How to revise for GCSE Chemistry

Key relationships:

• Moles: $n = \dfrac{m}{M}$
• Concentration: $c = \dfrac{n}{V}$ (with $V$ in dm³)
• Percentage yield: $\dfrac{\text{actual}}{\text{theoretical}} \times 100$

Example: how many moles are in 9.8 g of $\text{H}_2\text{SO}_4$? $M = 2(1) + 32 + 4(16) = 98$, so $n = 9.8 \div 98 = 0.1$ mol. Practise a few calculations every session.

Balance by counting atoms of each element on both sides and adjusting coefficients, never subscripts. Practise with reactions you need to know: combustion, neutralisation, metals with acids, displacement. Higher-tier students also need half equations and ionic equations.

Flashcards and blank-page recall work well for:

• Tests: hydrogen (squeaky pop), oxygen (relights a glowing splint), carbon dioxide (limewater turns milky), chlorine (bleaches damp litmus paper).
• Trends in the periodic table and properties linked to bonding.
• Organic chemistry: functional groups and their reactions.

For each required practical, know the method, variables, safety and how to process results. For longer answers, sequence your points logically and use precise terms (ions, electrons, bonds). See GCSE Science revision for the 6-mark technique.

Each session: five calculation questions, a set of flashcards, and one exam question marked against the mark scheme. Space topics over weeks and mix them. See how to study chemistry and get help with tricky calculations from our chemistry homework help.

4.0 g of sodium hydroxide (NaOH) is dissolved to make 250 cm³ of solution. What is the concentration in g/dm³, and in mol/dm³?

1. Convert the volume: $250 \text{ cm}^3 = 0.25 \text{ dm}^3$.
2. Mass concentration: $4.0 \div 0.25 = 16$ g/dm³.
3. Moles: $M_r$ of NaOH $= 23 + 16 + 1 = 40$, so $n = 4.0 \div 40 = 0.1$ mol.
4. Concentration: $0.1 \div 0.25 = 0.4$ mol/dm³.

The most common slip is forgetting to convert cm³ to dm³ (divide by 1000).

• Changing subscripts instead of coefficients when balancing.
• Using the wrong relative formula mass: write out the atoms, such as $\text{CaCO}_3 = 40 + 12 + 3(16) = 100$.
• Mixing up oxidation and reduction. OIL RIG: oxidation is loss of electrons, reduction is gain.
• Losing state symbols or units in the final answer.
• Describing a test without giving the result (for example, not saying the limewater turns milky).

You're given a periodic table in the exam, and it holds more answers than many students realise: relative atomic masses for calculations, the group number (the number of outer electrons for main groups), and the period (the number of electron shells). Practise using it to work out formulae, such as magnesium (group 2) and chlorine (group 7) forming $\text{MgCl}_2$, and to explain trends down a group.