The conductivity of a strong electrolyte
Decreases on dilution
Conductivity decreases on dilution — option 1.
The key is the definition. Conductivity (specific conductance), symbol κ, is the conductance of a solution held between two electrodes one centimetre apart with an area of one square centimetre — that is, the conductance of one cubic centimetre of the solution. It therefore depends on how many ions are present in that fixed volume. Dilution spreads the same ions through more water, so a unit volume contains fewer of them, and κ falls.
| Quantity | Defined per | On dilution | Why |
|---|---|---|---|
| Conductivity, κ | Unit volume | Decreases | Fewer ions in each cubic centimetre |
| Molar conductivity, Λm | One mole of electrolyte | Increases | The same one mole of ions is carried by more solvent and moves more freely |
The two are related by
\(\Lambda_{m}=\dfrac{\kappa\times1000}{C}\)
where C is the concentration in mol/L. As C falls, κ falls too — but not as fast — so the ratio rises. Both statements are true at once, and confusing them is the commonest error in this topic.
The contrast between strong and weak electrolytes shows up in the molar conductivity :
| Strong electrolyte (KCl, NaCl, HCl) | Weak electrolyte (CH3COOH, NH4OH) | |
|---|---|---|
| Ionisation | Complete at all concentrations | Partial; degree of ionisation rises sharply on dilution |
| Λm on dilution | Rises slowly and linearly, because only ion-ion interference is relieved | Rises steeply near infinite dilution, because more molecules ionise |
| Λm° obtained by | Extrapolating the Debye-Hückel-Onsager plot of Λm against √C | Cannot be extrapolated; found from Kohlrausch’s law of independent migration of ions |
Hence, the answer is that conductivity decreases on dilution.
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