Which of the following best describes the only temperature and pressure at which all three states of a substance coexist in equilibrium?
a) the triple point
b) the double point
c) the melting point
d) the max-energy point

explain the answer

A) The triple point
The triple point of a system is the thermodynamic point where the pressure, composition and temperature are maintained at a specific value and all three phases of that substance, solid, liquid and gas, coexist. This equilibrium is disturbed if any of the above mentioned variables are changed. The degree of freedom at the triple point is described as being zero.

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