Four unlabelled solids and a colleague with two rules
A school laboratory is clearing out a store cupboard and finds four unlabelled white solids, W, X, Y and Z. The technician records the data below.
| Solid | Melting point / °C | Solubility in water | Conductivity of solid | Conductivity of melt or of solution |
|---|---|---|---|---|
| W | 801 | 36 g per 100 g of water | none | melt conducts; solution conducts |
| X | 122 | very soluble | none | solution conducts weakly |
| Y | 1713 | insoluble | none | none |
| Z | 78 (sublimes) | slightly soluble | none | none |
The solids are needed for a lesson on ionic and covalent bonding, so each must be classified before use. A colleague insists on two rules: any solid melting above 1000 °C must be ionic, and any solid that dissolves in water must contain ions. On that basis the colleague wants to label Y and X as ionic and wants to discard W as 'obviously wrong'. None of the four solids is a metal, and no further chemical tests may be done before the lesson.
1. Assign each solid to a bonding type and justify each assignment with at least two independent observations from the table.
Read the worked answer
W - ionic. Melting point 801 °C shows a strong lattice; the solid does not conduct but both the melt and the solution do, which is exactly the signature of ions that are locked in place in the lattice and become mobile only when it is broken or dissolved. Solubility plus conductivity of the solution together are decisive; solubility alone would not be.
X - simple molecular (hydrogen-bonded). Low melting point, no conduction as a solid or as a pure liquid, so no free ions and no delocalised electrons. The weak conductivity of its solution must come from partial ionisation or reaction with water - behave like a weak acid - not from an ionic lattice.
Y - giant covalent. Very high melting point and hardness with no conduction in any state: no mobile ions (rules out ionic) and no delocalised electrons (rules out metallic). A network of strong covalent bonds must be broken to melt it, hence 1713 °C.
Z - simple molecular. Sublimes at 78 °C and never conducts: only weak intermolecular forces hold the particles together.
2. Evaluate the colleague's two rules using X and Y specifically, and state which single piece of evidence most strongly contradicts the 'Y is ionic' label.
Read the worked answer
Both rules fail as tests for ionic bonding.
Melting point measures the energy needed to separate the particles. Ionic lattices and giant covalent networks both need many strong bonds broken, so both melt high; 1713 °C is equally consistent with a network solid such as SiO2. The rule also fails in the other direction: some ionic solids melt or sublime low, for example AlCl3 (about 180 °C) because it forms molecular Al2Cl6 dimers with strong covalent character, while some molecular solids melt above 100 °C purely because of hydrogen bonding, as X does at 122 °C.
Solubility is equally unreliable. Many molecular solids that can hydrogen-bond to water (sugars, urea, X) dissolve freely, and many ionic solids are effectively insoluble (AgCl, BaSO4, CaCO3) because the lattice energy is not recovered by hydration.
The decisive evidence against Y being ionic is that the molten solid does not conduct. Any ionic solid must conduct when molten, because the ions become free to move. That single observation overrides the melting point.
3. Describe one further test, using only heating and simple apparatus, that would confirm the assignment of W but not that of Y, and state the expected observations.
Read the worked answer
Heat a small sample of each solid strongly in a crucible with a Bunsen burner and, once molten, dip two graphite electrodes connected to a low-voltage supply and a lamp into the liquid.
For W, the solid melts at about 800 °C to a mobile liquid. The lamp lights and gas bubbles appear at both electrodes: at the cathode the metal is deposited (a grey bead of sodium, if W is a sodium salt) and at the anode a non-metal is released (chlorine, detected by its smell or by damp indicator paper). This electrolysis is only possible if the melt contains mobile ions, so it is positive proof of ionic bonding.
For Y, no such test is possible: a giant covalent solid does not melt in a Bunsen flame (about 1500 °C maximum) and does not conduct, so no electrolysis occurs. The absence of a melt, rather than a positive result, is what supports the network assignment.
Note that this test adds no new information for W beyond the table - it confirms the existing conductivity evidence by showing the charge carriers really are ions.