Fig. 4.1 shows a kettle containing water placed on the burner of a gas cooker. [Figure 4.1] The gas burner is lit at time t = 0. At t = 250s the temperature of the water is 100°C, the boiling point of water.

1. (a)State what is meant by boiling point. ...............................................................................................................................................[1][1]
2. (b)Fig. 4.2 shows how the temperature of the water changes with time t. [Figure 4.2]
3. (b)(i)The kettle contains 1.5kg of water which has a specific heat capacity of 4200J/(kg°C). Using Fig. 4.2, determine the increase in the internal energy of the water between t = 0 and t = 250 s. increase in internal energy = ..............................................................[3][3]
4. (b)(ii)Thermal energy (heat) is transferred to the water at a constant rate but the temperature of the water increases at a rate that is not constant, as shown in Fig. 4.2. Explain why the temperature increases in this way. ............................................................................................................................................... ............................................................................................................................................... ...............................................................................................................................................[2][2]
5. (c)When the temperature reaches 100°C, the kettle is left on the burner. Thermal energy is still supplied to the water. The water boils as the molecules form bubbles and rise to the surface.
6. (c)(i)State what happens to the temperature of the water. ...............................................................................................................................................[1][1]
7. (c)(ii)Explain, in terms of the molecules, why it is necessary to supply thermal energy in order to keep the water boiling. ............................................................................................................................................... ............................................................................................................................................... ...............................................................................................................................................[2][2]

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