State the meaning of boiling point.
The kettle holds $1.5\,\text{kg}$ of water with a specific heat capacity of $4200\,\text{J kg}^{-1}\,^{\circ}\text{C}^{-1}$. Using Fig. 4.2, work out how much the internal energy of the water increases between $t = 0$ and $t = 250\,\text{s}$.
Thermal energy (heat) is delivered to the water at a constant rate, yet the water's temperature rises at a rate that is not constant, as shown in Fig. 4.2. Explain why the temperature rises in this manner.
Once the temperature reaches $100^{\circ}\text{C}$, the kettle is kept on the burner. Thermal energy continues to be supplied to the water. The water boils because molecules form bubbles and move to the surface. State what happens to the temperature of the water.
Explain, in terms of the molecules, why thermal energy has to be supplied to maintain boiling.