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TGO1: A closer look at change of state
Notes on video of lead melting
Students should notice that
a) below the melting point, all the lead is solid and the temperature is rising steadily
b) at the melting point, the temperature stays constant as the lead melts: there is lead in both the solid and liquid states.
c) once all the lead has melted the temperature of the liquid lead rises steadily again.

Note: Students find temperature –time graphs difficult, but you can use the video to help them relate the features of the graph to what is happening. Use the slider button on the video to emphasise how the horizontal part of the graph starts when lead first starts to melt, and continues until all the lead is melted.

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Notes on ‘Think for Yourself’ question

Lead has a melting point of 327°C. If we let a sample of lead at 450°C cool down, what would a graph showing temperature against time look like? Describe what would be happening to the sample.
At the start, all the lead would be in the liquid state and the temperature would drop rapidly.
At the melting point, the temperature would remain the same until all the sample had turned from liquid to solid
state. Once all the lead was in the solid state, the temperature would begin to drop again.

Note: You could ask students to produce a sketch graph for this first (on individual whiteboards for example), then ask one or two students to describe what their graph shows.

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