The energy involved in change of state
2 marks · EasyDescribe what a temperature–time graph would look like for a substance that is being heated steadily as it melts from solid to liquid.
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A rising section followed by a flat (horizontal) section where temperature remains constant as it melts.
Must show: initial rise, flat section at melting
Also accept: Temperature increases, then stays the same while the substance melts, then increases again after all is melted.
Common mistakes: Describes a continuous rise without mentioning the horizontal (constant) section.
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Marking points
B1 · 1 mark · Mentions temperature rises before melting.
B2 · 1 mark · Mentions temperature remains constant while melting occurs (flat section).
Worked solution
The graph rises steadily while the solid is heated, then becomes horizontal (flat) at the melting point where the temperature does not change until all the solid has melted.