Top 200 Secondary 4 Chemistry Vocabulary

Secondary 4 Chemistry vocabulary is the language of particles, reactions, evidence and quantities. Here are 200 clearly explained Chemistry terms, arranged so that readers can move from atomic structure to chemical equations, mole calculations, practical tests, organic chemistry and environmental decisions. Each term is a tool for understanding what is happening, not just a word to memorise.

Students sitting the 2026 GCE O-Level Chemistry will see subject code 6092; the corresponding 2027 SEC G3 Chemistry code is K324. Combined Science is offered under different subject codes. This is a curated independent learning glossary for upper-secondary Chemistry, including a few helpful extension terms, not a list issued or prescribed by MOE or SEAB.

This guide provides:

200 distinct vocabulary entries with readable definitions in ten subject groups.
Examples showing what changes when a student reads a Chemistry question accurately.
Practical investigation language, including observations versus conclusions.
A four-week revision method and self-checking questions.
Direct links to the official 2026 and 2027 syllabus listings.

Top 200 Secondary 4 Chemistry Vocabulary Terms and Meanings

Read the list from the start if you are repairing foundations. If one specific topic is difficult, jump to that cluster and link the words to an actual experiment, equation or diagram. A term such as concentration is much more useful when you can distinguish it from strength.

1. Practical investigations and observing evidence — terms 1–20

An observation is not an inference. ‘The blue colour disappeared’ is an observation; ‘the reactant was used up’ is a possible explanation that needs further evidence.

observation — A description of what is detected or measured.
inference — An explanation proposed from observations and evidence.
hypothesis — A testable suggestion about a possible relationship.
variable — A factor that may take different values.
independent variable — The factor deliberately changed by the investigator.
dependent variable — The measured response to a change.
control variable — A factor deliberately kept the same.
fair test — An investigation that changes the target factor while controlling others.
accuracy — How close a measurement is to an accepted value.
precision — How closely repeated measurements agree.
repeatability — Consistency when the same method and conditions are used.
reproducibility — Consistency when a method is repeated under changed conditions.
calibration — Checking or adjusting an instrument against a known reference.
measurement — Determining a quantity using an instrument or agreed method.
hazard — Something capable of causing harm.
risk — The chance and possible severity of harm in context.
filtration — Separating an insoluble solid from a liquid using a filter.
evaporation — Changing a liquid into vapour, often to recover a dissolved solid.
simple distillation — Separating a volatile liquid by boiling and condensation.
paper chromatography — Separating substances by movement through paper with a solvent.

2. Atoms, elements and the periodic table — terms 21–40

An element is defined by proton number, not by its mass. Isotopes share atomic number but differ in neutron number.

atom — The smallest particle of an element retaining its chemical identity.
element — A substance containing only one type of atom.
compound — A substance made of different elements chemically combined.
mixture — Two or more substances together without chemical bonding between them.
nucleus — The small positively charged centre of an atom.
proton — A positively charged particle in an atomic nucleus.
neutron — An uncharged particle in an atomic nucleus.
electron — A negatively charged subatomic particle.
atomic number — The number of protons in an atom’s nucleus.
mass number — The total number of protons and neutrons.
isotope — Atoms of the same element with different numbers of neutrons.
ion — An atom or group of atoms with a net electric charge.
cation — A positively charged ion.
anion — A negatively charged ion.
electron shell — A region of atomic energy levels occupied by electrons.
valence electron — An electron in the outermost occupied shell.
electronic configuration — The arrangement of electrons in an atom or ion.
periodic table — A chart arranging elements by atomic number.
group — A vertical column of elements in the periodic table.
period — A horizontal row of elements in the periodic table.

3. Chemical bonding, structure and properties — terms 41–60

Structure is the link between bonding and observable properties. Do not say ‘ionic compounds have no electrons’; ask whether charge carriers can move.

ionic bonding — Electrostatic attraction between oppositely charged ions.
covalent bonding — Bonding in which atoms share pairs of electrons.
metallic bonding — Attraction between positive metal ions and delocalised electrons.
electrostatic force — Attraction or repulsion between electrical charges.
ionic lattice — A repeating three-dimensional arrangement of oppositely charged ions.
molecular structure — A structure consisting of discrete molecules.
giant covalent structure — A large network of atoms joined by covalent bonds.
metallic structure — An arrangement of metal ions among mobile electrons.
delocalised electron — An electron not confined to one particular bond or atom.
melting point — Temperature at which a solid becomes liquid at stated pressure.
boiling point — Temperature at which a liquid boils at stated pressure.
electrical conductivity — Ability to allow electric current to pass.
solubility — Maximum amount of a substance that dissolves under stated conditions.
valency — A combining capacity used when determining chemical formulae.
chemical formula — Symbols and numbers identifying the elements and proportions present.
molecular formula — Actual numbers of each atom type in one molecule.
empirical formula — Simplest whole-number ratio of atoms of each element.
structural formula — A representation showing how atoms are connected.
formula unit — The simplest repeating ratio represented by an ionic formula.
allotrope — A different structural form of the same element in one physical state.

4. Reactions and equations: describe change precisely — terms 61–80

Balance equations by adjusting coefficients, never by changing subscripts in chemical formulae. The formula represents the substance; the coefficient represents the amount.

reactant — A substance consumed or changed during a reaction.
product — A substance formed by a chemical reaction.
word equation — A reaction described using substance names.
chemical equation — A reaction represented using chemical formulae.
balanced equation — An equation with the same number of each atom type on both sides.
stoichiometric coefficient — A number before a formula showing reaction proportion.
subscript — A small lower number showing atoms in a chemical formula.
conservation of mass — Total mass remains constant in a closed reacting system.
exothermic reaction — A reaction transferring energy to its surroundings.
endothermic reaction — A reaction taking energy from its surroundings.
activation energy — The minimum energy barrier for a reaction pathway.
catalyst — A substance increasing reaction rate without being consumed overall.
thermal decomposition — Breaking a compound down through heating.
complete combustion — Burning with sufficient oxygen for full oxidation of the fuel.
incomplete combustion — Burning with insufficient oxygen for full oxidation.
neutralisation — Reaction in which an acid and base neutralise each other.
precipitation reaction — A reaction forming an insoluble solid from solutions.
oxidation — Loss of electrons; also gain of oxygen in suitable contexts.
reduction — Gain of electrons; also loss of oxygen in suitable contexts.
displacement reaction — A more reactive element replacing a less reactive one in a compound.

5. Moles, quantities and solution calculations — terms 81–100

A mole is a counting unit. State what is being counted and use the molar mass, volume or concentration appropriate to the question.

mole — An amount containing Avogadro’s number of specified entities.
Avogadro constant — About 6.022 × 10^23 entities per mole.
relative atomic mass — Weighted mean mass of an atom relative to carbon-12 scale.
relative molecular mass — Sum of relative atomic masses in a molecule.
relative formula mass — Sum of relative atomic masses in a formula unit.
molar mass — Mass of one mole of a specified substance.
molar volume — Volume occupied by one mole of gas under stated conditions.
concentration — Amount of solute in a stated volume of solution.
molarity — Amount concentration expressed in moles per cubic decimetre.
mass concentration — Mass of solute per stated solution volume.
stoichiometry — Quantitative ratios among substances in a reaction.
limiting reactant — The reactant exhausted first, limiting possible product.
excess reactant — Reactant remaining after the limiting one is used up.
percentage yield — Actual yield divided by theoretical yield, multiplied by 100.
theoretical yield — Maximum predicted product under the stated model.
actual yield — Amount of product actually obtained.
gas volume — The measured or calculated space occupied by a gas.
solute — The substance dissolved in a solvent.
solvent — The liquid or other medium that dissolves a solute.
solution — A homogeneous mixture formed when substances dissolve.

6. Acids, bases, salts and pH — terms 101–120

Strong is not the same as concentrated. A strong acid ionises largely completely; a concentrated solution contains a relatively large amount of solute per volume.

acid — A substance that produces hydrogen ions in aqueous solution.
base — A substance that reacts with an acid to form a salt and water.
alkali — A water-soluble base producing hydroxide ions in solution.
salt — An ionic compound formed when hydrogen in an acid is replaced by another cation.
pH — A scale related to hydrogen ion activity in solution.
indicator — A substance changing appearance according to chemical conditions.
universal indicator — An indicator mixture displaying different colours over a pH range.
strong acid — An acid that ionises essentially completely in water.
weak acid — An acid that only partially ionises in water.
neutral solution — A solution that is neither acidic nor alkaline at the reference conditions.
titration — Adding one solution accurately to react with a known volume of another.
end-point — The observed signal used to stop a titration.
equivalence point — The point where reactants meet the required reaction ratio.
hydrogen ion — A positively charged hydrogen species, written H+ in school equations.
hydroxide ion — The negatively charged ion OH−.
dilute solution — A solution with relatively low solute concentration.
concentrated solution — A solution with relatively high solute concentration.
soluble salt — A salt that dissolves appreciably in the stated solvent.
insoluble salt — A salt with very low solubility in the stated solvent.
water of crystallisation — Water incorporated in a crystal structure in a fixed ratio.

7. Metals, reactivity and electrolysis — terms 121–140

Learn electrode names by the reaction occurring there: oxidation at the anode and reduction at the cathode. Do not memorise electrode polarity without its context.

reactivity series — An ordering of metals by relative tendency to react.
alkali metal — A reactive Group 1 metal.
alkaline earth metal — An element of Group 2 in the periodic table.
transition metal — A metal from the transition-element region of the periodic table.
halogen — An element in Group 17 with characteristic nonmetal reactivity.
noble gas — A generally unreactive element in Group 18.
metal — An element usually capable of conducting heat and electricity.
non-metal — An element without typical metallic properties.
alloy — A mixture of a metal with other elements.
corrosion — Chemical deterioration of a material through its environment.
rusting — Corrosion of iron involving oxygen and water.
ore — A naturally occurring material from which a useful substance can be extracted.
extraction — Obtaining a substance from its naturally occurring materials.
electrolysis — Using electric current to drive a nonspontaneous chemical change.
electrolyte — A substance with mobile ions that carries current.
electrode — A conductor through which current enters or leaves an electrolyte.
anode — The electrode at which oxidation occurs.
cathode — The electrode at which reduction occurs.
electroplating — Depositing a metal coating by electrolysis.
sacrificial protection — Protecting a metal by letting a more reactive metal corrode first.

8. Qualitative analysis and what test observations mean — terms 141–160

Gas and ion tests need both the reagent and the expected observation. Record actual evidence before deciding what ion or gas might be present.

gas test — A procedure for identifying a gas through a characteristic response.
lighted splint — A burning wooden splint used in selected gas tests.
glowing splint — A non-flaming hot splint that can relight in oxygen.
limewater — A calcium hydroxide solution used to test for carbon dioxide.
effervescence — Visible bubbling caused by gas release.
precipitate — An insoluble solid formed within a liquid mixture.
aqueous — Dissolved in water, often written (aq).
reagent — A substance added to cause or reveal a reaction.
flame test — Using characteristic flame colours to help identify some metal ions.
cation test — A procedure helping identify a positively charged ion.
anion test — A procedure helping identify a negatively charged ion.
ammonium test — A procedure checking for ammonium ions through characteristic chemistry.
chloride test — A procedure for identifying chloride ions under appropriate conditions.
sulfate test — A procedure for identifying sulfate ions under appropriate conditions.
carbonate test — A procedure checking for carbonate ions by acid reaction.
sodium hydroxide test — Using sodium hydroxide solution to observe selected ion reactions.
aqueous ammonia — Ammonia dissolved in water, used in selected qualitative tests.
residue — Material retained after a separation process.
filtrate — Liquid that has passed through a filter.
crystallisation — Producing solid crystals from a solution or melt.

9. Organic chemistry: structure determines reactions — terms 161–180

Organic names describe structure. Use the functional group to infer a reaction; avoid treating all hydrocarbons as interchangeable.

hydrocarbon — A compound containing only hydrogen and carbon.
saturated hydrocarbon — A hydrocarbon without carbon-carbon multiple bonds.
unsaturated hydrocarbon — A hydrocarbon containing a carbon-carbon multiple bond.
homologous series — A family with the same functional group and related chemical properties.
functional group — An atom or group responsible for characteristic organic reactions.
alkane — A saturated open-chain hydrocarbon with only single carbon bonds.
alkene — A hydrocarbon containing at least one carbon-carbon double bond.
alcohol — An organic compound with a hydroxyl group bonded to carbon.
carboxylic acid — An organic compound containing the carboxyl group.
ester — An organic compound containing a carboxylate-derived functional group.
polymer — A large molecule made from repeating structural units.
monomer — A small molecule used to form a polymer.
polymerisation — A process joining many monomers into larger chains or networks.
addition reaction — A reaction adding atoms across a multiple bond.
substitution reaction — A reaction replacing an atom or group with another.
cracking — Breaking larger hydrocarbons into smaller compounds.
fermentation — A process in which microorganisms convert substrates into products.
ethanol — An alcohol with molecular formula C2H5OH.
ethene — An alkene with molecular formula C2H4.
ethanoic acid — A carboxylic acid with formula CH3COOH.

10. Chemistry beyond the laboratory: air, energy and society — terms 181–200

Everyday claims about fuel, air and climate become clearer when you identify the exact chemical species, processes and measurement assumptions.

greenhouse gas — An atmospheric gas that absorbs and re-emits infrared radiation.
greenhouse effect — Warming caused by atmospheric gases affecting outgoing radiation.
carbon dioxide — CO2, a combustion product and important atmospheric gas.
carbon monoxide — CO, a toxic gas from incomplete combustion of carbon fuels.
sulfur dioxide — SO2, a gas that can contribute to acid deposition.
particulate pollution — Tiny airborne solids or droplets that affect air quality.
air pollution — Harmful substances present in the atmosphere.
water pollution — Contamination of water that harms its quality or use.
acid rain — Precipitation acidified by certain atmospheric pollutants.
fossil fuel — Coal, oil or natural gas formed from ancient organic material.
renewable resource — A resource that replenishes on a useful timescale when managed.
carbon footprint — An estimate of greenhouse-gas emissions associated with an activity.
climate change — Long-term changes in climate patterns and statistics.
photosynthesis — A process using light energy to make organic substances.
respiration — Cellular reactions that release usable energy from food molecules.
recycling — Collecting and reprocessing materials for further use.
circular economy — An approach reducing waste by extending material use.
potable water — Water meeting standards for safe drinking.
desalination — Removing dissolved salts from water.
Haber process — Industrial synthesis of ammonia from nitrogen and hydrogen.

Chemistry Worked Examples: From Word to Explanation

1. Is this sample an element, compound or mixture?

Pure copper is an element, because it contains one chemical element. Water is a compound, because its hydrogen and oxygen are chemically combined in a fixed proportion. Seawater is a mixture, because it contains dissolved salts and other substances without a single fixed chemical formula. A mixture may be homogeneous to the eye; that does not make it a compound.

2. Why does a balanced equation matter?

The simple reaction C + O₂ → CO₂ shows that one mole of carbon reacts with one mole of oxygen molecules to form one mole of carbon dioxide under the idealised reaction. Using approximate molar masses, 12 g of carbon and 32 g of oxygen form 44 g of carbon dioxide when both react completely. The mass totals agree. Do not change CO₂ to CO₃ just to make an equation balance: that would name a different substance.

3. Strong versus concentrated acid

A strong acid ionises essentially completely in water, while a weak acid ionises only partly. A dilute solution has relatively little dissolved acid in a given volume; a concentrated solution has more. The first pair of words describes degree of ionisation. The second describes amount per volume. A dilute strong acid and a concentrated weak acid are both conceptually possible.

4. Reading a qualitative test accurately

If carbon dioxide is bubbled through clear limewater, a white cloudiness can appear as calcium carbonate forms. The observation is that the liquid turns milky. The inference is that carbon dioxide may be present, given the test conditions. In a practical answer, state the reagent, what was done and what was seen before naming the suspected gas.

5. Anode and cathode are reaction names

At the anode, oxidation happens; at the cathode, reduction happens. In an electrolytic cell, an external power source drives the reactions. The sign of an electrode cannot be inferred safely from its name without identifying the cell type. This is why understanding oxidation and reduction is stronger than memorising a diagram without labels.

6. Mole calculation with clear units

For an illustrative sample containing 0.50 mol of sodium chloride, the mass is amount × molar mass. Taking molar mass as approximately 58.5 g mol⁻¹, the mass is 0.50 × 58.5 = 29.25 g. A good solution names the quantity, writes the relationship and retains units. Depending on the examination’s data and rounding instructions, the final reported precision may differ.

7. Chemistry of air and daily choices

Carbon dioxide and carbon monoxide are different compounds. Carbon dioxide contributes to the greenhouse effect; carbon monoxide is highly toxic and can form during incomplete combustion. A claim that all ‘carbon emissions’ are chemically identical is therefore inaccurate. Readers need the actual species and its context before assessing risk or policy.

How to Learn Chemistry Vocabulary Without Rote Overload

Students sometimes know the name of a reaction but cannot recognise it when presented as a diagram or observation. The solution is not a longer memorisation list. Build a three-way link: word → particle model → observable consequence.

For ionic bonding, the word describes attraction between oppositely charged ions. The model shows a repeating lattice. The consequence is that many ionic compounds have high melting points and conduct electricity when molten or suitably dissolved, because ions are then mobile. This explanation is more durable than a rule learned without the reason.

For chromatography, the word describes a separation technique. The model considers different interactions with mobile and stationary phases. The observation is that components move different distances under suitable conditions. It is wrong to say that chromatography always proves chemical identity on its own; matching migration can support identification under specified conditions but may not be conclusive.

For reaction rate, connect particle collisions with measurable change. Higher temperature usually increases reaction rate for the same reaction conditions because a larger fraction of collisions have enough energy to overcome the activation barrier. Do not simply write “hotter means more particles”; the number of particles need not increase.

Quick diagnosis: Ask a student to distinguish an atom from an ion, concentration from strength, an observation from an inference and a reactant from a product. These four pairs reveal whether a vocabulary gap is leading to a conceptual gap.

Four-Week Secondary 4 Chemistry Vocabulary Revision Plan

Week 1 — Practical evidence and particles: Learn entries 1–50. Each day draw or explain one particle diagram and write one observation without interpretation.
Week 2 — Reactions and quantities: Learn entries 51–100. Balance equations before doing calculations; label moles, masses and volumes clearly.
Week 3 — Acids, metals and practical tests: Learn entries 101–150. Compare similar terms side by side and practise deciding what evidence identifies a product.
Week 4 — Organic and environmental transfer: Learn entries 151–200. Explain how structural features affect properties, then apply chemical distinctions to a real-life claim about materials, energy or air.

At the end of each week, randomly select 20 terms. For each term, ask for an accurate meaning, a diagram or equation, and a short explanation. Do not penalise a student for knowing a correct idea in simple language; refine the terminology after the concept is clear.

Common Chemistry Mistakes This Glossary Helps Repair

Confusing melting with dissolving: Melting changes a solid to a liquid; dissolving mixes a solute into a solvent.
Calling a mixture a compound: Physical mixing does not necessarily form chemical bonds.
Mixing up strength and concentration: Acid ionisation and amount per volume are different properties.
Changing subscripts to balance equations: Adjust whole-substance coefficients instead.
Claiming every gas is identified by one observation: Use the correct test and consider alternative explanations.
Omitting conditions: Temperature, pressure and solvent can matter to a chemical statement.

Frequently Asked Questions

Is this the official MOE/SEAB 200-word Chemistry list?

No. The guide is an original editorial selection. SEAB publishes syllabuses, and those documents govern the actual assessable content for each examination year and subject.

What is the difference between pure Chemistry and Combined Science Chemistry?

They are different examination subjects with different syllabuses and codes. For 2027, G3 Chemistry is K324, while Science (Physics, Chemistry) is K326 and Science (Chemistry, Biology) is K328. Students should not assume that every glossary topic has the same required depth in all routes.

Do I need every advanced word in these 200 entries?

No. Start with the words your current syllabus and school topics require, then use extension vocabulary to deepen your understanding where it helps. Being able to explain one chemical process accurately matters more than reciting 20 uncertain definitions.

What if the student remembers definitions but performs poorly in practical questions?

Practise identifying variables, choosing suitable observations and recording results before making interpretations. Use actual school-approved practical instructions and lab safety rules rather than experimenting without supervision.

Why is a word list useful when Chemistry is a science subject?

Chemistry questions rely on precise verbs and distinctions. A student who misreads “explain”, “state”, “excess”, “dilute” or “ion” may choose a wrong method despite having learned the relevant process.

Can this guide replace textbooks and teachers?

No. It works as a reference and repair tool alongside school lessons, the year-specific syllabus, supervised practical work and appropriate exercises.

Official Syllabuses and Related Reading

Editorial update: 6 November 2026. Check your examination year and subject route before treating any entry as required syllabus content.

Top 200 Vocabulary for Secondary 4 Chemistry | O-Level and SEC 2027 Key Definitions, Experiments and Mole Calculations