States of Matter

Air is not a single substance — it is a mixture of several gases. Understanding what air is made of and what each component does helps us understand breathing, weather, and life on Earth.

Human activity has changed the composition of Earth's atmosphere in ways that harm both the environment and human health. Understanding air pollutants and their effects is the first step toward solving these problems.

Matter changes from one state to another when energy (heat) is added or removed, causing particles to move faster or slower — these changes have specific names like melting, freezing, evaporation, and condensation.

Density is a measure of how much mass is packed into a given volume. Dense materials have particles packed closely together. Density explains why some objects sink and others float, and why gases are much lighter than solids or liquids.

Diffusion is the net movement of particles from an area of high concentration to an area of low concentration. It happens because particles are always moving randomly, and explains how smells travel, how oxygen moves into blood, and how substances move in and out of cells.

Gas pressure is caused by gas particles colliding with the walls of their container. Pressure increases when particles move faster (higher temperature) or are squeezed into a smaller space (lower volume). Understanding gas pressure is essential in everyday applications from tyres to weather systems.

Gases have no fixed shape or volume — their particles move rapidly in all directions and spread out to fill whatever container they are in.

Liquids have a fixed volume but no fixed shape — their particles are close together but free to move past each other, allowing liquids to flow and take the shape of their container.

Every substance has a specific melting point and boiling point — temperatures at which it changes state. These fixed temperatures are used to identify pure substances.

Solids have a fixed shape and volume because their particles are tightly packed in a regular arrangement and can only vibrate in place.

Water is one of the most remarkable substances on Earth. Its unique physical and chemical properties make life possible and shape our entire planet.