Heating curves, melting and boiling points

In the flat parts of the curve, the substance is still gaining energy, but the temperature does not change. Where does the energy go? Temperature tells us how much kinetic energy the particles have on average. During melting, the energy supplied is used to pull the particles apart from their fixed places, weakening the attractions between them. So the potential energy of the particles rises, while their average kinetic energy, and the temperature, stay the same. When all the ice has melted, extra energy speeds up the particles again and the temperature rises. The same happens during boiling, but now the particles are pulled far apart into a gas. The energy does not disappear: it is stored as potential energy. A common mistake is to say the water is not warming because it is not getting energy; in fact it keeps receiving energy. Another is to think steam is the white cloud over a kettle: steam is invisible, and the cloud is tiny droplets of liquid water.