Four unlabelled solids: which evidence is actually decisive?
Four solids are supplied unlabelled, each in a labelled vial, with the following data collected by a previous class:
A: melting point 660 °C; conducts electricity in the solid state; can be hammered into thin sheets; insoluble in water. B: melting point 801 °C; does not conduct as a solid; conducts when molten; dissolves in water to give a conducting solution; shatters when struck. C: sublimes above 3500 °C; conducts in the solid state; feels slippery and flakes when pressed with a spatula; insoluble in water and in hexane. D: melting point 114 °C; does not conduct in any state; dissolves in hexane but not in water; vapour irritates the eyes.
A student is asked to classify each solid as metallic, ionic, simple molecular or giant covalent. He argues that C cannot be giant covalent because it conducts electricity, and that A must be ionic because, like B, it conducts electricity. He also claims that the low melting point of D proves its bonds are weak.
1. Classify A, B, C and D, and for each state which observation is decisive and which observations are only consistent with the classification.
Read the worked answer
A: metallic. Decisive: malleability (layers of ions slide without breaking the bonding). Conductivity as a solid and the moderate melting point are consistent with a metal, but they are not decisive on their own, since graphite behaves similarly in those two respects.
B: ionic. Decisive combination: does not conduct when solid, conducts when molten, and gives a conducting aqueous solution. This is the signature of a lattice of fixed ions that only become mobile when the lattice is broken. High melting point and brittleness are consistent: the lattice resists melting and shatters when layers are forced past one another because like charges meet.
C: giant covalent (graphite). Decisive: extremely high sublimation temperature plus insolubility in every solvent plus the slippery, flaky feel of layers sliding. Conductivity in the solid state is consistent but does not exclude a network solid.
D: simple molecular (iodine). Decisive: low melting point, no conductivity in any state, solubility in a non-polar solvent. Non-conduction is genuinely decisive here only because it is combined with a low melting point, which shows there are no ions or delocalised electrons that could be freed.
2. Evaluate the student's two claims: that C cannot be giant covalent because it conducts, and that A must be ionic because it conducts.
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Claim 1 is wrong. Conductivity requires mobile charged particles, not free electrons specifically; in graphite each carbon is bonded to three others with three sigma bonds and contributes one electron to a delocalised pi system spread over each layer. Those delocalised electrons carry current in the plane of the layer. Giant covalent structures are therefore not automatically insulators (diamond and silicon dioxide are, graphite is not). The feature that actually separates A from C is mechanical: A is malleable, whereas C flakes and cleaves because the layers are held to one another only by weak dispersion forces.
Claim 2 is also wrong, and for the same reason: conductivity is not diagnostic by itself. A conducts as a solid, which is inconsistent with an ionic lattice, where ions are locked in position and cannot migrate. Ionic solids conduct only when molten or dissolved. Adding the malleability evidence settles it: ionic crystals are brittle, metals are not.
The general lesson is that no single property identifies a structure type. A defensible classification needs at least one item of evidence about mobile charge carriers and one about the strength and directionality of the bonding.
3. Explain why C conducts electricity while diamond does not, and predict and justify what happens to the electrical conductivity of B as it is heated from 25 °C to 850 °C.
Read the worked answer
In graphite, each carbon forms three sigma bonds (approximately sp² hybridised) and the fourth valence electron occupies a p orbital perpendicular to the layer. These p orbitals overlap to give a delocalised band extending over the layer, so electrons are free to move and carry current. In diamond, each carbon forms four sigma bonds to four neighbours in a tetrahedral network; all four valence electrons are localised in bonding molecular orbitals, so there is a large energy gap to any mobile state and the solid is an insulator.
For B: at 25 °C, conductivity is essentially zero, because the ions are held in fixed positions in the lattice and only a negligible number of defects allow ion migration. On heating towards 801 °C the conductivity rises slightly as lattice defects become more mobile, then at the melting point the lattice collapses and the ions become free to migrate. Conductivity jumps sharply at the melting point and the molten liquid conducts well. This melting-point discontinuity in conductivity is the classic evidence for an ionic structure, and it can be demonstrated by connecting the sample in series with a lamp and a power supply.