What Are the States of Matter? Solids, Liquids and Gases Explained

The states of matter are the different forms in which matter can exist. They are determined by how particles are arranged and how much energy they have. There are three states of matter that students encounter in everyday life:

Solid

In a solid, particles are packed tightly together in a fixed arrangement. They vibrate in place but cannot move freely. This gives solids a definite shape and a definite volume. Examples: ice, rock, wood, metal.

Liquid

In a liquid, particles are close together but not rigidly fixed. They can slide past each other. This means liquids have a definite volume but no fixed shape — they take the shape of whatever container holds them. Examples: water, juice, mercury.

Gas

In a gas, particles are widely spread out and move freely and rapidly in all directions. Gases have no fixed shape and no fixed volume — they expand to fill any container. Examples: air, steam, oxygen, carbon dioxide.

A fourth state, plasma, exists in stars and lightning — rarely encountered in everyday life.

In essence, it is a question about how particles behave under different conditions of temperature and pressure.

Matter does not stay in one state forever. Adding or removing heat energy causes the states of matter to change. These transitions have specific names:

• Melting — solid becomes liquid (e.g., ice melting into water at 0°C)
• Freezing — liquid becomes solid (e.g., water freezing at 0°C)
• Evaporation / Boiling — liquid becomes gas (e.g., water boiling to steam at 100°C)
• Condensation — gas becomes liquid (e.g., steam condensing on a cold mirror)
• Sublimation — solid becomes gas directly without passing through liquid (e.g., dry ice — solid CO₂ — sublimes at room temperature)

Changes of state are physical changes, not chemical ones. The substance remains the same — only the arrangement of its particles changes. When steam condenses into water, the water molecules are identical to those that evaporated.

Each substance has a fixed melting point and boiling point — constants used to identify materials.

The particle model of matter explains the states of matter at a microscopic level. The key ideas are:

1. All matter is made up of tiny particles (atoms or molecules).
2. Particles are always moving.
3. Particles attract each other; the strength of attraction varies.
4. The higher the temperature, the more energy the particles have — and the faster they move.

In a solid, particles vibrate around fixed positions. The attractive forces are strong enough to keep them locked in place.

In a liquid, particles have more energy. They move enough to slide past each other, but attractive forces still keep them close.

In a gas, particles have so much energy that they overcome attractive forces entirely. They move rapidly and independently, spreading out to fill their container.

When you heat a solid, you give its particles more energy. Eventually they have enough energy to break free from their fixed positions — melting occurs. Add more heat and liquid particles gain enough energy to escape from the liquid surface entirely — evaporation or boiling occurs.

This particle model is the foundation of chemistry and physics at secondary school. Everyday examples — a drying puddle, steam over a hot drink — help make these concepts tangible. See For parents for more activity ideas.