WJEC GCSE Chemistry Revision
GCSE Chemistry · Wales
Revising for WJEC GCSE Chemistry? Below is every topic in the course, in specification order, with a short summary. Sign up free to get the full notes, quizzes, flashcards and exam questions for each one — filtered to the WJEC course so you only revise what's on your exam.
WJEC GCSE Chemistry topics
1 The nature of substances and chemical reactions
Substances can be classified into elements, compounds, and mixtures. An **element** is a pure substance consisting of only one type of atom, represented by a chemical symbol (e.g., O for Oxygen, Fe for Iron). A **compound** is a pure substance made of two or more different elements chemically bonded together in a fixed ratio (e.g., H₂O, CO₂). Compounds have properties distinct from their constituent elements. A **mixture** consists of two or more substances (elements or compounds) not chemically bonded together. Their components retain their individual properties and can be separated by physical means.
2 Atomic structure and the periodic table →
Atoms are the basic building blocks of all matter. They consist of a central nucleus containing positively charged **protons** and neutral **neutrons**, surrounded by negatively charged **electrons** orbiting in shells. The number of protons defines the element (atomic number, Z). In a neutral atom, the number of electrons equals the number of protons. The mass number (A) is the sum of protons and neutrons. Understanding these subatomic particles is crucial for comprehending chemical reactions and properties.
3 Chemical bonding, structure and properties
Ionic bonds form between metals and non-metals. Metals lose electrons to form positive ions (cations), and non-metals gain electrons to form negative ions (anions). These oppositely charged ions are held together by strong electrostatic forces, forming a giant ionic lattice structure. This type of bonding typically results in compounds with high melting and boiling points, and they conduct electricity when molten or dissolved in water.
4 Bulk and surface properties including nanoparticles
Nanoparticles are tiny particles, typically between 1 and 100 nanometres (nm) in size, containing only a few hundred atoms. To put this into perspective, a human hair is about 80,000 nm thick. Their incredibly small size gives them a very large surface area to volume ratio compared to larger particles of the same material. This unique characteristic significantly alters their properties from the bulk material, making them exhibit different chemical and physical behaviours. They are not visible to the naked eye and require powerful microscopes to be observed.
5 Chemical calculations
The **relative atomic mass (Ar)** of an element is the weighted average mass of an atom of the element, taking into account its isotopes, relative to 1/12th the mass of a carbon-12 atom. You can find this on the periodic table. The **relative formula mass (Mr)** of a compound is the sum of the relative atomic masses of all the atoms in its chemical formula. For example, the Mr of H₂O is (2 x Ar of H) + (1 x Ar of O) = (2 x 1) + (1 x 16) = 18. These are unitless quantities.
6 Acids, bases and salts
Acids are substances that produce hydrogen ions (H⁺) when dissolved in water. They typically have a pH less than 7. Common strong acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃). Bases are substances that neutralise acids. Alkalis are soluble bases that produce hydroxide ions (OH⁻) when dissolved in water, giving a pH greater than 7. Examples include sodium hydroxide (NaOH) and potassium hydroxide (KOH).
7 Metals and their extraction
Metals are typically found on the left and centre of the periodic table. They generally have high melting and boiling points, are good conductors of heat and electricity, and are malleable (can be hammered into shapes) and ductile (can be drawn into wires). Most metals are solids at room temperature, with the exception of mercury. These properties are due to their metallic bonding structure, where delocalised electrons are free to move throughout the lattice of positive metal ions, allowing for thermal and electrical conductivity.
8 Chemical reactions and energy
Chemical reactions involve energy changes. Exothermic reactions release energy into the surroundings, often as heat, causing the temperature of the surroundings to increase. Examples include combustion and neutralisation. The products have less energy than the reactants. Endothermic reactions absorb energy from the surroundings, causing the temperature to decrease. Examples include photosynthesis and thermal decomposition. The products have more energy than the reactants. Both types of reactions are crucial for understanding energy transfers.
9 Rate of chemical change
The rate of a chemical reaction measures how quickly reactants are used up or products are formed. A fast reaction produces products rapidly, while a slow reaction takes a long time. For example, burning wood is a fast reaction, whereas rusting iron is a very slow one. Understanding reaction rates is crucial in industry to control processes and in everyday life, like food preservation. Chemists measure reaction rates by monitoring changes in concentration, mass, or volume over time, or by observing changes in properties like pH or temperature.
10 Reversible reactions and industrial processes
Unlike many reactions that proceed to completion, reversible reactions can go in both the forward and reverse directions. This means products can react to reform the original reactants. The double arrow (⇌) is used in chemical equations to signify a reversible reaction. For example, in the Haber process, nitrogen and hydrogen react to form ammonia, but ammonia can also decompose back into nitrogen and hydrogen. Understanding this dynamic equilibrium is crucial for controlling industrial processes.
11 Organic chemistry →
Organic chemistry focuses on compounds containing carbon atoms, often bonded to hydrogen, oxygen, nitrogen, or halogens. Hydrocarbons are the simplest organic compounds, made only of carbon and hydrogen. They are classified as saturated or unsaturated. Saturated hydrocarbons contain only single carbon-carbon bonds, while unsaturated hydrocarbons contain at least one carbon-carbon double or triple bond. Alkanes are saturated hydrocarbons, forming a homologous series with the general formula CnH2n+2. Understanding these basic building blocks is crucial for grasping more complex organic structures and reactions.
12 Chemical analysis →
Flame tests are used to identify certain metal ions (cations) based on the characteristic colour they produce when heated in a Bunsen flame. A small sample of the compound is introduced into a hot flame on a clean nichrome wire loop. The heat causes electrons to jump to higher energy levels; as they fall back, they emit light of specific wavelengths, resulting in a unique colour for each metal ion. For example, lithium produces a crimson flame, sodium an orange flame, and copper a blue-green flame. Barium gives a pale green, and potassium a lilac flame.
More WJEC GCSE Science
Topic summaries are ScienceForge originals, not official WJEC material. Check the official WJEC specification for the definitive content list.