Q.The aqueous solution of sugar does not conduct electricity. However, when sodium chloride is added to water, it conducts electricity. How will you explain this statement on the basis of ionisation and how is it affected by concentration of sodium chloride?

The key idea is Molar Conductivity — the conducting power of all ions produced from one mole of electrolyte dissolved in a given volume.

1. Sugar (sucrose) is a covalent molecule; it dissolves in water as neutral molecules. No ions are formed, so there are no charge carriers — the solution does not conduct electricity.

2. Sodium chloride is an ionic compound. In water it ionises completely into Na⁺ and Cl⁻ ions. These free-moving ions carry charge, so the solution conducts. …

The Core Idea: Why Some Solutions Conduct and Others Don’t

Electricity in a solution is carried by ions — charged particles that can move under an electric field. A substance that dissolves without forming ions cannot conduct, no matter how much you dissolve. Sugar (sucrose, C₁₂H₂₂O₁₁) is a covalent molecule; when it dissolves, it simply disperses as neutral molecules. No ions, no conduction.

Sodium chloride (NaCl), on the other hand, is an ionic crystal. In water, the polar water molecules pull the Na⁺ and Cl⁻ ions apart from the lattice. This process is called ionisation (or, more precisely, dissociation). Once free, these hydrated ions can drift toward oppositely charged electrodes, completing the circuit.

Step-by-Step Explanation

1. Ionisation of NaCl in Water

When NaCl is added to water, the following dissociation occurs:

Each formula unit yields one Na⁺ and one Cl⁻. Because NaCl is a strong electrolyte, this dissociation is essentially complete at all moderate concentrations — every dissolved NaCl molecule becomes two ions.

2. Why Sugar Does Not Conduct

Sugar molecules are held together by covalent bonds. When sugar dissolves, the molecules separate from each other but the bonds within each molecule remain intact. No charged species are produced:

The solution contains only neutral sugar molecules and water. Without ions, there is nothing to carry charge — so the solution is an insulator.

3. Effect of NaCl Concentration on Conductivity

Conductivity depends on two factors: the number of ions per unit volume and how fast they can move. As you change the concentration of NaCl, both factors change.

(a) At higher concentration

More NaCl means more Na⁺ and Cl⁻ ions per unit volume. You might expect conductivity to keep rising linearly — but it doesn’t. The ions are crowded together, and their mutual electrostatic attractions (ion–ion interactions) slow them down. Each ion is surrounded by an ionic atmosphere of opposite charge, which drags on it as it moves. The result: molar conductivity ($\Lambda_m$) — the conductivity per mole of electrolyte — decreases as concentration increases.

(b) At lower concentration (dilution)

As you dilute the solution, the ions spread farther apart. The ionic atmosphere weakens, and the drag on each ion reduces. Ions move more freely, so molar conductivity increases. At infinite dilution, ions move independently of each other, and $\Lambda_m$ reaches a maximum value called $\Lambda_m^\infty$ (limiting molar conductivity).