By Chris Deziel, Updated Mar 24, 2022
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If the ambient temperature around a piece of ice rises, the ice’s temperature climbs accordingly. However, this rise halts the moment the ice reaches its melting point—32 °F (0 °C). At that instant the ice undergoes a phase transition, converting to liquid water while its temperature remains fixed until all of it has melted. A simple experiment demonstrates this: leave a cup of ice cubes in a hot car and monitor the temperature with a thermometer. The icy water stays at 32 °F until it is fully melted; thereafter, the temperature rises rapidly as the remaining liquid continues to absorb heat from the car’s interior.
When you heat ice, its temperature rises until it reaches 32 °F, then stays constant while melting. The added heat breaks the crystal lattice bonds rather than increasing kinetic energy.
Heating ice increases the kinetic energy of its molecules, causing them to vibrate more rapidly. Until the melting point is reached, this extra energy merely amplifies vibration; the molecules cannot yet break the lattice bonds that keep them in a solid structure. Once the ice reaches 32 °F, the molecules acquire enough energy to detach from the lattice. All the heat energy supplied is therefore consumed by the phase transition, not by raising the kinetic energy of the liquid. Consequently, the temperature of the water remains at 32 °F until every crystal has melted.
The same principle applies to boiling water. It will heat up to 212 °F (100 °C), but will not exceed that temperature until every droplet has become vapor. As long as liquid water remains in the pan, its temperature stays at 212 °F regardless of the intensity of the heat source.
You might assume that a mixture of ice and water would warm uniformly, but in reality the temperature near the ice remains locked at the melting point. In a large container of water with an ice cube, the bulk of the water can rise above 32 °F, yet the immediate environment of the ice stays at that constant temperature. This equilibrium occurs because as ice melts, some of the surrounding water refreezes, balancing the heat flow. The net result is that the overall temperature does not increase until all ice has disappeared.
Introducing more heat can still produce a linear temperature rise; the ice will melt faster, and the temperature of the remaining liquid will climb. However, the heat required to break the lattice bonds dominates until the phase change is complete.
Pressure also plays a crucial role. By confining steam in a sealed vessel, you raise the boiling point, allowing water to stay liquid at temperatures above 212 °F. This is the principle behind pressure cookers and industrial steam boilers.
