Classifying Solids by Particles and Intermolecular Forces
Three unlabeled white solids are tested in a lab. Solid A melts at 801 °C, dissolves in water, and the resulting solution conducts electricity; when A is melted with a Bunsen burner, the melt also conducts. Solid B melts with decomposition near 186 °C, dissolves readily in water, but its aqueous solution does not conduct. Solid C melts at 69 °C, does not dissolve in water, dissolves in hexane, and neither its solid nor its hexane solution conducts. The student labels A as ionic, B as a polar molecular solid, and C as a nonpolar molecular solid. The instructor asks her to defend the labels using the particles present and intermolecular forces, and to explain why solid A does not conduct while molten A does. The student also wonders why B dissolves in water but C does not, even though both are molecular solids with low melting points relative to A.
1. Evaluate the student's labels for A, B, and C. For each solid, identify the type of particle and the main force responsible for its properties.
Read the worked answer
A is ionic. The particles are cations and anions in a crystal lattice. The strong electrostatic attractions between oppositely charged ions explain the very high melting point and the brittle solid. The ions are not free to move in the solid, so solid A does not conduct. When melted, the ions become mobile and the melt conducts.
B is a polar molecular solid. The particles are molecules with polar O–H groups, so hydrogen bonding with water is possible. That explains why B dissolves in water. The molecules are neutral, so the aqueous solution does not conduct. The melting point is much lower than A's because intermolecular forces, not ionic bonds, must be overcome.
C is a nonpolar molecular solid. The particles are nonpolar molecules held together mainly by dispersion forces. It does not dissolve in water because it cannot form favorable hydrogen bonds with water, but it dissolves in nonpolar hexane. It does not conduct because there are no ions and no mobile charged particles.
2. Explain why solid A does not conduct electricity but molten A does. Include the role of charged particles.
Read the worked answer
Conduction requires mobile charged particles. In solid A, the ions are locked in fixed positions in the crystal lattice. They can vibrate, but they cannot move from place to place, so there is no steady flow of charge. That is why solid A does not conduct.
When A melts, the lattice breaks down and the ions become free to move throughout the liquid. The cations and anions can now migrate toward oppositely charged electrodes, carrying current. That is why molten A conducts.
Dissolving A in water has a similar effect: the ions separate and become hydrated, so the aqueous solution also conducts. The key is not whether the substance is 'charged' overall - the crystal is neutral - but whether charged particles can move.
3. The student is surprised that B dissolves in water but C does not, since both are molecular solids with low melting points. Explain this difference in terms of intermolecular forces, not just 'like dissolves like.'
Read the worked answer
Dissolving depends on whether the solute particles can form favorable attractions with water molecules that are strong enough to compensate for the attractions being broken.
B has polar O–H groups. Water molecules can hydrogen-bond to those groups. The energy required to separate B molecules from one another is at least partly recovered by the new B–water hydrogen bonds, so B dissolves.
C is nonpolar and has no O–H or other strongly polar groups. Its molecules interact with one another mainly by dispersion forces. Water molecules strongly hydrogen-bond to each other, and C cannot replace those interactions. The energy cost of creating a cavity in the water is not repaid by C–water attractions, so C does not dissolve. Hexane is nonpolar and can interact with C through dispersion forces, so C dissolves in hexane.
The phrase 'like dissolves like' summarizes the result, but the actual reason is the balance of intermolecular forces.