Short on time? This page is the recap. For explanations, the diagram and worked examples, use the full study guide.
Recap
- The phase of a substance depends on a balance between interparticle forces (pulling particles together) and thermal motion (pushing them apart).
- Crystalline solid: particles in a regular, repeating 3-D pattern. Amorphous solid (such as glass): no repeating pattern. In both, particles vibrate but do not move past one another.
- Liquid: particles touch, move, collide and swap neighbours. Hydrogen bonding or polarity can give short-range order; higher temperature disrupts it.
- Gas: particles far apart, moving in straight lines between collisions. Spacing and collision frequency depend on temperature, pressure and volume.
- Particles keep their size and identity in every phase. Melting and boiling overcome forces between particles, not bonds inside molecules.
Key relationships
| Relationship | Meaning | Use it to |
|---|---|---|
| Molar volume = M ÷ density | volume of 1 mol of the substance | compare phases quantitatively |
| V(solid) ≈ V(liquid) | particles touch in both | explain why melting changes volume only a little |
| V(gas) ≫ V(liquid) | particles separate completely | explain why boiling changes volume hundreds or thousands of times |
| spacing ratio = ∛(volume ratio) | length scales as the cube root of volume | estimate how far apart gas particles are |
Assumptions behind the models
- Each circle stands for one whole particle (atom, molecule or ion), drawn the same size in every phase.
- A 2-D drawing stands for a 3-D arrangement.
- Density data are for a stated temperature and pressure; gas densities change a lot with conditions.
Mistakes to avoid
- Gaps in a liquid. Liquid particles touch; draw them almost as close as in the solid.
- Gas in a heap. Spread gas particles through the whole box.
- Changing the particle. Same size, same atoms, in all phases.
- Splitting molecules. Boiled Br₂ is still Br₂, not Br atoms.
- “Solid particles are still.” They vibrate about fixed positions.
- “All solids are crystalline.” Amorphous solids have no long-range order.
- Phase diagrams. Interpreting them is not assessed in this course; spend your time on particle models.
Quick self-check
- Name one crystalline and one amorphous solid. (For example, table salt and glass.)
- Why do the solid and liquid of one substance have similar molar volumes? (Particles are in contact in both phases.)
- A gas has 1000 times the molar volume of its liquid. Roughly how many times farther apart are its particles? (∛1000 = 10 times.)
- What happens to the spacing of gas particles when the gas is compressed at constant temperature? (It decreases, and collisions become more frequent.)
Next: practice questions.