Specific heat of fusion. Melting and hardening of amorphous solids
The amount of heat required to turn 1 kg of a crystalline substance at its melting point completely into liquid is called the specific heat of fusion λ, unit J/kg. To melt a body of mass m at its melting point we need Q = λm; the same amount of heat is released on solidification. For ice λ ≈ 336 kJ/kg = 3.36·10⁵ J/kg: this is about the energy needed to heat 1 kg of water by 80 °C (4200·80 ≈ 336 000 J). That is why melting ice cools its surroundings strongly and snow melts slowly in spring. Amorphous solids (glass, resin, wax) soften gradually when heated and liquefy slowly; they have no definite melting temperature and no λ. This is why glass can be shaped by blowing. When going from ice to water and then to steam, the total heat is the sum of the separate stages.
Observation: take two ice pieces of equal size: put one in a glass of lukewarm water and the other in room air. Which melts faster? Why does ice melt faster in water, and why does the water cool? Use lukewarm, not boiling, water.