Q.Assertion: Critical temperature of CO$_2$ is 304 K, it can be liquefied above 304 K. Reason: For a given mass of gas, volume is directly proportional to pressure at constant temperature.

Thomas Andrews plotted pressure-volume isotherms of carbon dioxide at a series of fixed temperatures and produced the first complete picture of how a real substance's P, V and T relate across both its gaseous and liquid states. At low temperature, each isotherm falls as a smooth curve (gas only), flattens into a horizontal plateau where liquid and gas coexist at constant pressure, and then rises steeply once the gas has fully condensed to liquid (which compresses far less than a gas does). As temperature rises, this flat coexistence plateau shrinks; at $31.1\,^\circ\text{C}$ (for CO$_2$) it shrinks to a single point, where gas converts directly to liquid with no separate two-phase stretch at all. Above this temperature, no pressure -- however large -- can liquefy the gas. …