Conductivity Evidence for Bonding Type in Three Unknown Chlorides
Three white crystalline solids, labelled X, Y and Z, are supplied with no other information. A student tests each: (i) melting point, (ii) electrical conductivity of the pure solid, (iii) conductivity of the molten liquid, and (iv) conductivity of the aqueous solution. Results: X melts at 801 °C, does not conduct as a solid, conducts when molten and in water. Y melts at 78 °C (sublimes readily), does not conduct as a solid, does not conduct when molten, but its aqueous solution conducts and gives a white precipitate with silver nitrate that dissolves in ammonia. Z melts at 714 °C, does not conduct as a solid, conducts when molten, and its aqueous solution conducts but gives no precipitate with silver nitrate. The student concludes all three are ionic because their aqueous solutions conduct.
1. Evaluate the student's conclusion for each substance and assign the most likely bonding type, justifying with the conductivity and melting-point evidence.
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The student's reasoning is flawed: conductivity of an aqueous solution only shows that mobile ions exist in water - it does not prove the solid was ionic. Dissolving a molecular covalent substance that reacts with water can also produce ions.
X - high melting point (801 °C), non-conducting solid, conducting melt and solution: classic ionic behaviour. Ions are locked in the lattice as a solid but mobile when molten or dissolved. Likely an ionic chloride such as NaCl.
Y - low melting/sublimation temperature, non-conducting solid and non-conducting melt, but conducting solution with a AgCl-type precipitate: this is a simple molecular covalent chloride. The melt does not conduct because there are no free ions; the solution conducts because hydrolysis produces ions (e.g., HCl). Consistent with a covalent chloride like one that hydrolyses.
Z - high melting, conducting melt, but no chloride precipitate: ionic, but the anion is not chloride. The absence of a AgNO3 precipitate means the anion is something else (e.g., a sulfate or nitrate). So Z is ionic but was mislabelled as a chloride.
The decisive test is conductivity of the pure molten substance, not the solution.
2. Explain why the molten-state conductivity test is more diagnostic than the solution test, using the concept of mobile charge carriers.
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Electrical conduction in these solids requires mobile charged particles. In an ionic solid, ions are held in fixed lattice positions and cannot migrate, so the solid does not conduct. On melting, the lattice breaks down and the ions become free to move, so the liquid conducts. This molten-state conductivity therefore directly demonstrates the presence of ions that were present in the solid lattice - it is a property of the substance itself.
In contrast, an aqueous solution introduces a new component: water. Water can react with a covalent molecular substance (hydrolysis) to generate ions that were not present in the original solid. For example, a covalent chloride reacting with water produces hydrated protons and chloride ions, so the solution conducts even though the pure substance was molecular. Thus solution conductivity conflates the substance's own structure with its reaction with the solvent.
Because the molten test removes the solvent variable and directly probes whether the pure substance dissociates into mobile ions, it distinguishes ionic from molecular covalent solids far more reliably.