eli5 Why does changing the state of matter transfer more energy than a normal amount without the state of change?
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There’s intrinsic energy in the structure of matter in its different phases in addition to its “normal” thermal energy. To cross the temperature boundary between phases, that energy must also be overcome to change the temperature of the material, otherwise the phase change will absorb/release heat and counter the change in temperature.
Consider the classic example of a glass of ice water: Under normal circumstances, a glass of water with ice in it will drop to 0 C. If a small amount of heat is removed, the temperature will still remain at 0 C, as the water will freeze. This formation of ice crystals releases energy into the surrounding water, returning it to its equilibrium point of 0 C. If a small amount of heat is added, the temperature will still stay at 0C, as the ice crystals will melt, absorbing that heat from the surrounding water.
In ideal conditions, the glass of water will remain at 0 C until all the ice has melted, or all of the water has frozen. Only after there is no more ice to form or melt will the mass begin to increase or decrease in temperature: No more phase change is occurring, so there is no moderating effect on the temperature of the water in the glass
This same effect happens to boiling water: When water is heated above 100 C, the formation of steam takes energy from the mass, cooling it back to 100C, but forming water vapor. Technically the same effect occurs during condensation, with the forming liquid water dumping heat into the surroundings. This is a part of the reason why steam burns are so nasty, as heated water vapor dumps additional energy into the flesh as it condenses.
This is useful for the added energy capacity, but also for that aforementioned moderating effect: Heating with boiling water or cooling with ice will naturally gravitate to specific temperatures. So long as we control for variables that can change these temperatures (for example, salt on ice, or high altitude boiling), we can maintain consistent temperatures on demand.
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