Short on time? This page is the recap. For explanations, the particle diagram and worked examples, use the full study guide.
Recap
- Chemical bonds (covalent, ionic, metallic) hold atoms together inside particles or lattices. They are strong.
- Intermolecular attractions (London dispersion, dipole–dipole, hydrogen bonding, ion–dipole) act between separate particles. They are much weaker.
- A chemical process breaks and/or forms chemical bonds, so new substances appear.
- A physical process changes only intermolecular attractions. Phase changes, dissolving a molecular solid and separating mixtures are physical.
- Quick test: compare the formulas before and after. A new formula means atoms were rearranged, so the change is chemical. The same formula in a new state or spacing means a physical change.
Key relationships
| Process | What changes | Classification |
|---|---|---|
| Melting, boiling, subliming, condensing | Attractions between molecules | Physical |
| Dissolving sugar | Sucrose–sucrose hydrogen bonds replaced by sucrose–water ones | Physical |
| Electrolysis of water; metal + acid; burning | Bonds broken and new bonds formed | Chemical |
| Dissolving HCl gas | H–Cl bond breaks, H₃O⁺ forms | Usually chemical |
| Dissolving NaCl | Ionic bonds break, ion–dipole attractions form | Either, with reasoning |
Energy scale (water): about 41 kJ mol⁻¹ to boil, about 920 kJ mol⁻¹ to break both O–H bonds. More than 20 times as much.
Assumptions behind the classification
- The labels “physical” and “chemical” describe typical cases. Some processes, such as dissolving a salt, sit on the boundary.
- An observation is interpreted through a particle-level model. The model, not the observation alone, decides.
Mistakes to avoid
- Saying bubbles in boiling water are H₂ and O₂.
- Treating any gas or colour change as proof of a reaction.
- Using reversibility as the test.
- Saying melting a molecular solid breaks covalent bonds.
- Giving a label without a bond-level reason.
- Making a claim from one observation when the data offer several.
Quick self-check
- Liquid nitrogen boils away in an open flask. Physical or chemical? (Physical: N₂ molecules stay intact; only London forces are overcome.)
- Which attraction forms when KBr dissolves in water? (Ion–dipole attractions between K⁺ or Br⁻ and water molecules.)
- A heated solid loses mass and gives a gas that turns limewater cloudy. What kind of change? (Chemical: CO₂ is a new substance made by breaking and forming bonds.)
Next: practice questions.