The Three States of Matter
Everything around us — water, air, rocks, and even our bodies — is made of matter. Matter can exist in three main states: solid, liquid, and gas. The state of a substance depends on how its particles are arranged and how much energy (heat) they have.
In a solid, particles are packed tightly together in a regular pattern. They vibrate in place but cannot move around freely. This is why solids have a definite shape and volume. In a liquid, particles are close together but can move past each other. Liquids have a definite volume but take the shape of whatever container they are in. In a gas, particles are spread far apart and move very quickly in all directions. Gases have no fixed shape or volume — they expand to fill any container.
There is actually a fourth state of matter called plasma, found in stars and lightning. It is made of super-hot, charged particles. But for everyday chemistry, we focus on the three familiar states: solid, liquid, and gas.
Changing States
Matter can change from one state to another when energy (usually heat) is added or removed. These changes are called changes of state and they are physical changes — the substance remains the same, just in a different form.
- Melting: solid → liquid (ice melting into water)
- Freezing: liquid → solid (water freezing into ice)
- Evaporation/Boiling: liquid → gas (water boiling into steam)
- Condensation: gas → liquid (steam cooling back into water droplets)
- Sublimation: solid → gas directly (dry ice — solid CO₂ — turning into gas)
Each substance has a specific melting point and boiling point. Water melts at 0°C and boils at 100°C at sea level (standard atmospheric pressure).
Heating and Cooling Curves
When you heat ice, the temperature rises until it reaches 0°C — then it stays at 0°C while it melts, even as you keep adding heat. All the extra energy goes into breaking the bonds between particles rather than raising the temperature. Once all the ice has melted, the temperature starts rising again. The same happens at the boiling point.
This shows that changes of state require energy. Melting and evaporating absorb energy, while freezing and condensing release energy. This is why sweating cools you down — when sweat evaporates from your skin, it absorbs heat from your body.
Remember the changes of state with a simple sentence: "My Favourite Expedition Conquers Summits" — Melting, Freezing, Evaporating, Condensing, Sublimating. Draw a diagram with arrows going up (adding heat) and down (removing heat)!
Particle Theory
The particle theory of matter explains the states of matter. It states that:
- All matter is made of tiny particles.
- Particles are always moving.
- Particles have spaces between them.
- Higher temperature means particles move faster.
This simple model explains why gases expand when heated (particles move faster and spread out more), why liquids flow (particles can slide past each other), and why solids keep their shape (particles are locked in place).
A common mistake is to think that "melting" and "dissolving" are the same thing. Melting is a change of state caused by heat (ice → water). Dissolving is when a substance mixes into a liquid — no change of state occurs.
Real world: industrial processes — steel production
Iron is extracted from ore in a blast furnace at temperatures above 1500°C. At this temperature, iron melts (solid → liquid) and flows out at the base, while impurities float on top as slag. Casting the liquid iron into moulds and cooling it produces solid steel. Industrial metallurgy is entirely based on controlling state changes through temperature.
Real world: weather and the water cycle
The water cycle is a continuous series of state changes. Liquid water in oceans and lakes evaporates (liquid → gas, absorbing heat). Water vapour rises, cools, and condenses into liquid droplets forming clouds (gas → liquid, releasing heat). Droplets fall as rain or, in freezing conditions, water freezes (liquid → solid). Meteorologists understand precipitation patterns through the particle model and energy of state changes.
Real world: dry ice in food transport and special effects
Dry ice is solid CO₂ (melting point −78.5°C at atmospheric pressure). Unlike water ice, it undergoes sublimation directly from solid to gas, leaving no liquid residue — ideal for transporting food and medical supplies. On stage, the cool CO₂ gas sinks and flows along the floor, creating the "smoke on the stage" effect. Sublimation has direct practical and creative applications.
Practice Problems
What happens to particles in a liquid compared to a solid?
Show answer
What is evaporation and give one everyday example?