WJEC GCSE Chemistry 1.1 The Nature of Substances and Chemical Reactions Exam Questions

About WJEC GCSE Chemistry 1.1 The Nature of Substances and Chemical Reactions

This opening topic of WJEC GCSE Chemistry sets up the language and calculations used throughout the course. You need to distinguish elements, compounds and mixtures, work out formulae, and choose separation techniques such as filtration, distillation and chromatography. You must write word and balanced symbol equations, calculate relative formula masses and percentage composition, and use moles to find reacting masses, empirical formulae and percentage yield. These skills appear in almost every later topic, from limestone to titrations. Examiners frequently set multi-step mole calculations and ask why the actual yield is lower than the theoretical yield.

What you need to know

• Substances Elements contain one type of atom; compounds contain two or more elements chemically combined in fixed proportions; mixtures contain substances not chemically combined, which can be separated by physical methods.
• Separation Filtration separates an insoluble solid from a liquid; evaporation or crystallisation recovers a dissolved solid; simple distillation separates a solvent from a solution; fractional distillation separates miscible liquids; paper chromatography separates dyes. Rf = distance moved by substance / distance moved by solvent.
• Equations Write word equations and balanced symbol equations with state symbols (s), (l), (g) and (aq). Balance by changing numbers in front of formulae, never the formulae themselves.
• Relative masses Relative formula mass (Mr) is the sum of the relative atomic masses (Ar) of all atoms in the formula. Percentage of an element = (Ar x number of atoms / Mr) x 100.
• Moles Moles = mass / Mr. Use the ratio of moles in the balanced equation to find reacting masses. Empirical formulae come from dividing the mass (or percentage) of each element by its Ar and finding the simplest whole number ratio.
• Yield Percentage yield = (actual yield / theoretical yield) x 100. The actual yield is lower because of incomplete reactions, losses when separating or transferring the product, and side reactions.

Common mistakes in 1.1 The Nature of Substances and Chemical Reactions

• Balancing equations by changing subscripts, such as writing H2O2 instead of 2H2O. Only the large numbers in front of formulae may change.
• Forgetting to multiply Ar by the number of atoms, e.g. using 16 for O3. In Fe2O3 the Mr is (2 x 56) + (3 x 16) = 160.
• Using the masses from the equation directly without converting to moles. Convert mass to moles, use the equation ratio, then convert back to mass.
• Rounding too early in multi-step calculations. Keep full values until the final answer, then give an appropriate number of significant figures.

Try three 1.1 The Nature of Substances and Chemical Reactions exam-style questions