The figure below shows a household hot water system.

(a) State the process by which the hot water rises from the boiler to the hot water tank.

Convection

(b) Through which pipe, P or Q, does the heated water rise from the boiler to the hot water tank? Explain your choice.

Pipe P. Hot water is less dense than cold water. Thus, hot water in the boiler will rise, and exit the boiler through pipe P to reach the hot water tank. In the hot water tank, the colder water, being denser, will sink, and exit through pipe Q to reach the boiler.

(c) State the purpose of pipe R.

Pump cold water to the lower half of the water tank to be transferred to the boiler via pipe Q for boiling/heating.

(d) What is the name and purpose of pipe S.

Pipe S is the overflow pipe attached to the tap of the hot water tank in case the temperature of the water becomes too high and cause a large expansion of the hot water. The excess water will then flow back into the cistern.