Add heat to ice and its temperature climbs — until it reaches 0 °C, where it stops while the ice melts. Boiling does the same at 100 °C. Those flat spots are latent heat: energy going into changing the state, not raising the temperature.
A phase change is a substance switching between solid, liquid and gas: ice melting to water, water boiling to steam, and the reverse when it cools. What makes phase changes special is that they swallow or release a large amount of energy while the temperature refuses to budge.
On the left, a beaker on a hot plate shows all three states at once — ice, water and steam. On the right, the heating curve plots temperature against the heat added. The sloping sections are the substance simply warming up; the two flat plateaus are the phase changes, marked at 0 °C (melting) and 100 °C (boiling). The energy those plateaus represent is latent heat.
Latent heat is what makes steam so good at carrying energy, why sweating cools you down, and how a refrigerator moves heat out of a warm kitchen. Any device that evaporates or condenses a fluid relies on it.
At the start the beaker holds ice. While it is cold the added energy goes straight into making the ice molecules vibrate faster, so the temperature climbs — the first sloping section of the curve.
At 0 °C the temperature stops. Now the energy is spent prising the molecules out of the rigid ice lattice into a liquid, rather than speeding them up, so the thermometer holds steady while solid and liquid sit side by side. This first plateau is the latent heat of fusion.
Once every last bit of ice has melted, added energy has nowhere to go but into raising the temperature again. The curve slopes upward once more as the water warms from 0 °C towards 100 °C.
At 100 °C there is a second flat plateau, this time longer. The energy is being used to separate the water molecules completely and send them off as vapour — the latent heat of vaporisation, far larger because full separation costs much more than the loosening of melting.
With all the water turned to steam, the curve slopes up one last time: the same free molecules now simply move faster, so the temperature of the vapour rises.
Latent heat is the energy absorbed or released during a phase change — melting, freezing, boiling or condensing — that does not change the temperature. It goes into rearranging the particles rather than speeding them up, so the thermometer stands still while the state changes.
Because the energy being added is used to break the rigid ice lattice apart into liquid, not to make the molecules move faster. Until all the ice has melted, extra energy only shifts the balance between solid and liquid, so the temperature holds at the melting point.
Melting only loosens the lattice so the molecules can slide past each other; boiling has to pull the molecules completely apart and send them off as a gas. That full separation costs far more energy, so the boiling plateau is much longer than the melting one.
Yes. The boiling point depends on pressure: lower the pressure and water boils at a lower temperature, which is how cooking works faster in a pressure cooker and slower on a high mountain. The 100 °C value is just what applies at normal sea-level pressure.
Temperature measures how hot something is; heat is the energy transferred because of a temperature difference. During a phase change, a lot of heat moves in or out while the temperature — how hot it is — stays the same.
Refrigerators and air conditioners move heat by evaporating and condensing a fluid; sweat cools you down by evaporating; steam heating and ice packs both rely on the large latent heat of water.