What causes ice to change into liquid water?

• A. The absorption of heat energy.
• B. The release of heat energy.
• C. A sudden drop in temperature.
• D. The addition of pressure.

Answer: (A)

Melting is an endothermic phase change driven by energy input. Ice changes into liquid water because it absorbs heat energy. That added energy breaks the orderly hydrogen-bonded structure and lets the molecules move past each other, turning a solid into a liquid. The energy goes into overcoming the lattice, known as the latent heat of fusion, and the temperature stays at the melting point (about 0°C for pure ice) until all the ice has melted. Once melted, further heating raises the temperature of the liquid.