Which of the following liquids is a bad conductor of electricity?

Question

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Correct — D, None of the above. All three named liquids conduct, so no bad conductor is on the list. A liquid conducts electricity only if it contains ions free to move, and NCERT's rule of thumb is that 'most liquids that conduct electricity are solutions of acids, bases and salts' — which is precisely what each of these three is. Salted water is the textbook case: dissolve common salt in distilled water and the sodium chloride separates into Na⁺ and Cl⁻ ions, and NCERT Class VIII states flatly that the resulting salt solution 'is a conductor of electricity'. It is the same chemistry that makes tap water conducting, since water from taps, hand pumps and wells carries dissolved mineral salts, and the same reason the chapter opens by warning against touching an electrical appliance with wet hands. Lemon juice is not a matter of inference at all — NCERT's Table 11.1, 'Good/Poor Conducting Liquids', is printed with its first row already filled in: lemon juice, compass needle deflects, GOOD CONDUCTOR. Lemon juice is roughly five per cent citric acid, and an acid in water releases H⁺ ions, which is exactly how current is carried through it. Orange juice is the same class of liquid — citric and ascorbic acid plus dissolved potassium and other mineral salts — and it is what the familiar fruit-battery demonstration runs on. With (a), (b) and (c) all conducting, the fourth option is not a fallback but the only answer that survives. One thing to hold honestly: this answer is ours, derived rather than taken from any published key, and its weakest link is orange juice, which NCERT never classifies by name — its status rests on its composition and on the fruit-battery evidence, not on a printed line.

Why the other options are wrong — (a) The strongest conductor on the list, not the weakest. Sodium chloride is an ionic compound: in the solid state its ions are locked in a lattice and it does not conduct, but dissolved in water the Na⁺ and Cl⁻ ions become free to migrate, carrying charge between the electrodes. NCERT builds the whole chapter on this contrast — distilled water is free of salts and is a poor conductor, and adding a pinch of common salt to it turns it into a good one. Candidates who pick this are usually thinking of pure water rather than salt water.  (b) Orange juice conducts, though less strongly than brine. It carries citric acid and ascorbic acid, both of which release hydrogen ions in water, together with dissolved potassium and other mineral salts — the same ingredients that make a citrus fruit work as the electrolyte in a fruit battery, where two dissimilar metal electrodes pushed into an orange or a lemon will drive a small current. The trap is a relative reading: because a torch bulb may not visibly glow in juice, candidates conclude it is not conducting.  (c) The one option NCERT rules out in print. Its Class VIII table of good and poor conducting liquids has lemon juice entered as the worked example — the compass needle deflects, and the classification given is 'Good Conductor'. Lemon juice is about five per cent citric acid by volume with a pH near 2, and NCERT Class X demonstrates separately that an acid solution conducts because the current is carried through it by ions. Choosing this option means contradicting the textbook line the question is drawn from.

Concept: Conduction in a metal and conduction in a liquid are different mechanisms. In a metal, current is carried by free electrons drifting through a fixed lattice of positive ions. In a liquid, there are no free electrons to speak of; current is carried by ions — charged atoms or groups of atoms — physically moving through the fluid, positive ions towards the negative electrode and negative ions towards the positive. A liquid therefore conducts only if something in it has dissociated into ions, which is why acids, bases and salts dissolved in water conduct while covalent substances that dissolve without ionising do not. NCERT demonstrates the contrast directly: dilute hydrochloric and sulphuric acid make the bulb glow, but 'glucose and alcohol solutions do not conduct electricity', because they release no ions. NCERT also cautions against treating the categories as absolute: 'under certain conditions most materials can conduct', which is why it prefers 'good conductor' and 'poor conductor' to 'conductor' and 'insulator'. Passing a current through a conducting solution is not passive, either — it drives chemical change at the electrodes, which is what electrolysis is.

State PCS 2023 Q19 — Science & Technology PYQ with Answer

State PCS 2023 Q19 — Science & Technology PYQ with Answer