The definitions you must know for SPM Chemistry

Definition questions are among the most predictable marks in SPM Chemistry, and among the most carelessly lost. A definition is a fixed answer: the mark scheme lists the exact points, and a vague paraphrase that “means the right thing” often scores nothing. This is a curated list of the definitions worth knowing word for word, grouped by chapter, with a note on the wording that carries the mark. Keep the glossary open beside it to see each term in context.

Matter, atoms and the mole

• Proton (nucleon) number, the number of protons in the nucleus of an atom.
• Nucleon (mass) number, the total number of protons and neutrons in the nucleus.
• Isotopes, atoms of the same element with the same proton number but different numbers of neutrons (hence different nucleon numbers).
• Relative atomic mass, the average mass of one atom of an element compared to one-twelfth the mass of one atom of carbon-12.
• Relative molecular mass, the mass of one molecule compared to one-twelfth the mass of one atom of carbon-12.
• Mole, the amount of substance that contains as many particles as the number of atoms in 12 g of carbon-12; that number is the Avogadro constant. The key idea examiners want is that a mole is a count of particles, not a mass. The whole of the mole concept rests on this.

Bonding and the Periodic Table

• Ionic bond, the electrostatic force of attraction between oppositely charged ions, formed by the transfer of electrons from a metal to a non-metal.
• Covalent bond, a bond formed by the sharing of electrons between non-metal atoms.
• Valence electrons, the electrons in the outermost occupied shell of an atom, which determine how it bonds.
• Proton number order, across the Periodic Table, elements are arranged in order of increasing proton number.

Acids, bases and salts

• Acid, a substance that ionises in water to produce hydrogen ions, H⁺. The phrase “in water” matters: acidic properties only show when water is present.
• Alkali, a base that is soluble in water and ionises to produce hydroxide ions, OH⁻.
• Strong acid, an acid that ionises completely in water; a weak acid ionises only partially.
• Neutralisation, the reaction between an acid and a base to form salt and water only.
• Salt, a compound formed when the hydrogen ion of an acid is replaced by a metal ion or an ammonium ion.
• Basicity of an acid, the number of hydrogen ions that one molecule of the acid can produce on ionisation.

Rate of reaction

• Rate of reaction, the change in the quantity of a reactant or product per unit time.
• Activation energy, the minimum energy that colliding particles must have in order to react.
• Catalyst, a substance that changes the rate of a chemical reaction without itself undergoing a permanent chemical change. Note “without being used up” and “changes the rate”, not “speeds up” alone.

Thermochemistry

• Exothermic reaction, a reaction that releases heat to the surroundings, so the temperature rises and ΔH is negative.
• Endothermic reaction, a reaction that absorbs heat from the surroundings, so the temperature falls and ΔH is positive.
• Heat of neutralisation, the heat released when one mole of water is formed from the neutralisation of an acid by an alkali.
• Heat of combustion, the heat released when one mole of a substance is completely burned in excess oxygen.

Redox

• Oxidation, the loss of electrons, or an increase in oxidation number (also gain of oxygen or loss of hydrogen).
• Reduction, the gain of electrons, or a decrease in oxidation number (also loss of oxygen or gain of hydrogen).
• Oxidising agent, a substance that oxidises another and is itself reduced.
• Reducing agent, a substance that reduces another and is itself oxidised.

The electron-transfer wording is the safest for full marks; the deeper detail sits in redox equilibrium.

How to make definitions stick

Do not merely reread these, that builds recognition, not recall. Cover the definition, say it aloud from the term alone, then check against the exact wording and repair any missing keyword. Pay attention to the load-bearing phrases: “in water” for an acid, “completely” for a strong acid, “one mole of water” for heat of neutralisation, “without itself undergoing a permanent change” for a catalyst. Those phrases are usually where the mark lives. If your child can explain a concept but keeps losing the definition mark, it is almost always a missing keyword, and it clears quickly with targeted feedback. Our teachers coach SPM Chemistry online, one-to-one in English from RM50 an hour, with a paid one-hour trial to start, drilling definitions to the exact wording the mark scheme rewards.