Fig 6Heating 1 kg of icesteady heat inice warms · −20 °C
Heat 1 kg of ice at a steady rate and watch the temperature. It rises while the ice warms, then stops at 0 °C while the ice melts, because the heat is going into breaking hydrogen bonds instead of speeding up molecules. It rises again through the liquid, then stops at 100 °C for far longer while the water boils, because boiling has to pull every molecule completely free of its neighbours. Turning the water to steam takes over five times the energy of heating it from 0 to 100 °C.