JEE Main 2020 · 9 January · evening · Single correct · +4 / −1
Amongst the following, the form of water with the lowest ionic conductance at 298 K is :
- (a) saline water used for intravenous injection
- (b) sea water
- (c) distilled water
- (d) water from a well
Answer and solution
Answer (c) distilled water
Solution
Chemistry ma’am · 3 steps
Conductance comes from dissolved ions, so the water with the fewest ions conducts least.
- What carries current. In water, current is carried by ions moving through the liquid. More dissolved ions means higher conductance.
Form of water Dissolved ions Conductance Saline for injection about 0.9% NaCl, so plenty of Na⁺ and Cl⁻ high Sea water about 3.5% salts, the most ions here highest Distilled water almost none, only H⁺ and OH⁻ from self-ionisation, each about 10⁻⁷ mol L⁻¹ lowest Well water dissolved minerals such as Ca²⁺, Mg²⁺, HCO₃⁻ moderate - Why distilled is lowest. Distillation leaves the dissolved salts behind. What remains are the very few ions from H₂O ⇌ H⁺ + OH⁻, far too few to carry much current.
- So. Distilled water has the lowest ionic conductance at 298 K, option (c).
The idea to keep. Pure water is a very poor conductor. Its conductance comes almost entirely from dissolved salts, so ranking waters by conductance means ranking them by how much salt they hold.
Answer from the NTA final answer key. Solution by PrepWiser.