Which phase change corresponds to fusion?

• A. Liquid to gas
• B. Gas to solid
• C. Solid to liquid
• D. Solid to gas

Answer: (C)

Fusion is the melting process: a solid absorbs heat and becomes a liquid. At the melting point, the particles gain enough energy to overcome some of the forces holding them in a rigid, fixed arrangement, so they can move past each other and form a liquid. A familiar example is ice turning into water as it warms.

So, the phase change where a solid becomes a liquid is fusion. Other transitions involve different changes: liquid turning into gas is vaporization, solid turning into gas is sublimation, and gas turning into solid is deposition (the reverse of sublimation).