The phase diagram for water is shown in figure. If the temperature of a certain amount of ice is increased by following the path represented by the dashed line from A to B in the phase diagram, which of the graphs of temperature as a function of heat added is correct? Treat water vapour as an ideal gas.

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TestHub · Accepted Answer

The path A to B passes through the triple point at constant low pressure.
Ice first warms up to $0^{\circ} C$ .
At the triple point ice changes directly into vapour (sublimation), so the temperature stays at $0^{\circ} C$ during this change.
No liquid phase appears. Afterwards the vapour is heated, and the temperature rises steeply since vapour has a small specific heat.
This matches graph (B).

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