Final answer:
The lever rule is used to calculate the proportions of two different phases in equilibrium within the two-phase region of a phase diagram, not above the critical point, at the triple point, or solely at the boiling point.
Step-by-step explanation:
The lever rule is a tool used in materials science and thermodynamics to determine the relative amounts of different phases in a two-phase equilibrium system. The question at hand involves understanding where the lever rule applies on a phase diagram, specifically whether it's used below the triple point, above the critical point, at the boiling point, or in the two-phase region.
In a phase diagram, the boundaries or lines represent the conditions of temperature and pressure where two phases are in equilibrium. For instance, at the boiling point inside a closed container, the liquid and gas phases are in equilibrium with each other because the rates of boiling and condensation are equal. However, it is within the two-phase region of the phase diagram, where two distinct phases, such as liquid and vapor or liquid and solid, exist in equilibrium, that the lever rule can be applied to determine the proportions of each phase.
The lever rule does not apply above the critical point because there is no distinction between the phases, and it does not apply at the triple point because all three phases are in equilibrium, not just two. Therefore, the correct answer to the question is that the lever rule allows for the calculation of the relative amounts of two phases in equilibrium in the two-phase region of a phase diagram.