Question

How the conductivity varies on decreasing concentration for both weak and strong electrolytes ?

• A. It increases for weak electrolyte and decreases for strong electrolyte.
• B. It decreases for weak electrolyte and increases for strong electrolyte.
• C. It increases for both weak and strong electrolytes.
• D. It decreases for both weak and strong electrolytes.

Hint:

Conductivity depends on ions per unit volume, which falls on dilution for both types. Only molar conductivity increases.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Conductivity (\( \kappa \)) is the conductance of a solution of unit length and unit area.
It depends on the number of ions present in each unit volume of the solution.
Molar conductivity (\( \Lambda_m \)) is different and is defined as \( \kappa \times 1000 / c \).

Step 2: Effect of dilution on the number of ions:
When we decrease the concentration, we add more solvent.
The number of ions in a fixed volume goes down.

Step 3: Strong electrolytes:
A strong electrolyte is fully ionised at all concentrations.
On dilution no new ions form, but the ions per unit volume fall, so \( \kappa \) decreases.

Step 4: Weak electrolytes:
On dilution the degree of dissociation rises, so more ions form from the same amount of solute.
But the solute itself is spread over a bigger volume, and the fall in concentration outweighs the gain in dissociation. So the ions per unit volume still fall and \( \kappa \) decreases.

Step 5: Check the options:
(A), (B) and (C) say that conductivity increases for at least one type of electrolyte. This is wrong, because it is molar conductivity that rises on dilution, not conductivity.

Final Answer:
Conductivity decreases on dilution for both weak and strong electrolytes, option (D).
\[ \boxed{\text{decreases for both}} \]