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A solid is heated at a constant rate, and its temperature is plotted against the heat supplied. On this graph, segment DE is the final sloped portion (after the substance has fully vaporised). What does the slope of DE represent? (Heat is on the horizontal axis, temperature on the vertical axis.)

1. latent heat of liquid.
2. latent heat of vapour.
3. heat capacity of vapour.
4. inverse of heat capacity of vapour.

Correct answer: inverse of heat capacity of vapour.

Solution

On a temperature-vs-heat graph, a sloped (non-flat) segment corresponds to a single phase; its slope dT/dQ equals 1/C, i.e. the inverse of the heat capacity of that phase (vapour for DE).

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