How does conductivity and molar conductivity vary with decrease in concentration of solution?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Conductivity decreases and molar conductivity increases.
Conductivity (κ) is the conductance of a unit volume of solution, placed between electrodes of unit area kept a unit distance apart. It depends on the number of ions present in that unit volume.
Molar conductivity (Λm) is the conductance of the volume V of solution that contains one mole of electrolyte:
\( \displaystyle \Lambda_m=\frac{\kappa}{c}=\kappa\,V \)
On dilution, conductivity decreases while molar conductivity increases. For a strong electrolyte the rise in Λm is slow, because interionic attractions become weaker; for a weak electrolyte it is steep at low concentration, because the degree of dissociation increases.
Why the other options are incorrect
Option (1) Conductivity and molar conductivity both decrease. — Incorrect. Conductivity does decrease, but molar conductivity increases on dilution.
Option (2) Conductivity increases and molar conductivity decreases. — Incorrect. This is the change that happens when concentration increases, not decreases.
Option (4) Conductivity and molar conductivity remain unchanged. — Incorrect. Both depend on concentration, so both change on dilution.
NCERT (2026-27 edition):
Variation of Conductivity and Molar Conductivity with Concentration – “Both conductivity and molar conductivity change with the concentration of the electrolyte. Conductivity always decreases with decrease in concentration both, for weak and strong electrolytes. This can be explained by the fact that the number of ions per unit volume that carry the current in a solution decreases on dilution. … Molar conductivity increases with decrease in concentration. This is because the total volume, V, of solution containing one mole of electrolyte also increases. It has been found that decrease in κ on dilution of a solution is more than compensated by increase in its volume.”