Which phase change involves the input of heat to convert a solid to a liquid?

• A. Melting
• B. Deposition
• C. Condensation
• D. Freezing

Answer: (A)

This question tests understanding of phase changes and how heat flow drives them. When heat is added to a solid, its particles gain kinetic energy. At the melting point, enough energy overcomes the forces holding the solid together, allowing the structure to break down and become a liquid. This endothermic process is called melting (fusion).

In contrast, the other options involve releasing heat as a substance moves to a more ordered, lower-energy state: deposition (gas to solid), condensation (gas to liquid), and freezing (liquid to solid).

So the phase change described—input of heat to convert a solid to a liquid—is melting.