Latent heat of fusion of water

Melting a kilogram of ice at 0 °C takes 333.55 kJ and produces a kilogram of water at 0 °C: the energy goes entirely into breaking the crystal lattice, not into raising temperature. Put another way, the heat that melts ice would otherwise warm the same mass of water by 80 K, which is why an iced drink holds its temperature so stubbornly and why glaciers lag the seasons.

Joseph Black identified latent heat in Glasgow in the 1760s by the simple observation that ice in a warm room takes hours to melt while staying at 0 °C the whole time — the first clear evidence that heat and temperature are different quantities, and the conceptual groundwork James Watt built the separate condenser on a decade later. In refrigeration the constant sets the ton: 12 000 BTU/h is the rate needed to melt one short ton of ice in 24 hours.