Question at position 6 Which of the following statements about a PT diagram is NOT true? Which of the following statements about a PT diagram is NOT true? It is possible to have more than one triple point if a solid has multiple crystalline structuresAt the critical point there are three phases in equilibriumAt the isobar of 1 atm under standard conditions, the melting and boiling points for a substance can be read from the PT diagram The liquid-solid phase line has a negative slope for water

Question

BlackTom AI · Accepted Answer

When approaching the topic of PT diagrams, we assess what each statement asserts about phase boundaries and critical phenomena.
Option 1: It is possible to have more than one triple point if a solid has multiple crystalline structures. In principle, polymorphism introduces multiple solid phases, each with its own relationships to liquid and gas. While this can complicate diagrams, it is not inherently ruled out that a substance with several crystal forms could exhibit multiple triple points corresponding to different solid forms coexisting with liquid and gas. This option is believable and not obviously false.
Option 2: At the critical point there are three phases in equilibrium. The critical point is defined as the end point of the liquid–gas boundary where distinct liquid and gas phases cease to exist as separate phases; beyond this point, they become a single supercritical fluid. By definition, there are not three coexisting phases at the critical point, so this statement is incorrect.
Option 3: At the isobar of 1 atm under standard conditions, the melting and boiling points for a substance can be read from the PT diagram. A PT diagram shows phase boundaries, and the intersections of the solid–liquid line with an isobar give the melting point, while the intersections of the liquid–gas line with the same isobar give the boiling point. Therefore, this statement is true.
Option 4: The liquid-solid phase line has a negative slope for water. Water is well-known to have a negative slope for the solid–liquid boundary because ice is less dense than liquid water, causing the melting line to slope downward with increasing pressure. This is a true attribute of water’s PT diagram.

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