Which of the following substances does not conduct electricity?

Step 1: Analyze the question We need to find the substance that does not conduct electricity. That means we're looking for an insulator.

Step 2: Concept Electrical conduction needs free charge carriers:

• Free electrons (as in metals and graphite)
• Free mobile ions (as in electrolytes: acids, bases, salt solutions)

Substances without free electrons or mobile ions are insulators.

Step 3: Check each option

• A. Graphite: A non-metal that conducts. Each carbon atom bonds to only three others, leaving one delocalised electron per atom free to move. It conducts well (it is used for electrodes and in dry cells).
• B. Glass: A covalent/amorphous solid with no free electrons and no mobile ions at room temperature. It is a good insulator.
• C. Sulphuric acid: In solution it ionises into H⁺ and SO₄²⁻ ions, which carry charge. It is a strong electrolyte and conducts.
• D. Table salt solution: NaCl dissolved in water gives free Na⁺ and Cl⁻ ions, so it conducts.

Step 4: Answer Only glass lacks free charge carriers, so it does not conduct electricity.

Tip: Remember the "two carriers" rule: conductors have free electrons (metals, graphite) or free ions (electrolytes). Common insulators to memorise: glass, rubber, plastic, wood, dry air, and pure distilled water. Also remember that graphite is the classic exception among non-metals, and examiners love to test it.