Practice question · Multiple choice
At the triple point all three phases coexist, and it occurs at one specific temperature AND pressure. Why is there no freedom in either?
Hints
- Count the constraints and count the variables.
- Compare with a phase BOUNDARY, where two phases coexist along a line.
Show the answer
B. Because three phases impose two conditions on two variables
Why
Gibbs’ phase rule in miniature. Two phases give one constraint and hence a line; three give two constraints and hence a point, which is why water’s triple point is stable enough to define a temperature scale.
Practise States of matter and phase behaviour
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