States of Matter and Phase Changes

3. Gases (Vapour): Expansion and Randomness

Gases (often referred to as vapour when a substance is in its gaseous state at temperatures below its critical point) are characterized by high energy and high disorder. The particles in a gas are far apart and move rapidly in random directions. Because there is very little attraction between the particles, gases possess neither a definite shape nor a definite volume. They will expand to fill the entire volume of any container they are placed in.

Phase Changes: The Role of Energy

A phase change is the transition of a substance from one state of matter to another. These transitions are not caused by the creation of new matter, but by the addition or removal of thermal energy (heat).

The relationship between energy and phase changes is fundamental:

• Increasing Temperature: Adding thermal energy increases the kinetic energy of the particles. As they move faster, they can overcome the intermolecular forces that keep them in a structured state.
• Decreasing Temperature: Removing thermal energy reduces the kinetic energy of the particles. As they slow down, the attractive forces pull them into more ordered, stable arrangements.

Summary of Phase Transitions

Exam Tip: Vaporization vs. Boiling Remember that vaporization is a general term for a liquid turning into a gas. However, it can happen in two distinct ways: evaporation, which occurs only at the surface of the liquid, and boiling, which occurs throughout the entire volume of the liquid when its vapor pressure equals the external pressure.

Particle Movement and Diffusion

Once particles are in motion—particularly in liquids and gases—they exhibit a phenomenon known as diffusion. Diffusion is the movement of particles from an area of high concentration to an area of low concentration.

This movement is driven by a concentration gradient, which is the difference in the concentration of particles between two specific regions.

Worked Example: Ink in Water

Imagine you add a single drop of blue ink into a clear glass of water:

1. Initial State: The area where the drop hits the water has a very high concentration of ink particles. The rest of the water has a concentration of zero.
2. The Gradient: A steep concentration gradient exists between the drop and the surrounding water.
3. The Process: Due to their kinetic energy, the ink particles move randomly. Because there is more "room" (lower concentration) in the surrounding water, the particles naturally spread out.
4. Equilibrium: Eventually, the ink will be spread evenly throughout the glass, and the concentration will be uniform.

Did You Know? While diffusion is the movement of particles from high to low concentration, there is a specific type of movement called osmosis. Osmosis is the movement of particles from a lower concentration to a higher concentration through a semipermeable membrane.

Summary Table of Particle Properties

To help with your revision, use this table to compare the three main states: