Particle Theory, States of Matter & Diffusion

Particle theory is a model for explaining matter. In this topic, students use particle arrangement, motion, energy and attraction to explain observable properties.

• represent solids, liquids and gases with particle models
• relate heating and cooling to particle motion and changes of state
• compare the effect of particle attractions in the three states
• explain shape, volume and compressibility
• connect particle ideas to density and melting point
• explain diffusion in liquids and gases

Core reasoning pattern: observation → particle arrangement/motion/spacing → scientific explanation.

Heating generally increases average particle kinetic energy. Cooling decreases it. During a physical change of state, the substance stays the same; particle arrangement and motion change.

• Melting: solid → liquid; particles gain enough energy to leave fixed positions and move past one another.
• Freezing: liquid → solid; particles lose energy and become held in a more fixed arrangement.
• Evaporation: higher-energy particles escape from a liquid surface into the gas state.
• Boiling: gas formation occurs throughout the liquid at the boiling point under the stated conditions.
• Condensation: gas → liquid; particles lose energy and become much closer together.

Important: heating does not make individual particles grow. The model changes spacing and motion, not particle size.

Density

For the same substance, the gas state is usually far less dense than the liquid because the particles are much farther apart. Across different materials, density also depends on particle mass and packing, so spacing alone is not enough.

Compressibility

A gas-filled syringe compresses much more than a water-filled syringe because gas particles have large average gaps that can be reduced.

Melting point

In a simple Year 7 model, stronger attractions generally require more energy to disrupt the solid arrangement, so they can be associated with a higher melting point.

Diffusion is the net spreading of particles from a region of higher concentration toward lower concentration because particles are continually moving randomly.

• Vinegar or perfume vapour spreads through air because gas particles move and mix.
• Food colouring spreads through water because particles in the liquid are moving.
• Diffusion generally happens faster at higher temperature because average particle kinetic energy is greater.
• Diffusion is generally faster in gases than liquids because gas particles are much farther apart and move more freely through the available space.

AC9S7U05: use particle theory to describe particle arrangement, motion and attraction and relate these to substance properties.

• E1: model changes in particle arrangement during changes of state.
• E2: relate particle motion and energy to particle distance in different states.
• E3: compare attractive effects in solid, liquid and gas states.
• E4: connect absorbed or released heat energy to particle motion.
• E5: explain state properties using particle theory.
• E6: investigate density, melting point and compressibility using particle ideas.
• E7: explain diffusion in liquids and gases.

• “Particles stop moving in solids.” They still vibrate.
• “Particles expand when heated.” Average spacing and vibration change; particle size does not.
• “The gaps in a particle diagram are air.” The gaps represent separation in a simplified model.
• “Boiling and evaporation are the same.” Evaporation is a surface process; boiling occurs throughout a liquid at its boiling point under the stated conditions.
• “A change of state makes a new substance.” Melting, freezing, evaporation, boiling and condensation are physical changes.

Particle diagrams are models: colours, sizes and distances are not literal or to scale.

1. Why can a liquid flow while keeping nearly the same volume?
   Answer

   Particles stay close but can move past one another.

2. Why are gases easy to compress?
   Answer

   There are large average gaps between gas particles.

3. What happens to particle motion when a substance is heated before a state change?
   Answer

   Average motion or vibration increases.

4. Why does perfume spread through a room?
   Answer

   Moving gas particles diffuse through the air.

5. Why does food colouring usually spread faster in warm water?
   Answer

   Particles have greater average kinetic energy, increasing diffusion rate.

6. What changes during melting: particle identity or arrangement?
   Answer

   Arrangement and motion change; particle identity is preserved.

7. What is density if 240 g occupies 300 mL?
   Answer

   0.80 g/mL.

8. Why can a sealed rigid gas container develop higher pressure when heated?
   Answer

   Faster particles collide more often and more forcefully with the walls.

• AC9S7U04 — Forces, motion and gravity
• AC9S7U06 — Pure substances, mixtures and separation
• All Year 7 Science topics