Fig. 9.1 shows the structure of a water cooler that is used to supply cold water to the workers in a hot office. [Figure 9.1]

1. (a)The reservoir of water in the plastic container is at room temperature. It does not mix with the cold water in the tank below. Explain why.[2]
2. (b)(i)When the tap is opened, water at room temperature from the reservoir flows down into the tank. Cold water from the tank flows through the plastic pipe and out of the tap. Cold liquid from the refrigeration unit is pumped through the copper pipe and thermal energy passes through the copper to this liquid. Suggest why this pipe is made from copper.[1]
3. (b)(ii)Explain, in terms of free electrons, how thermal energy is transferred through the copper.[3]
4. (b)(iii)As the water near the copper pipe cools, it begins to mix with the rest of the water in the tank. Explain how this happens.[3]
5. (c)The tap is opened and water at 21 °C flows from the reservoir into the tank. The specific heat capacity of water is 4200J/(kg°C). Calculate the thermal energy that is removed from 250 g of this water to reduce its temperature to 7°C.[3]
6. (d)(i)Describe the motion of the molecules in a liquid which is at a uniform temperature throughout.[2]
7. (d)(ii)State what happens to the motion of the molecules in a liquid as the temperature of the liquid decreases.[1]

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