Chemistry · Grades 4–7
Particles, states and changes
Use particle models without treating the spaces between particles as air.
Learn the idea
Particle model
A model represents matter as particles. Heating can change their motion and separation without enlarging the particles themselves.
States
Solids retain shape, liquids flow with a nearly fixed volume, and gases spread through an available container.
Changes
Melting changes arrangement, not the chemical identity of a pure substance.
Limits
Diagrams exaggerate sizes and spacing; they are models, not photographs.
Worked examples
When pure ice melts, which identity stays the same?
Melting changes the physical state of water, not its molecular identity.
The water molecules remain water.
Which sample normally fills the entire available container?
Gas particles spread throughout the available space.
A gas.
Check your understanding
Why can a gas be compressed much more than a liquid?
- Its particles are generally much farther apart.
- Each particle is hollow.
- Its particles vanish under pressure.
Correct this claim: heating makes each molecule swell to explain every expansion.
Which change is physical for pure water?
- Reaction with a metal.
- Decomposition into hydrogen and oxygen.
- Freezing.
Answers and marking notes
Answers and marking notes
- Its particles are generally much farther apart.
Compression mainly decreases the separations between gas particles.
- Heating usually increases particle motion and can increase average separation; molecules are not simply inflated balls.
A response should distinguish particle size from separation.
- Freezing.
Freezing changes state while retaining water as the substance.
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.