Question

What are the most favourable conditions to liquefy a gas? (a) High temperature and high pressure (b) Low temperature and high pressure (c) Low temperature and low pressure (d) High temperature and low pressure

Short answer

(b) Low temperature and high pressure are the most favourable conditions to liquefy a gas.

Step-by-step solution

Understand Liquefaction of Gases

The process of turning a gas into a liquid is known as liquefaction. This process generally depends on temperature and pressure. The liquefaction of gases typically occurs when the gas is cooled or compressed (or both) to the point where kinetic energy between molecules is reduced enough for intermolecular forces to take hold and form a liquid.

Consider the Effects of Temperature

Lower temperatures lead to a decrease in the kinetic energy of the gas molecules. This reduced kinetic energy makes it more likely for the molecules to come closer together and form a liquid.

Consider the Effects of Pressure

Higher pressure forces the molecules of a gas closer together, which can lead to the formation of a liquid. This pressure works in tandem with low temperatures to overcome the kinetic energy of the molecules.

Choose the Correct Option

Given the dependence of liquefaction on temperature and pressure, the most favorable conditions to liquefy a gas would be low temperature, to reduce kinetic energy, and high pressure, to force molecules closer together.

Key concepts

States of Matter

The universe is composed of matter which primarily exists in three distinct forms: solids, liquids, and gases. These are known as the states of matter, and they differ in the behavior and arrangement of their constituent particles. Solids have a fixed shape and volume with particles locked in a rigid lattice structure. Liquids, meanwhile, have a definite volume, but they can change shape to fit their container because their particles are closely packed but movable. Gases have neither a fixed shape nor a fixed volume, as their particles move freely and are spaced far apart, filling any container they are in. The transformation between these states, such as the liquefaction of gases, involves altering temperature and pressure conditions to change the energy and arrangement of the particles.

Understanding these states lays the groundwork when considering how a gas can transform into a liquid under certain conditions—a cornerstone in fields such as cryogenics, where gases are liquified for storage and transport.

Intermolecular Forces

Intermolecular forces are the forces that hold molecules together. These forces are significantly weaker than the intramolecular forces that keep a molecule intact but are critical in determining the physical properties of substances. There are several types of intermolecular forces, including dispersion forces, dipole-dipole interactions, and hydrogen bonds. The strength of these forces impacts a substance's melting and boiling points, vapor pressure, and, importantly for our discussion, the conditions required to liquefy a gas.

When a gas is subjected to low temperatures and high pressure, the kinetic energy of the gas molecules decreases, and the molecules come closer together. Then intermolecular forces start to dominate, leading to the formation of a liquid. These forces help explain why reducing temperature and increasing pressure are necessary for the liquefaction process, as they enable the molecules to overcome their kinetic energy and form a more ordered, liquid state.

Kinetic Molecular Theory

The Kinetic Molecular Theory (KMT) is a fundamental concept that explains the behavior of gases and how their molecular motion relates to temperature and pressure. According to this theory, gas molecules are in a constant state of random motion and they possess kinetic energy that is directly proportional to the temperature of the gas. Under high temperatures, the molecules move very rapidly, which prevents them from coming together despite the attractive intermolecular forces. When the temperature is lowered, the kinetic energy of these molecules decreases and their movement slows down, making them more susceptible to the attractive forces that can pull them closer to form a liquid.

Thus, understanding KMT helps clarify why low temperatures (which reduce molecular movement) and high pressure (which forces molecules together) are the most favourable conditions to liquefy a gas. This theory offers insight into the dynamic relationship between the states of matter and provides the foundation for exploring gas behavior under various conditions.