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TGO1:
A closer look at change of state
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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.
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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Science Enhancement Programme CD-ROM 2005 |
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user
guide |
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