Particles — atoms, elements, compounds, kinetic theory and states of matter
Atoms, elements and compounds
All substances are made of atoms — the smallest particle of an element that retains the chemical properties of that element. An element contains only one type of atom and cannot be broken down further by chemical means. A compound is formed when atoms of two or more different elements bond together in fixed ratios.
Key distinctions for OCR J258:
| Term | Definition | Example |
|---|---|---|
| Element | One type of atom only | Oxygen (O₂), iron (Fe) |
| Compound | Two or more elements chemically combined | Water (H₂O), sodium chloride (NaCl) |
| Mixture | Two or more substances not chemically joined | Air, seawater |
A mixture can be separated by physical means (filtration, distillation, chromatography) because the components are not bonded. A compound requires a chemical reaction to separate its elements.
Kinetic theory of matter
The kinetic theory explains the properties of solids, liquids and gases in terms of particle arrangement, movement and energy:
Solids
- Particles arranged in a regular, close-packed lattice.
- Particles vibrate about fixed positions — they do not move freely.
- Strong forces of attraction hold particles together.
- Definite shape and volume; very low compressibility.
Liquids
- Particles close together but in a random arrangement.
- Particles can move past each other (flow), giving liquids a definite volume but no fixed shape.
- Weaker forces of attraction than in solids.
Gases
- Particles far apart, moving rapidly and randomly in all directions.
- Negligible forces of attraction between particles.
- No definite shape or volume; highly compressible.
- Gas pressure arises from particles colliding with container walls.
Particle energy and temperature
The average kinetic energy of particles is directly proportional to temperature (in kelvin). Heating a substance increases particle speed and the frequency and force of collisions.
Changes of state
Changes of state are physical changes — no new substance is formed and the change is reversible:
- Melting (solid → liquid): particles gain enough energy to overcome the forces holding them in a lattice.
- Boiling/evaporation (liquid → gas): particles gain enough energy to escape the surface.
- Condensing (gas → liquid): particles lose energy; forces of attraction pull them together.
- Freezing (liquid → solid): particles lose energy; a lattice forms.
- Sublimation (solid → gas directly): e.g. solid carbon dioxide (dry ice).
Melting point and boiling point are fixed for a pure substance. Impurities lower the melting point and widen the melting range — used in PAG C3 (paper chromatography of inks to check purity, and melting point analysis to confirm identity).
Diffusion
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration due to random particle motion. It is evidence for kinetic theory.
- Gases diffuse faster than liquids because gas particles move faster.
- Higher temperature → faster diffusion (particles have more kinetic energy).
- Lighter particles (lower molecular mass) diffuse faster than heavier ones at the same temperature (Graham's Law — Higher tier).
OCR PAG C1 (Making and testing a salt) and PAG C4 (Investigating rates) both require students to understand that reaction rates depend on particle collision frequency, which connects directly to kinetic theory.
Common OCR J258 examiner traps
- Confusing "gas pressure increases because particles move faster" with "particles get bigger" — particles do not change size; only their speed and collision frequency change.
- Stating that particles in a solid "don't move" — they vibrate; only their position is fixed.
- Calling evaporation a chemical change — it is always physical; the substance is unchanged.
- Describing a mixture as having a fixed melting point — only pure substances have sharp melting points.
- Forgetting that diffusion can occur in liquids as well as gases (just more slowly).
AI-generated · claude-opus-4-7 · v3-ocr-chemistry