A chemical reaction forms new substances. Possible observations include a gas being released, a precipitate forming or a change in temperature or colour. An observation alone is not always enough to identify the reaction; use the substances, conditions and products given in the question.
Balance atoms, not formula subscripts
In an ordinary chemical reaction, atoms are rearranged rather than created or destroyed. A balanced equation has the same number of atoms of each element on both sides. Change coefficients in front of formulae, not the subscripts within them.
A useful method is to count each element, balance a convenient element first, and recount after every change. Reduce whole-number coefficients to their simplest ratio when appropriate.
- H₂O and H₂O₂ are different substances.
- State symbols such as (s), (l), (g) and (aq) describe the physical state; (aq) means dissolved in water.
Recognise common reaction patterns
A combination reaction forms one product from two or more reactants. A decomposition reaction splits a compound into simpler substances. In displacement, a more reactive element can replace a less reactive element from a suitable compound.
In double displacement, ions exchange partners; some such reactions form an insoluble precipitate. A reaction may also be described in other ways, such as oxidation or reduction, so categories are not always mutually exclusive.
Put it into practice
A worked example
Balance Mg + O₂ → MgO and name the broad reaction type.
- There are two oxygen atoms on the left but one on the right.
- Write 2MgO on the right, then 2Mg on the left.
- Recount: two magnesium and two oxygen atoms on each side.
Answer: 2Mg + O₂ → 2MgO. This is a combination reaction.
Try it yourself
Have a go before revealing the answer.
Balance H₂ + O₂ → H₂O.
Show answer
2H₂ + O₂ → 2H₂O.
Answer: 2H₂ + O₂ → 2H₂O.
Classify Fe + CuSO₄ → FeSO₄ + Cu.
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A displacement reaction: iron displaces copper from copper sulphate solution under suitable conditions.
Answer: A displacement reaction: iron displaces copper from copper sulphate solution under suitable conditions.
Watch out for these
- Never alter a correct chemical formula just to make atom counts match.
- A coefficient multiplies every atom in the formula that follows it.
Continue with official resources
Match these concept notes to your current textbook and syllabus. They are independent explanations, not official board publications.