[Figure 9.1] shows a boiling liquid at its boiling point, trapped in a cylinder by a piston. There is an electric heater in the liquid which is connected to a power supply. The thermal energy produced by the heater gradually turns the boiling liquid into a gas.

1. (a)State what is meant by boiling point.[2]
2. (b)Explain, in terms of molecules, why it is necessary to supply energy in order to turn a liquid at its boiling point into a gas.[3]
3. (c)In order to determine the power of the electric heater, two meters are connected into the circuit. One meter measures the current in the heater and the other meter measures the potential difference (p.d.) across it. On the circuit in Fig. 9.1, draw symbols to show the two meters used and where they are connected.[2]
4. (d)(i)The current in the heater is 2.0A and the p.d. across it is 6.0 V. Calculate the power of the heater.[2]
5. (d)(ii)Calculate the thermal energy supplied to the liquid by the heater in 1.0 minute.[2]
6. (d)(iii)The specific latent heat of vaporisation of the liquid is 9.0 × 10⁵J/kg. Calculate the mass of liquid that vaporises every minute.[2]
7. (e)The piston is free to move in the cylinder. As the liquid boils, the piston is pushed upwards in the cylinder at a constant speed. The volume occupied by the gas just above the liquid increases. Discuss whether the upward force on the piston changes as the piston moves upwards at constant speed.[2]

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