States of Matter and Particle Theory: ACARA Science Guide
All physical matter is made of tiny atoms and molecules in constant motion. Temperature changes alter molecular kinetic energy.
Everything in our universe that has mass and takes up volume is matter. The Particle Model of Matter explains that all substances are made of microscopic particles that are constantly moving.
In ACARA Science (Years 5 to 8), students learn that states of matter are determined not by the identity of atoms, but by the strength of forces between them and their thermal kinetic energy.
State Changes and Phase Transitions
| Phase Transition | Starting State | Ending State | Thermal Action |
|---|---|---|---|
| Melting | Solid (Ice) | Liquid (Water) | Heat absorbed; particles vibrate fast enough to break fixed lattice |
| Freezing | Liquid (Water) | Solid (Ice) | Heat released; particles slow down and lock into fixed positions |
| Evaporating / Boiling | Liquid (Water) | Gas (Steam / Vapor) | Heat absorbed; surface particles gain enough energy to escape as gas |
| Condensation | Gas (Vapor) | Liquid (Droplets) | Heat released; gas cools and particles coalesce into liquid |
| Sublimation | Solid (Dry Ice) | Gas (CO₂ vapor) | Direct transition from solid to gas without becoming liquid |
Why Gases are Compressible but Liquids are Not
Because gas particles have vast empty spaces between them, you can easily compress air inside a bicycle pump or syringe by pushing the plunger.
In contrast, particles in liquids and solids are already packed tightly touching one another. Liquids cannot be compressed, which is the foundational engineering principle behind vehicle hydraulic brakes!
✨ Key Takeaways
- Solids possess fixed shape and volume; particles vibrate in locked lattice positions.
- Liquids maintain fixed volume but flow to take the shape of their container.
- Gases have neither fixed shape nor fixed volume; particles move rapidly in all directions.
- Temperature is simply a measurement of the average kinetic energy of a substance's particles.
Open oven temperature converter →