Building Chemistry Vocabulary for IGCSE

Chemistry has its own language. Words that mean one thing in everyday English mean something different and precise in chemistry. “Strong” does not mean “powerful” — it means “fully ionised.” “Volatile” does not mean “explosive” — it means “having a low boiling point and evaporating easily.” Students who use everyday language in exam answers lose marks because the examiner marks against scientific vocabulary. Building this vocabulary is not optional decoration. It is a core exam skill.

Why everyday language costs marks

Consider this exam answer: “The substance melted because the bonds broke.”

In everyday English, this sounds reasonable. In chemistry, it is wrong or incomplete. Which bonds? In an ionic compound, the electrostatic forces between ions are overcome, but the ionic bonds within the lattice do not “break” in the way a covalent bond within a molecule breaks. In a simple molecular substance, it is the intermolecular forces that are overcome during melting, not the covalent bonds within the molecules.

The mark scheme for “explain why methane has a low melting point” says something like: “weak intermolecular forces between molecules, little energy needed to overcome them.” A student who writes “weak bonds” scores zero because the examiner cannot tell whether they mean intermolecular forces or covalent bonds.

Precision is the difference between scoring and not scoring.

Categories of chemistry vocabulary

Syllabus definitions

These are terms the syllabus explicitly requires you to define. They appear in command-word questions like “Define the term ‘isotope’” or “What is meant by ‘relative atomic mass’?”

High-priority definitions to learn exactly as the syllabus states them:

• Element: A substance that cannot be broken down into simpler substances by chemical means; it contains only one type of atom.
• Compound: A substance made of two or more elements chemically combined in a fixed ratio.
• Mixture: Two or more substances not chemically combined.
• Atom: The smallest particle of an element that can take part in a chemical reaction.
• Ion: An atom or group of atoms with a positive or negative charge.
• Isotope: Atoms of the same element with the same number of protons but different numbers of neutrons.
• Relative atomic mass (Ar): The average mass of naturally occurring atoms of an element, on a scale where carbon-12 has a mass of exactly 12.
• Mole: The amount of substance that contains 6.02 x 10^23 particles (Avogadro’s number).
• Catalyst: A substance that increases the rate of a chemical reaction without being used up.
• Exothermic: A reaction that releases energy to the surroundings.
• Endothermic: A reaction that absorbs energy from the surroundings.

Learn these word-for-word. Use flashcards with the term on one side and the definition on the other. Test in both directions: given the term, produce the definition; given the definition, name the term.

Observation words

Chemistry exams require specific observation language. Students who write “it changes colour” lose marks. Students who write “the solution changes from blue to colourless” score.

Key observation terms:

• Effervescence / fizzing: not “bubbling” (although some schemes accept it)
• Precipitate: an insoluble solid formed when two solutions are mixed
• Decolourises: becomes colourless (used for bromine water test with alkenes)
• Turns milky / cloudy: used for the limewater test for CO2
• Pungent: a sharp, unpleasant smell (used for describing ammonia or chlorine)

Process words

These describe what happens at the particle level:

• Ionisation: the process of forming ions (not “ionisation” as in radiation — context matters)
• Dissociation: breaking apart into ions when dissolved or molten
• Displacement: a more reactive element takes the place of a less reactive one
• Decomposition: a compound breaks down into simpler substances
• Neutralisation: an acid reacts with a base to form a salt and water

Confusable pairs

These are the terms students most often mix up:

Term	Meaning	Common confusion
Strong acid	Fully ionised	Confused with concentrated
Concentrated acid	High amount of solute per volume	Confused with strong
Atom	Neutral particle	Confused with ion or molecule
Molecule	Two or more atoms covalently bonded	Confused with compound
Compound	Chemically combined elements	Confused with mixture
Oxidation	Loss of electrons (or gain of oxygen)	Confused with reduction
Reduction	Gain of electrons (or loss of oxygen)	Confused with oxidation
Exothermic	Releases energy, temperature rises	Confused with endothermic
Endothermic	Absorbs energy, temperature drops	Confused with exothermic

Card every confusable pair. The exam is designed to test whether you can distinguish between them.

How to build vocabulary throughout the course

In class

When your teacher uses a scientific term, write it down immediately with its definition. Do not wait until revision season. A running glossary built across the year is more complete than one assembled from memory in the final weeks.

While reading

When the textbook uses a term you cannot define precisely, stop and look it up. Write the definition in your glossary. Then use the term in a sentence about the topic you are studying. Using a word in context strengthens memory more than reading the definition alone.

From past papers

Mark schemes are the best vocabulary teachers. Every time you compare your answer to the scheme and find a term you did not use, add it to your glossary. The scheme shows you exactly which words score.

Daily practice

Pick five terms from your glossary each morning. Write the definitions from memory. Check. This takes three minutes and, combined with spaced repetition, makes the entire vocabulary permanent.

Using vocabulary in exam answers

The mark scheme rewards specific terminology. When answering questions:

1. Use the scientific term, not the everyday equivalent. Write “exothermic” not “gives off heat.” Write “electrolyte” not “the liquid that conducts.”
2. Define terms the first time you use them if the question asks “explain.” Writing “the reaction is exothermic (releases energy to the surroundings)” shows the examiner you know what the word means.
3. Be precise about particles. “Atoms” are not the same as “molecules” are not the same as “ions.” Using the wrong particle type in a bonding or electrolysis answer loses the mark.
4. Match the command word to the depth of vocabulary. “State” needs one or two technical terms. “Explain” needs the terms plus the reasoning behind them.

Scientific vocabulary is the currency of the exam. Every term you learn precisely is a potential mark scored.