latent heat
Stir a glass of iced water and check the temperature: it sits stubbornly at 0°C and refuses to budge while any ice remains, even in a warm room. Heat is clearly flowing in from the room, yet the thermometer ignores it. Where is that heat going? It is being quietly swallowed to melt the ice. That hidden, temperature-silent energy is called latent heat.
Latent heat is the energy a substance must absorb or release to pass through a first-order transition, like melting or boiling, without any change in temperature. The energy does not make the molecules move faster — instead it goes into breaking or forming the bonds that hold one phase together, rearranging the matter from one form into another. To melt ice you must pay in energy to loosen the crystal; when water freezes, that same energy is handed back out to the surroundings.
Latent heat matters enormously in everyday life and engineering: it is why steam scalds far worse than boiling water, why sweating cools you, and how refrigerators and heat pumps shuttle warmth around. The word 'latent' means hidden, and that is the key idea — the energy is real and large, but it hides from the thermometer because it pays for rearrangement, not for raising the temperature. Crucially, only first-order transitions have latent heat; continuous transitions have none.
Steam at 100°C burns far more severely than water at 100°C, even at the same temperature. The reason is latent heat: when steam touches your skin and condenses back to water, it dumps its large latent heat all at once, delivering a far bigger jolt of energy than hot water alone ever could.
Steam scalds worse than boiling water because condensing releases a large latent heat on contact.
The presence of latent heat is the cleanest way to tell a first-order transition from a continuous one. If energy is absorbed or released at a fixed temperature during the change, it is first-order; if not, it is continuous. Boiling has latent heat; a magnet losing its magnetism does not.