Chemistry: Particles, Reactions and Quantitative Thinking
Core ideas
Chemistry at this level rests on the particle model: matter is made of atoms, atoms combine into molecules and ions, and observable properties follow from how particles are arranged and how strongly they attract each other. You should be able to name the three states of matter and explain changes of state in terms of energy and particle motion, distinguish elements, compounds and mixtures, and interpret a chemical formula (subscripts count atoms within a unit; coefficients in front of a formula count whole units).
Chemical reactions are described by word and symbol equations, which must be balanced because atoms are conserved. Recognise common reaction types: combination, decomposition, combustion, and simple acid–base neutralisation. Linking reaction types to observations (gas evolved, precipitate formed, colour change, temperature change) is frequently examined.
Traps to watch
- Confusing mass number with atomic number, or assuming atoms of the same element always have equal neutrons (isotopes).
- Forgetting to balance equations, or changing subscripts instead of coefficients.
- Mixing up 'melting' and 'dissolving'; dissolving is a physical process forming a mixture, not a change of state.
- Treating a reaction as 'using up' mass when a gas escapes.
Worked example
Balance: __H₂ + __O₂ → __H₂O. Count oxygen first: O₂ supplies two O atoms, so place 2 before H₂O. That gives four H atoms on the right, so place 2 before H₂. Check: 4 H and 2 O each side. Rule of thumb: balance the element that appears in the fewest formulas first, and leave single-element substances until last.