Question

Compare and contrast the phase diagrams of water and carbon dioxide. Why doesn’t ${\mathbf{CO}}_2$have a normal melting pointand a normal boiling point, whereas water does? Theslopes of the solid/liquid lines in the phase diagrams of ${\mathbf{H}}_{\mathbf{2}}\mathbf{O}$ and${\mathbf{CO}}_2$ are different. What do the slopes of the solid/liquid lines indicate in terms of the relative densities of the solid and liquid states for each substance? Howdo the melting points of role="math" localid="1663781921156" ${\mathbf{H}}_{\mathbf{2}}\mathbf{O}$and ${\mathbf{CO}}_2$depend on pressure? How do the boiling points of ${\mathbf{H}}_{\mathbf{2}}\mathbf{O}$and${\mathbf{CO}}_2$ depend on pressure? Rationalize why the critical temperature for${\mathbf{H}}_{\mathbf{2}}\mathbf{O}$ is greater than that for${\mathbf{CO}}_2$.

Short answer

In this case, we will be differentiating the physical properties of CO₂ and water using a phase diagram.

Step-by-step solution

Step 1:Triple point of carbon dioxide 

Carbon dioxide has a triple point of $-{56.6}^0\mathrm{C}$and 5.1 atm temperature and pressure, respectively, while water has $0^0\mathrm{C}$and 0.00604 atm.

At normal boiling point (1atm), ${\mathrm{CO}}_2$does not exist at 1atm; when temperature increases, the${\mathrm{CO}}_2$ (ice) as in the solid phase, directlysublimes into the gas at the temperature of$-78.5°\mathrm{C}$ . For${\mathrm{CO}}_2$ , the slope of the solid-liquid equilibrium line is positive; as pressure rises, the ${\mathrm{CO}}_2$ converts into liquid from solid. Hence, the melting point of $C{O}_2$ increases. The slope of the solid/liquid line of${\mathrm{CO}}_2$ ispositive,while water isnegative. The density of ${\mathrm{CO}}_2$(ice) rises as the pressure increases while in water. Also, as the pressure increases, the liquid phase transforms into the solid phase. Therefore, the density of ice is lower than the density of water.

The liquid phase is denser than the solid phase.

The melting points of H2O and CO2 depend on pressure

The melting point of ice and water depends on thepressure. In water, as pressure rises, the solid form starts to melt into the liquid because theslope is negative in the graph. If you raise the temperature at a constant 0.00604 atm, the ice does not melt, and the solid sublimes into water vapor.

The boiling points of H2O and  CO2 

The boiling point of ice and water depends on the pressure. From the phase diagram, we conclude that at constant pressure, as the temperature of CO₂ (ice) increases, the solid phase directly sublimes into the gas phase. In the case of water, as the pressure is constant, the rising temperature indicates the solid converts into the liquid phase.

The critical temperature (CP) is defined as the temperature at which the vapor cannot be converted into the liquid phase, whatever the pressure will be.

The CP depends upon the intermolecular forces between the molecules. The water has high intermolecular forces, which means hydrogen bonding, but CO₂ exhibits London dispersion forces.