The graph in Fig. 1 shows a heating curve for a 0.20 kg sample of a wax used to make decorative candles. A heater supplies energy at a constant rate. The wa...
Question 1 Report
The graph in Fig. 1 shows a heating curve for a 0.20 kg sample of a wax used to make decorative candles. A heater supplies energy at a constant rate. The wax starts as a solid. During section BC, the temperature does not change even though electrical energy continues to enter the wax. The candle maker wants to know why a liquid layer appears at this stage. The vertical axis is temperature and the horizontal axis is heating time. The apparatus includes a power supply, an ammeter to measure current and a heatproof glass beaker.
(a) What happens to the average kinetic energy of the wax particles from A to B? [3]
(b) What happens to the energy transferred to the wax from B to C? [3]
(c) Sketch, on the diagram, a curve that could show the wax cooling from D until it is solid. [3]
(d) Which section shows the liquid wax being heated? [3]
Answer Details
A heating curve shows how temperature changes as energy is supplied. Sloping sections mean the particles' average kinetic energy changes; a horizontal section means a change of state is occurring.
- (a) From A to B, the average kinetic energy increases [1]. The particles move or vibrate faster [1] because the temperature increases [1].
- (b) From B to C, the transferred energy is used to overcome forces or bonds between particles [1]. The particles separate or change arrangement [1] as solid wax becomes liquid. This energy is the latent heat of fusion, or melting [1], so it does not increase the particles' average kinetic energy and the temperature stays constant.
- (c) The cooling curve must fall from D, have a horizontal section at the same temperature as BC while the wax freezes, then fall again once it is solid [3].
- (d) CD [3] shows liquid wax being heated. The wax has already melted at C, and its temperature then rises as the liquid gains kinetic energy.
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