During melting, which of the following is true?

• A. The solid changes to a liquid and the temperature remains constant.
• B. The liquid changes to a gas.
• C. The gas changes to a solid.
• D. The solid changes to a gas.

Answer: (C)

Melting is the phase change from a solid to a liquid that occurs at the melting point. When a solid melts, the added heat goes into breaking the forces that hold the particles in a rigid lattice, turning the solid into a liquid without raising the temperature (at least for a pure crystalline solid). This temperature stay-flat during the melting process until all of the solid has melted, after which heating raises the temperature of the liquid. The other options describe different transitions: gas turning into a solid is deposition, liquid turning into a gas is vaporization, and solid turning into a gas is sublimation. So the true description of melting is that the solid changes to a liquid while the temperature remains constant.