Conductivity of Aqueous Solutions

Introduction

An aqueous solution conducts current when charged particles can move through it. More dissolved ions often increase conductivity, but the relationship is not a simple count of formula units. Ion charge, mobility, temperature and the geometry of the measuring cell also affect the observed current.

Core explanation

Place two electrodes in a salt solution and connect them to a low-voltage circuit. The external metal wires carry electrons. Within the liquid, positive ions migrate toward the electrode connected to the negative terminal, and negative ions migrate toward the positive electrode. These motions transport charge and complete the circuit. Without enough mobile ions, the liquid conducts poorly even if it contains a large mass of neutral dissolved molecules.

Concentration is one factor. Diluting a given salt solution usually reduces the number of charge carriers in a fixed volume, reducing its measured conductivity under comparable conditions. Yet doubling salt formula concentration need not exactly double conductivity because ion interactions and mobility can change with concentration. Different salts at equal formula molarity can produce different numbers of ions per formula unit; NaCl provides two ions in the simple model, while CaCl₂ provides three. Their ionic charges and mobilities also differ.

Temperature usually changes ion mobility and therefore conductivity. A warmer sample can show a different reading even if its chemical composition has not changed. A meaningful comparison either keeps temperature fixed or corrects for it using the instrument's defined method. Electrode spacing and area also affect the resistance of a test cell. A conductivity instrument uses a cell constant or calibration to report a material property rather than raw current through arbitrary geometry.

Conductivity alone cannot identify a dissolved substance. Several mixtures can yield similar readings. An ion-rich solution may contain Na⁺ and Cl⁻, other salts, acids or bases. A dissolved gas can also react with water and contribute ions. To identify a species, combine conductivity with chemical tests or an analytical instrument suited to that substance.

Pure water has a small intrinsic ion concentration and real water usually contains traces of dissolved material, so a classroom bulb test may show “off” or “very dim” rather than proving exactly zero current. Likewise, a weak electrolyte can carry current at a lower level than a strong electrolyte under matched conditions. The measurement is continuous, while classroom categories simplify interpretation.

Step-by-step reasoning

1. Identify ions available in the liquid and whether they are mobile. 2. Predict qualitative conductivity from ion amount, charge and mobility. 3. Control concentration, temperature, electrode geometry and applied conditions in comparisons. 4. Distinguish a raw circuit current from a calibrated conductivity value. 5. Avoid claiming chemical identity from conductivity alone.

Visual explanation

Draw a beaker with cations moving left and anions right between electrodes. Beside it draw two equal-size beakers with few and many ion symbols. Label both as ionic conductors but note that actual current also depends on mobility and apparatus.

Real-world analogy

Road traffic depends on both the number of vehicles and how easily they move. More vehicles can increase total movement, but crowding can slow each vehicle. Ion conduction likewise depends on number and mobility, though electrostatic transport follows different laws.

Real-world example

A water-treatment system may use a conductivity probe to monitor overall dissolved-ion changes. A rising reading can prompt further testing, but it does not specify which contaminant or salt increased.

Why?

Why can two equal-molar salt solutions have different conductivity? Their formulas can yield different ion counts and charges, and those ions can move at different rates in water.

Common misconception

“Conductivity directly gives the concentration of one named ion.” A probe responds to all mobile charge carriers and cell conditions, so specific-ion concentration needs more information.

Worked example

Compare ideal 0.10 M NaCl and 0.10 M CaCl₂. The first provides formal 0.10 M Na⁺ and 0.10 M Cl⁻; the second provides 0.10 M Ca²⁺ and 0.20 M Cl⁻. CaCl₂ gives more formal ions per litre in this model, but an exact conductivity ratio cannot be computed from those counts alone because mobility and interactions matter.

Quick check

1. What carries current through the solution between electrodes? Answer: Mobile positive and negative ions carry charge through the liquid, while electrons carry current in the external metal wires.

Exam focus

Explain charge carriers and state controlled variables in a fair comparison. Do not equate brightness of a bulb with a unique chemical concentration.

Advanced insight

Molar conductivity can vary with concentration, partly because ions interact. Conductivity meters often calibrate with a known standard solution and report carefully temperature-compensated results.

Summary

Aqueous conductivity arises from mobile ions. Their amount, charge and mobility matter, as do temperature and apparatus geometry. It is useful for monitoring overall ionic content but not for identifying a unique solute.

Practice questions

1. Why does a sugar solution conduct poorly compared with a salt solution? Answer: Sugar molecules are mainly neutral, while a soluble salt supplies mobile ions that transport electrical charge. 2. Why control temperature in a conductivity comparison? Answer: Temperature changes ion mobility and can change the reading even if the solution composition stays the same. 3. Can equal formula molarity guarantee equal conductivity for NaCl and CaCl₂? Answer: No. Ion numbers, charges, mobility and interactions differ, so the measured values need not match.