Question

Consider cooling a gas so that it gets colder and colder. (a) Explain why this would eventually cause the gas to condense into a liquid. (b) Explain what would happen to this gas if you could somehow turn off all intermolecular forces.

Short answer

(a) Cooling a gas decreases the average kinetic energy of its molecules, causing them to slow down and come closer together. As the gas particles get closer, the intermolecular forces become significant and eventually overcome the kinetic energy, leading to gas condensation into a liquid. (b) If all intermolecular forces were turned off, the gas molecules wouldn't be attracted to each other and wouldn't be able to form a liquid, regardless of the temperature. The gas would remain in its gaseous state, as there would be no attractive forces to cause condensation.

Step-by-step solution

(a) Gaseous state and intermolecular forces

Gases consist of molecules that are widely separated from each another and move randomly in the container. The distance between the gas molecules is much larger than the size of the molecules themselves. Because of this, the intermolecular forces in the gas are relatively weak compared to those in liquids and solids.

(a) Cooling the gas

As the gas gets colder and colder, the average kinetic energy of the gas molecules decreases. The decrease in kinetic energy slows down the movement of the molecules, and the gas particles come closer together. As a result, the intermolecular forces between the gas molecules become significant.

(a) Gas condensation

When the kinetic energy of the molecules becomes significantly lower than the intermolecular forces, the gas particles can no longer move freely. Instead, they begin to gather together to form clusters or droplets, which is the process of condensation. This results in the gas changing its state and becoming a liquid.

(b) Removal of intermolecular forces

If we could somehow turn off all intermolecular forces, the gas molecules would no longer be attracted to each other. The force that causes the gas molecules to come together and form a liquid would be gone.

(b) Consequence for the gas

In absence of intermolecular forces, the gas molecules wouldn't be able to combine and form a liquid, regardless of how cold the temperature becomes. The gas would remain in its gaseous state, as there would be no attractive forces to cause the molecules to cluster together and condense into a liquid.

Key concepts

Intermolecular Forces

Understanding gas condensation hinges on the concept of intermolecular forces. These are the forces that occur between molecules and include attractions such as van der Waals forces, dipole-dipole interactions, and hydrogen bonding. In the gaseous phase, molecules are spread out with lots of empty space in between, which means that these forces have a minimal influence on their behavior. However, as a gas cools down, the molecules lose kinetic energy and begin to move closer together.

This change in proximity allows the intermolecular forces to become more significant, binding molecules closer together. Imagine this like a crowd of people who stand far apart in a large hall but start to huddle together as the room shrinks. At a certain point, the attractive forces overcome the remaining kinetic energy, and the molecules move from a free-flowing gas into a more structured liquid, which is essential for condensation to occur.

These intermolecular forces are crucial for maintaining the structure and properties of substances in different states. For example, water's high surface tension and relatively high boiling point compared to other similar molecules result from strong hydrogen bonds.

Kinetic Energy of Molecules

The kinetic energy of molecules is a significant player in the drama of phase changes. It refers to the energy that molecules possess due to their motion. In gases, molecules are constantly moving in random directions at high speeds, colliding with each other and the walls of their container. The temperature of the gas is directly proportional to the average kinetic energy of the molecules: higher temperature means more kinetic energy.

When you cool a gas, you're essentially taking away its kinetic energy, slowing the molecules down. As they slow, they don't have the energy needed to resist the pull of intermolecular forces. At a certain threshold, known as the condensation point, the kinetic energy is low enough that the gas molecules begin to stick together and form a liquid. It's like slowing down a swarm of bees until they cluster together to form a group, rather than buzzing around freely.

Phases of Matter

Matter exists primarily in three phases: solid, liquid, and gas. Each phase has its own unique properties determined by the arrangements and motions of its molecules. Solids have molecules that are tightly packed in a fixed arrangement, liquids have molecules that are close but can move past one another, and gases have molecules that are far apart and move freely and randomly.

These phases are not fixed; matter can transition from one phase to another through processes such as melting, freezing, boiling, and condensing, depending on temperature and pressure conditions. For instance, water can exist as ice, liquid water, or steam. Understanding the motion and arrangement of molecules in these phases helps us to comprehend not only everyday phenomena like water boiling on a stove but also the more complex behaviors of matter under extreme conditions, like the superfluidity of liquid helium.

State Change in Matter

A state change in matter occurs when a substance transitions from one phase to another, such as gas condensing into liquid. This involves a shift in the order and energy of molecules. When a substance like water vapor cools down, the decrease in kinetic energy allows the intermolecular forces to draw molecules closer, leading to the formation of liquid water.

State changes are part of a dynamic equilibrium, which means the rates of the forward and reverse changes can be equal. For example, at the boiling point, the rate of water molecules evaporating from the liquid phase equals the rate of molecules condensing back into the liquid. As students of chemistry and physics, grasping these basic principles arms us with the understanding needed to delve into topics such as thermodynamics and materials science, where controlling and manipulating phase changes is key to innovations in technology and industry.