Classifying Four Solids from Melting Point and Conductivity
Four solids are tested. W: white crystals, melting point 801 °C; does not conduct when solid; dissolves readily in water; the solution conducts. X: colorless crystals, melting point 43 °C; does not conduct as a solid or liquid; insoluble in water; soft and easily crushed. Y: silvery solid, melting point 1085 °C; conducts in both solid and liquid states; malleable. Z: hard, brittle solid, melting point 1710 °C; does not conduct in solid or liquid state; insoluble in water. A student classifies W as metallic because its solution conducts, X as ionic because it melts below 100 °C, Y as covalent network because it is malleable, and Z as ionic because it has a high melting point. The teacher says every classification is wrong. The tests were done on pure samples; the liquid-state conductivity test for X was repeated and gave the same result.
1. Classify each solid and identify the error in each of the student's four assignments.
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W: ionic. High melting point, water solubility, nonconducting solid, and conducting solution are the signature of a salt; mobile ions appear only after dissolution. X: molecular covalent. Low melting point, softness, and no conductivity in either state fit discrete molecules held by weak intermolecular forces. Y: metallic. Conducts as a solid and liquid and is malleable, which requires delocalized electrons and non-directional bonding. Z: covalent network. Very high melting point, hardness, brittleness, and no conductivity fit a continuous covalent lattice. The errors are: solution conductivity does not make W metallic; low melting point does not make X ionic; malleability does not make Y a covalent network; and a high melting point alone does not make Z ionic.
2. Explain at the particle level why solid W does not conduct but its aqueous solution does, and why Y conducts in both the solid and liquid states.
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In solid W the ions are held in fixed lattice positions, so there is no long-range motion of charge; the solid is an insulator. When W dissolves, water separates the ions and they become mobile, so the solution conducts. In Y, the metal lattice has delocalized valence electrons that can move throughout the solid; melting disrupts the ordered lattice but keeps the electron sea, so both solid and liquid metal conduct. This is why conductivity alone does not prove a substance is ionic: the state of matter and the charge carrier matter. The conducting species in W's solution are ions; in Y they are electrons.
3. Predict whether melted W and melted Y would conduct electricity, and explain how a melt-conductivity test would distinguish X from the others.
Read the worked answer
Molten W should conduct because the ionic lattice collapses into a liquid containing mobile ions; ionic melts are electrolytes. Molten Y should also conduct because metallic bonding persists in the liquid and delocalized electrons remain mobile. Molten X should not conduct appreciably because it consists of neutral molecules, not free ions or electrons. A melt-conductivity test therefore separates molecular X from ionic W and metallic Y, but it does not separate W from Y; for that, one would use other evidence such as malleability, solubility behavior, or the nature of the charge carriers.