Chemistry: Matter, Atoms, Reactions and Everyday Chemicals
Core concepts
Chemistry questions cover states of matter, atomic structure, the periodic table, chemical formulae and equations, acids, bases and salts, oxidation and reduction, and common industrial and household chemicals such as acids, alkalis, fertilizers and fuels.
Common traps
- Balancing equations: the number of atoms of each element must be equal on both sides; coefficients change, subscripts do not.
- Acid versus base indicators: litmus turns red in acid and blue in base; phenolphthalein is colourless in acid and pink in base.
- pH scale: below 7 is acidic, 7 neutral, above 7 basic; a change of one pH unit means a tenfold change in hydrogen ion concentration.
- Mixture versus compound: a compound has a fixed ratio and new properties; a mixture does not.
- Valency errors: write the symbol, then the valency, then cross them to get the formula.
Rule of thumb
For a salt, the metal (or ammonium) comes first and the non-metal or acid radical second, with valencies crossed. Sodium (valency 1) plus sulphate (valency 2) gives Na₂SO₄.
Worked example
Balance: H₂ + O₂ → H₂O. Two oxygen atoms on the left require two water molecules: H₂ + O₂ → 2H₂O. This gives four hydrogen atoms on the right, so use 2H₂: 2H₂ + O₂ → 2H₂O. Count each element on both sides as a final check.