The phase transition temperature

Heat some water and watch a thermometer immersed in it: the temperature rises steadily... up to 100 °C, where it stops abruptly. It remains stuck at 100 °C for as long as there is any liquid water left to evaporate, even if you turn the gas up to full under the pan.

The temperature plateau

This phenomenon is called a phase transition plateau. On a graph of temperature against time, it appears as a horizontal plateau:

température (°C)
100 |        ______________
    |       /              \___ (steam, it's rising)
    |      /
  0 |_____/
    +---------------------------> temps
       (liquid water that heats up)   (stage = spraying in progress)

None of the energy supplied during the plateau is used to raise the temperature: it is used to break the bonds that hold the water molecules together in the liquid state. It is only once all the liquid has turned into gas that the temperature begins to rise again.

A characteristic temperature of the substance

Every pure substance has its own melting and boiling points at a given pressure. Water melts at 0 °C and boils at 100 °C; iron melts at around 1538 °C; dinitrogen in the air boils at -196 °C. These values can also be used to identify an unknown substance.

The common misconception

‘If I heat it more intensely, the water will boil at a higher temperature. ” This is incorrect: heating more intensely makes water boil faster (the plateau phase lasts for a shorter time), but not at a higher temperature. The plateau temperature depends solely on the substance and the pressure, never on the heating power.