Question

Indicate whether each statement is true or false: (a) The critical pressure of a substance is the pressure at which it turns into a solid at room temperature. $(\mathbf{b})$ The critical temperature of a substance is the highest temperature at which the liquid phase can form. (c) Generally speaking, the higher the critical temperature of a substance, the lower its critical pressure. (d) In general, the more intermolecular forces there are in a substance, the higher its critical temperature and pressure.

Short answer

(a) False: The critical pressure of a substance is for the liquid-gas phase change, not for solid formation. (b) True: The critical temperature represents the highest temperature at which the liquid phase can form. (c) False: Higher critical temperatures do not necessarily imply lower critical pressures. The relationship is complex and depends on other factors. (d) True: Stronger intermolecular forces generally result in higher critical temperatures and pressures, as more energy is required to cause a phase change.

Step-by-step solution

Statement (a) Evaluation

The critical pressure of a substance is the pressure at which it turns into a solid at room temperature. This statement is false. The critical pressure of a substance is the pressure required to cause a phase change between its liquid and gas states at the critical temperature, which is above the solid phase but below the gas phase.

Statement (b) Evaluation

The critical temperature of a substance is the highest temperature at which the liquid phase can form. This statement is true. When a substance reaches its critical temperature, it is no longer possible to distinguish between its liquid and gas phases. Above the critical temperature, the substance exists only in the gas phase.

Statement (c) Evaluation

Generally speaking, the higher the critical temperature of a substance, the lower its critical pressure. This statement is false. There is no direct rule that suggests that a substance with a higher critical temperature will have a lower critical pressure. Instead, the relationship is complex and depends on other factors like molecular structure and intermolecular forces.

Statement (d) Evaluation

In general, the more intermolecular forces there are in a substance, the higher its critical temperature and pressure. This statement is true. Intermolecular forces are the attractive forces between molecules. The stronger these forces, the more resistant the substance is to undergoing phase changes. As a result, substances with stronger intermolecular forces typically have higher critical temperatures and pressures because they require more energy to cause a phase change.

Key concepts

Critical Pressure

Critical pressure is a fascinating concept in chemistry that refers to the minimum pressure needed for a substance to change from a liquid to a gas at its critical temperature. It is important to understand that critical pressure does not relate to solidifying a substance at room temperature. Critical pressure is essentially the point where distinct liquid and gas phases can coexist. Beyond this pressure and at the critical temperature, a substance cannot be condensed back into the liquid form by applying pressure alone, as it becomes a supercritical fluid. Supercritical fluids have unique properties and are used in various industrial applications, such as in supercritical fluid extraction. Thus, critical pressure is inherently tied to phase transitions and substance behavior at specific conditions rather than to solids.

Phase Change

Phase change refers to the transformation of a substance from one state of matter—solid, liquid, or gas—to another. Common phase changes include melting, freezing, boiling, and condensation. The critical temperature is vital as it is the highest temperature at which a substance can exist as a liquid. When studying phase changes, it is important to grasp that at the critical temperature and pressure, the liquid and gas phases become indistinguishable. Above the critical temperature, no matter how much pressure is applied, the substance remains in the gas phase. Understanding phase changes is crucial in fields ranging from meteorology, where the behavior of gases affects weather patterns, to engineering, where phase change materials are used for efficient thermal management.

Intermolecular Forces

Intermolecular forces play a central role in determining a substance's physical properties, including its critical temperature and pressure. These forces are the attractions and repulsions between molecules, arising from interactions involving charges, dipoles, or induced dipoles. Key types of intermolecular forces include:

• Van der Waals forces: Weak, often temporary forces arising from fluctuations in electron distributions.
• Dipole-dipole interactions: Occur between two polar molecules, aligning their oppositely charged ends.
• Hydrogen bonds: Strong dipole-dipole attractions happening between hydrogen and more electronegative atoms like oxygen or nitrogen.

The stronger the intermolecular forces in a substance, the higher its resistance to phase changes, leading to higher critical temperatures and pressures. This is because more energy is required to overcome these forces and induce a phase change. Understanding these forces can help predict and explain the behavior of substances under varying conditions.