The temperature and pressure at which a solid turns into a liquid is the...

• A. Heat of fusion.
• B. Sublimation point.
• C. Boiling point.
• D. Melting point.

Answer: (D)

The melting point is the temperature at which a solid changes into a liquid at a given pressure. At that temperature, solid and liquid phases are in equilibrium, so heat added goes into breaking the solid’s lattice rather than raising the temperature (latent heat of fusion). For pure crystalline substances this is a sharp point, though impurities or unusual pressure can broaden or shift it. The other terms refer to different concepts: heat of fusion is the energy required to melt, sublimation point is solid turning directly to gas, and boiling point is liquid turning to gas.