#### Question

Answer the following questions based on the *P–T *phase diagram of CO_{2}:

Describe qualitatively the changes in a given mass of solid CO_{2} at 10 atm pressure and temperature –65 °C as it is heated up to room temperature at constant pressure.

#### Solution 1

When the solid CO_{2} at – 65 °C is heated at 10 atm pressure, it is first converted into liquid. A further increase in its temperature brings it into the vapour phase. If a horizontal line at P = 10 atm is drawn parallel to the T-axis, then the points of intersection of line with the fusion and vaporization curve give the fusion and boiling points at 10 atm.

#### Solution 2

The fusion and boiling points are given by the intersection point where this parallel line cuts the fusion and vaporisation curves.

**Explanation**

When the temperature of a mass of solid CO_{2} (at 10 atm pressure and at –65°C) is increased, it changes to the liquid phase and then to the vaporous phase. It forms a line parallel to the temperature axis at 10 atm. The fusion and boiling points are given by the intersection point where this parallel line cuts the fusion and vaporisation curves.