Problem — The pot on the stove

Problem

A stove is lit under a pot of water. For ten minutes the temperature rises steadily. Then, from a certain moment on, the water keeps boiling but the thermometer no longer rises more than a degree. Explain in five lines why, even though the flame is unchanged, the temperature stops rising.

Solution

Heating phase. As long as the water is below the boiling point, the heat supplied by the flame increases the molecules’ average kinetic energy: the temperature therefore rises steadily, according to $Q = m \, c \, \Delta T$. Reaching boiling point. At atmospheric pressure water boils at $100\ ^{\circ}$C. From that moment all the heat supplied is absorbed as latent heat of vaporisation $L_v$: it goes into breaking the bonds between the molecules and turning the liquid into vapour, not into increasing their average kinetic energy. Why the thermometer stops rising. Temperature measures precisely that average kinetic energy, which stays constant during the change of state. The flame keeps supplying energy, but it leaves the pot as vapour instead of heating the remaining water. During boiling the heat supplied feeds the change of state at constant temperature, not a rise in temperature.

Links

Topics: Termologia Concepts: Calore latente e cambiamenti di stato