Q.Why does the conductivity of a solution decrease on dilution of the solution?

An electrolyte dissolved in a solvent such as water dissociates into free-moving cations and anions, and it is exactly these free ions that let the solution conduct an electric current when a field is applied — a fundamentally different conduction mechanism from the free-electron conduction of a metal. The conductivity cell — two platinum electrodes of area A, separated by distance l, immersed in the solution — obeys Ohm's law at constant temperature: $V = IR$, where R is the solution's resistance in ohms.

From this single relationship, three related quantities are built up. Resistivity (specific resistance), $\rho$, is defined through $R = \rho(l/A)$, and is the resistance of an electrolyte of unit length and unit cross-sectional area, with SI unit Ωm. Conductance, $C = 1/R$, is measured in siemens (S). Conductivity (specific conductance), $\kappa = 1/\rho$, is the conductance of exactly one cubic metre of the solution, with SI unit S m⁻¹ — a physical property of the solution itself, independent of the particular cell geometry used to measure it.

Conductance is genuinely sensitive to physical conditions: it falls as interionic attraction rises (more concentrated solutions, stronger attraction), rises with the solvent's dielectric constant (better charge screening), falls as viscosity rises (more drag on ion motion), and rises with temperature (faster-moving, less-attracted ions). Because a solution's raw conductance also depends on the amount of electrolyte present, comparing different electrolytes fairly requires standardising the amount considered — which is exactly the motivation for molar and equivalent conductivity, covered in the companion concept on Kohlrausch's law.

In practice, κ is measured using a Wheatstone-bridge circuit, but with one essential change from the metallic-wire version: an AC source (550 Hz–5 kHz) must replace the usual DC supply, because DC current would electrolyse the very solution being measured. The bridge is balanced (P·Q = R·S) to find the cell's resistance R, and the cell constant l/A — rarely measured directly — is instead calibrated once against a KCl solution of known κ and reused for all subsequent measurements with that same cell.