
CuSO₄ in water is acidic by hydrolysis.
Common mistakes
- Choosing electrolysis because copper sulphate is used in electrolysis.
- Thinking all salt solutions are neutral.
- Not knowing which acid and base form the salt.
Exam tip
Ask which parent (acid or base) is stronger; the solution takes its side.
Sources
- NCERT Class X — Science, chapter 'Acids, Bases and Salts'
- NCERT Class XI — Chemistry Part I, chapter 'Equilibrium' (hydrolysis of salts)