Edexcel IGCSE Chemistry (Triple Award) Past Papers

Common mistake: A frequent error is failing to apply the molar ratio from a balanced chemical equation during multi-step calculations. For instance, if an equation shows a 1:2 ratio between reactants, many candidates forget to multiply or divide their mole values accordingly, leading to incorrect final masses or concentrations.

How to solve it: Always check the balanced equation before starting a calculation. Explicitly write down the ratio (e.g., "1 mole of A reacts with 2 moles of B") to ensure you use it as a multiplier or divisor in your steps.

Common mistake: Many students incorrectly mention intermolecular forces when discussing ionic compounds or giant covalent structures like diamond and graphite. They also often fail to distinguish between breaking strong covalent bonds and overcoming weak intermolecular forces in simple molecular substances.

How to solve it: Remember that intermolecular forces exist only between molecules in simple molecular substances. To melt giant structures (ionic, metallic, or giant covalent), you must break strong chemical bonds, which requires significantly more energy.

Common mistake: When asked for an observation, students frequently state the name of the gas or substance formed (e.g., "hydrogen is produced") rather than what can actually be seen.

How to solve it: An observation must be a sensory description. Use terms like "fizzing", "effervescence", "bubbles", "solid disappears", or specific "colour changes".

Common mistake: In organic chemistry, students often draw structures with the wrong number of bonds. Common errors include carbon atoms with more or fewer than four bonds, or failing to show the bond between Oxygen and Hydrogen in an alcohol group.

How to solve it: Use a "check-list" for every atom: Carbon must have 4 bonds, Oxygen 2, Nitrogen 3, and Hydrogen 1. In a "displayed" formula, every single bond must be drawn as a line.

Common mistake: A major misconception is that energy is required to make bonds.

How to solve it: Memorise this rule: Bond breaking is endothermic (requires energy), and bond making is exothermic (releases energy). A reaction is exothermic if more energy is released when making bonds than is taken in to break them.