Many Chemistry mistakes are vocabulary mistakes in disguise. Two words sound close, the question uses one, and the answer needs the other. This glossary gives 17 terms, each with a plain meaning, a small example, the word it is most often confused with and a lesson to open.
The English, Malay and Chinese concept bridge shows the same terms in all three languages, with example sentences. The exam is written in English, so the English word is always the one to write in your answer.
Matter and structure
1. Element, compound, mixture. An element has one kind of atom, such as oxygen. A compound has different atoms chemically joined, such as water. A mixture has substances that are not joined, such as salt in water. Confused with: compound versus mixture. Hydrogen and oxygen gas mixed in a jar is a mixture. Burn them and you form a compound. See atoms, elements and isotopes.
2. Atom, ion, molecule. An atom is the smallest unit of an element. An ion is an atom or group with a charge, such as Na⁺. A molecule is a group of atoms joined by covalent bonds, such as H₂O. Confused with: molecule versus ion. NaCl is not made of molecules, it is a lattice of ions. See bonding and structure.
3. Relative atomic mass (Ar) and relative formula mass (Mr). Ar compares one atom with a reference standard, and Mr adds the Ar values for a whole formula. For CO₂, Mr = 12 + 16 + 16 = 44. Confused with: Ar versus Mr. Ar is for one element, Mr for a formula. Learn the routine in calculating a relative formula mass.
4. Mole and mass. A mole is an amount of substance. Mass is how heavy it is in grams. 44 g of CO₂ is 1 mol, but 44 g of H₂O is about 2.4 mol because its Mr is 18. Confused with: “moles” as a mass. See converting mass to amount.
Solutions and acids
5. Concentration. It tells you how much solute is dissolved in a volume of solution, such as mol/dm³ or g/dm³. 0.50 mol in 2.0 dm³ is 0.25 mol/dm³. Confused with: concentration versus amount. Amount is moles, concentration is moles per volume. See solution volume and concentration.
6. Solute, solvent, solution. The solute dissolves in the solvent, and together they make the solution. In salt water, salt is the solute and water is the solvent. Confused with: dissolving versus melting. Sugar in tea dissolves, while butter in a pan melts. See particle models.
7. Concentrated and strong. Concentrated describes a lot of acid in the solution. Strong describes an acid that ionises almost completely in water. Confused with: the two words as one. A dilute solution of a strong acid and a concentrated solution of a weak acid are both possible. See acidity and particles.
8. Acid, base, alkali. An acid gives H⁺ ions in water. A base neutralises an acid. An alkali is a base that dissolves in water. Confused with: base versus alkali. Copper oxide is a base but not an alkali, because it does not dissolve. See classifying a solution.
Reactions and change
9. Oxidation and reduction. Oxidation is gain of oxygen or loss of electrons, and reduction is loss of oxygen or gain of electrons. They happen together, which is why the word “redox” exists. Check which definitions your syllabus expects. Confused with: which one is which. See ordering metals from displacement.
10. Electrolyte and electrode. The electrolyte is the liquid or solution that conducts because of moving ions. The electrodes are the solid conductors that carry current in and out. Confused with: the two as one object. See electrolysis reasoning.
11. Exothermic and endothermic. An exothermic change gives out energy, and the surroundings warm up. An endothermic change takes in energy, and the surroundings cool down. Confused with: the direction of heat flow. Think of the reaction and its surroundings separately. See energy changes.
12. Rate and yield. Rate is how fast a product forms. Yield is how much product forms in total. Confused with: fast and high. A catalyst can make a reaction faster without changing the final amount. See extracting a rate and percentage yield.
13. Reversible reaction and dynamic equilibrium. A reversible reaction can go both ways. At equilibrium, forward and reverse reactions continue at the same rate, so amounts stay steady. Confused with: “the reaction has stopped”. It has not, it is balanced. See a dynamic state.
Organic chemistry and analysis
14. Saturated and unsaturated. A saturated molecule has only single carbon-carbon bonds, as in ethane. An unsaturated one has at least one double bond, as in ethene. Confused with: saturated solutions. The word means something different in the two topics. See saturated and unsaturated structures.
15. Homologous series and isomer. A homologous series is a family with the same functional group and a pattern in formulas. Isomers have the same molecular formula but different structures. Confused with: family versus same formula. See family versus isomer.
16. Residue and filtrate. After filtering, the residue stays on the paper and the filtrate passes through. Confused with: which one holds the dissolved substance. Dissolved solute goes through with the filtrate. See choosing a separation.
17. Observation and inference. An observation is what you see, hear or measure. An inference is what you conclude from it. “A gas turned limewater cloudy” is an observation, and “the gas is carbon dioxide” is an inference. Confused with: writing the inferred substance as if it were seen. See observation versus inferred identity.
How to use this glossary
Choose three pairs a week. For each, cover the page and write one sentence of your own that could not use the other word.
If a pair keeps returning in your mistake log, add it to the mistake log with a retest date. The Chemistry learning guide shows where each term fits in the topic map.
In online one-to-one Chemistry tuition, a teacher can ask you to define a term and then use it in a fresh example, which shows quickly whether the idea or only the word is secure.