To convert water at 32°F to ice at the same temperature, what must the refrigeration system remove?

• A. The latent heat of fusion
• B. The sensible heat
• C. The latent heat of vaporization
• D. No heat

Answer: (A)

The key idea is phase-change energy. To take water that’s already at its freezing point and turn it into ice at the same temperature, you must remove the energy stored in the liquid that allows it to change to a solid—the latent heat of fusion. At 32°F, water and ice coexist, so freezing requires extracting this latent energy without dropping the temperature further. For water, that latent heat of fusion is about 144 BTU per pound (roughly 333 kJ/kg). If you only remove sensible heat, the temperature would drop below 32°F before freezing begins; removing latent heat of vaporization would move the substance toward vapor, not solid ice. No heat removal would not drive the phase change at all.