Sodium chloride is heated in a laboratory furnace to investigate changes of state. A sample of the ionic compound is given energy until it melts. The studen...

Sodium chloride is heated in a laboratory furnace to investigate changes of state. A sample of the ionic compound is given energy until it melts. The student notes that the temperature remains constant for a short time while melting occurs, although the heater continues to transfer energy. The solid structure contains positive sodium ions and negative chloride ions in a regular lattice. This investigation is carried out with a very small sample because the melting temperature is high.

(a) Name the change of state taking place at the constant temperature. [1]
(b) Describe what happens to the energy transferred to sodium chloride while it melts. [2]
(c) Explain why sodium chloride needs much more energy to melt than a simple molecular substance. [2]

Fig. 1 is a Sankey diagram for a small petrol engine driving a water pump on a farm. The chemical energy supplied by the fuel in one minute is 1000 kJ. Some energy is transferred usefully to the moving pump. The rest is wasted, mainly as thermal energy to the engine, water and surrounding air, with a small amount as sound. The engine uses an oil lubricant to reduce friction, but it cannot stop all unwanted energy transfers.

(a) Name the useful energy store increased by the moving pump. [1]
(b) Use the diagram to calculate the percentage efficiency if 280 kJ is transferred usefully. [2]
(c) Describe two ways in which energy is wasted by the engine. [2]