Each phase of a material can exits only in certain regions of pressure and temperature . `P-T` phase diagrams, in which pressure is plotted versus temperature, show the regions corresponding to various phases and phase transformations . `P-V` diagrams, on the other hand , can be used to study pressure volume relationship at a constant temperature.
If the liquid and gaseous phases of a pure substances are heated together in a closed container, both the temperature and the vapor pressure will increase until a point is reached at which the two phases can no longer be distinguished from one another. The temperature and pressure at which this occurs are called the critical temperature and pressure. Exceeding either of these parameters, by itself ,will cause the `"gas"//"liguid"` phase transition to disappear. if the other variable is then changed as well, while the first variable is maintained above its critical point , a gradual transition will occur between the gaseous and liquid phases, with no clear boundary.(The liquid and solid phases, on the other hand , maintain a distinct boundary at all pressure above the triple point). Shown in figure is a combined `P-T` phase diagram for material `A` and `B`.

Which is true about the substance in figure?

Options

A. At `2` atm pressure and `220K` temperature, `A` is a gas and `B` is solid
B. At `6` atm pressure and `280K` temperature, `A` is a gas and `B` is a solid
C. At `5` atm pressure and `100K` temperature, `A` is a gas and `B` is a solid
D. At `4` atm pressure and `300K` temperature, both `A` and `B` are liquids

Answer

A

Explanation

From the phase diagram, at `2` atm & `220K, A` is gas &`B` is solid.