Identifying Four Unlabelled Solids from Property Data
A technician has four unlabelled white or shiny solids and a list of tests carried out on each. A: melting point 801 °C; does not conduct when solid, conducts when molten; dissolves in water and the solution conducts. B: melting point about 186 °C, with decomposition; does not conduct solid or molten; dissolves in water but the solution does not conduct; burns with a smoky yellow flame. C: melting point about 1710 °C; does not conduct solid or molten; insoluble in water; scratches glass. D: melting point 1083 °C; conducts both solid and molten; insoluble in water; malleable and ductile. The four stocks are known to be sodium chloride, sucrose, silicon dioxide and copper, but the labels have been lost. The technician also needs a crucible in which to melt sodium chloride at about 850 °C, and a material for the electrodes in an electrolysis demonstration.
1. Match each solid to its identity, using at least two pieces of evidence for each and explaining the conductivity pattern of A.
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A is sodium chloride. The high melting point, conduction when molten but not when solid, and conduction by its aqueous solution are the signs of a giant ionic lattice. In the solid the ions are held in fixed positions and cannot move; when molten, or when dissolved, the ions are free to move and carry charge, so conduction occurs.
B is sucrose. Low melting point with decomposition, no conduction in the solid, molten or dissolved state, solubility in water, and a smoky yellow flame are signs of a simple molecular solid. Dissolving separates neutral molecules, not ions, so the solution does not conduct.
C is silicon dioxide. Very high melting point, no conduction in any state, insolubility and hardness (scratching glass) indicate a giant covalent lattice.
D is copper. Shiny, malleable and ductile, conducting in both solid and molten states, with a high melting point, identifies a metal with delocalised electrons.
2. Explain why silicon dioxide has a very high melting point yet does not conduct, and why sucrose melts at a much lower temperature.
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Silicon dioxide has a giant covalent structure: each silicon atom is bonded to four oxygen atoms and each oxygen to two silicon atoms, and these strong covalent bonds extend throughout the crystal. Melting means breaking many strong covalent bonds rather than separating small units, so the melting point is very high. There are no free ions and no delocalised electrons, so it does not conduct, even when molten, because breaking the bonds does not produce mobile charge carriers.
Sucrose has a simple molecular structure. Within each molecule the atoms are joined by strong covalent bonds, but separate molecules are held to one another only by weak intermolecular forces. Melting breaks these weak forces, not the covalent bonds, so the melting point is low and the molecules survive intact. That is also why neither the solid nor its solution conducts.
3. Choose the most suitable of the four materials for the crucible and for the electrodes, justifying each choice and stating one limitation.
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Crucible: C, silicon dioxide. Its melting point (about 1710 °C) is far above the 850 °C needed, and unlike D it will not melt or soften in the flame. Limitation: it is brittle and can crack under thermal shock, so it must be heated gradually; molten sodium chloride can also attack it slowly, so it is not ideal for long use.
Electrodes: D, copper. It is the only electrical conductor among the four and is ductile enough to be shaped and connected. Limitation: copper is not inert. At the anode it is oxidised (Cu → Cu²⁺ + 2e⁻) and dissolves, contaminating the electrolyte and eroding the electrode, so it is only really suitable as the cathode. Graphite or platinum would be the proper choice, but neither is available here.