A container is filled with water at 20 °C and placed in a room that is also at 20 °C. The container can be fitted with an internal electric cooling unit, in one of the three labelled positions P, Q or R. The outside of the container can either be painted dull black, or be covered in shiny aluminium foil. In order to cool all the water as quickly as possible to 5 °C, in which position should the cooling unit be fitted, and should the outside of the container be dull black or shiny?

Answer:

Cooled water will become more dense and fall, causing a convection current in the container. The
cooling unit should be positioned at the top to ensure that all the water is cooled, including the
warmer, less dense water at the top.
The water starts at room temperature, and will soon become cooler than its surroundings. This
means that a dark surface would absorb more infra-red from the surroundings than it would emit.
A shiny surface reduces this effect and will help to keep the water cooler than the surroundings.
The answer is B