Which statement describes a property of a pure solid substance?

• A. It has a sharp melting point.
• B. It melts over a range of temperatures.
• C. It does not melt.
• D. It boils at 100°C.

Answer: (A)

Pure crystalline solids have a definite melting point because their particles form a uniform lattice with the same forces throughout. When you heat such a solid, the energy required to break the lattice is the same everywhere, so the whole solid changes to a liquid at a single temperature. If a solid contains impurities, the lattice is disrupted and parts melt at different temperatures, so you observe melting over a range. Saying a solid does not melt is not generally true for pure substances at high enough temperatures, and boiling at 100°C is specific to water at 1 atmosphere, not a general property of all pure solids.