Chemistry: Matter, Atoms and Reactions
Must-know concepts
States of matter and changes of state, elements, compounds and mixtures, atomic structure (protons, neutrons, electrons, atomic number, mass number), the periodic table (groups, periods, metals and non-metals), chemical formulae and balancing equations, acids, bases and salts, pH, oxidation and reduction, types of reaction (combination, decomposition, displacement, neutralisation), and water, air and corrosion.
Traps to watch
- Atom versus molecule versus ion: an atom is a single particle; a molecule is two or more atoms bonded; an ion carries charge.
- Atomic number versus mass number: atomic number counts protons; mass number counts protons plus neutrons.
- Balancing: change coefficients only, never subscripts. H₂ + O₂ → H₂O must become 2H₂ + O₂ → 2H₂O.
- Acid strength versus concentration: a dilute strong acid can be less corrosive than a concentrated weak acid.
- pH: below 7 is acidic, 7 neutral, above 7 basic. Do not confuse pH with litmus colour alone.
- Physical versus chemical change: no new substance means physical.
Rule of thumb
Count atoms on each side of an equation before and after balancing. For any unfamiliar reaction, name the reactants and products, then classify it - the classification usually reveals the answer.
Worked example
How many atoms are in 2 moles of water, H₂O? One molecule has 3 atoms (2 H + 1 O). One mole contains 6.022 × 10²³ molecules. Atoms = 2 × 3 × 6.022 × 10²³ = 3.61 × 10²⁴ atoms. The trap is forgetting to multiply by the number of atoms per molecule.