Which one of the following solutions is not capable of conducting electricity?
- (a)Copper sulphate
- (b)Sodium chloride
- (c)Sugar
- (d)Sodium hydroxide
Correct — C, Sugar. A solution conducts only if it contains mobile charged particles, and in solution that means ions. Sugar (sucrose) is a covalent, molecular compound — it dissolves freely in water because it hydrogen-bonds to it, but the molecules stay whole and no ions are set free, so a sugar solution carries essentially no current. The other three are electrolytes that break into ions the moment they dissolve.
- (a)Copper sulphate — Copper sulphate is an ionic salt that dissociates into Cu²⁺ and SO₄²⁻ in water. Its solution conducts well, which is exactly why acidified copper sulphate is the electrolyte used in copper refining and electroplating.
- (b)Sodium chloride — Common salt dissociates completely into Na⁺ and Cl⁻ in water. Brine is the textbook conducting solution — it is also why salty water is more dangerous around live wiring than pure water.
- (d)Sodium hydroxide — Sodium hydroxide is a strong base and ionises completely into Na⁺ and OH⁻, giving a strongly conducting solution.
Conduction in a metal is carried by free electrons; conduction in a solution is carried by ions drifting towards the electrodes. Substances that produce ions in water — salts, acids and alkalis — are electrolytes and conduct. Substances whose particles dissolve as intact neutral molecules are non-electrolytes and do not, however concentrated the solution. Sugar, alcohol, urea and glucose are the standard non-electrolytes.
Read the option list as a bonding question rather than an electricity question. Three of the four are ionic or fully ionising compounds; sugar is the only covalent molecular substance, so it is the only one that dissolves without producing ions. Two related facts guard against over-generalising in either direction — solid sodium chloride does NOT conduct, because its ions are locked in the crystal lattice until it is melted or dissolved, and pure distilled water conducts very poorly, since tap water owes its conductivity entirely to dissolved salts.
- Conduction in solution is by mobile ions, so a solution with no ions carries no current.
- Salts, strong acids and strong bases are electrolytes; sugar, alcohol, glucose and urea are non-electrolytes.
- Solid sodium chloride does not conduct because its ions are fixed in the lattice; molten or dissolved, it conducts freely.
- Distilled water is a very poor conductor — ordinary water conducts because of dissolved salts.
- Weak electrolytes such as acetic acid ionise only partly, so they conduct, but far less than a strong acid of the same concentration.
Only the covalent molecular solute fails to release ions — sugar, option (c).
- Assuming anything that dissolves in water must conduct.
- Forgetting that solid sodium chloride does not conduct even though its solution does.
- Treating glucose or alcohol as conducting because they contain hydrogen and oxygen.
Usually as an odd-one-out among four solutions, sometimes reversed into 'which of these conducts', and occasionally through an electrolysis or electroplating setting.
In a dry cell (battery), which of the following are used as electrolytes?
- (a) Ammonium chloride and Zinc chloride
- (b) Sodium chloride and Calcium chloride
- (c) Magnesium chloride and Zinc chloride
- (d) Ammonium chloride and Calcium chloride
Answer(a) Ammonium chloride and Zinc chloride
The same electrolyte idea in a device. A cell works only because its paste contains ionising salts — the property this NDA item tests by asking which solution lacks it.
Which one of the following is a covalent compound?
- (a) Calcium oxide
- (b) Sodium nitride
- (c) Silicon carbide
- (d) Zinc sulphide
Answer(c) Silicon carbide
The bonding half of the same reasoning, from the earlier session of the same year. Sugar fails to conduct in this item precisely because it is covalent and molecular rather than ionic.
In electrolytic refining of copper, the electrolyte is a solution of
- (a) acidified copper chloride.
- (b) acidified copper sulphate.
- (c) potassium chloride.
- (d) sodium sulphate.
Answer(b) acidified copper sulphate.
Copper sulphate appears in both papers for the same reason — it dissolves into ions and therefore conducts, which is what makes it usable as an electrolyte and what rules it out as the answer here.
- practice — not a real PYQ
Which one of the following will NOT conduct electricity?
- (a)Molten sodium chloride
- (b)Dilute hydrochloric acid
- (c)Solid sodium chloride
- (d)Copper sulphate solution
Answer(c) Solid sodium chloride — its ions are locked in the crystal lattice and cannot move until it melts or dissolves.
- practice — not a real PYQ
An aqueous solution conducts electricity because it contains
- (a)free electrons
- (b)free ions
- (c)free atoms
- (d)free neutrons
Answer(b) free ions — the ions drift towards the electrodes and carry the current.