Problem 75 - Chemistry The Science in Context

Question

Which phase of a substance (gas, liquid, or solid) is most likely to be the stable phase: (a) at low temperatures and high pressures; (b) at high temperatures and low pressures?

Step-by-Step Solution

Answer

Answer: (a) Under low temperatures and high pressures, the most likely stable phase of a substance is a solid. (b) Under high temperatures and low pressures, the most likely stable phase of a substance is a gas.

Step 1: Condition (a): Low Temperatures and High Pressures

At low temperatures, the kinetic energy of the particles in a substance is reduced. This means that the particles are moving more slowly, and as a result, the attractive forces between these particles become more dominant. Meanwhile, high pressure conditions indicate that the particles are compressed and forced closer together. Considering these factors, it is most likely that the stable phase of a substance under low temperatures and high pressures is a solid. The reason behind this is that solids have a more ordered particle arrangement, and their particles are held in fixed positions by the strong attractive forces between them, making solids the most stable phase under these conditions.

Step 2: Condition (b): High Temperatures and Low Pressures

At high temperatures, the kinetic energy of the particles in a substance is increased. This means that the particles are moving more rapidly and as a result, the attractive forces between these particles become less significant. Meanwhile, low pressure conditions indicate that the particles are not compressed and have more space to move around. Considering these factors, it is most likely that the stable phase of a substance under high temperatures and low pressures is a gas. The reason behind this is that gases have a more chaotic particle arrangement, and their particles are not held together by strong attractive forces. Gases are more stable under these conditions since their particles have enough energy and space to move freely.

Key Concepts

Phase Transitions

When we talk about phase transitions, as in the case of a substance's state changing between gas, liquid, or solid, we're referring to a fundamental transformation that occurs under varying temperature and pressure conditions.

Phase transitions are all about the competing influences of kinetic energy, which encourages movement and disorder, and intermolecular forces, which favor an organized and compact state. For instance, at low temperatures and high pressures—condition (a) as mentioned—the kinetic energy of the particles is low, so the molecules move less and are able to settle into a stable, orderly arrangement as a solid.

On the flip side, for condition (b), high temperatures increase the kinetic energy so much that the particles vibrate intensely, overcoming the intermolecular forces, and transition to a gaseous state where they're free to move independently since the influence of those forces is greatly diminished.

Kinetic Energy of Particles

The kinetic energy of particles is a measure of how much energy they have due to their motion. When a substance is heated, its particles gain energy and move faster. Higher kinetic energy implies that the particles can resist the pull of intermolecular forces to some degree.

As an example, a liquid can become a gas when its particles gain enough kinetic energy to break free from the attraction of neighboring particles—this is how boiling happens. Conversely, when cooled, the particles lose kinetic energy and move more slowly, allowing the intermolecular forces to dominate and potentially pull the particles into the structured, tight-knit configuration of a solid. Understanding this balance helps explain why, at low temperatures (condition (a)), a substance is most likely to be in a solid state, and why, at high temperatures (condition (b)), it would be gaseous.

Intermolecular Forces

Intermolecular forces are the forces that act between molecules or discrete parts of molecules. These include forces like hydrogen bonding, dipole-dipole interactions, and Van der Waals forces—all of which play a central role in determining a substance's stability at various phases.

At low temperatures and high pressures (condition (a)), these forces can effectively pull particles together into a fixed position typical of solids. However, in condition (b), where the temperatures are high and the pressures are low, the kinetic energy of the particles is so substantial that the intermolecular forces can't quite hold the particles together, leading to the particles moving about freely in a gaseous state. This illustrates why gases are the most stable phase at high temperatures and low pressures—the intermolecular forces are simply not enough to oppose the high kinetic energy and maintain a liquid or solid state.