NCEA L2 Chemistry key terms

The amount of substance containing 6.02 × 10²³ particles. Used as the counting unit that links mass, concentration and gas volume in calculations.

The number of particles in one mole, 6.02 × 10²³ mol⁻¹. Converts between number of particles and amount in moles.

The mass of one mole of a substance in grams per mole, found by adding relative atomic masses. Bridges mass and moles in calculations.

The average mass of an atom of an element compared with one-twelfth the mass of a carbon-12 atom. Given on periodic tables as Ar.

Atoms of the same element with the same number of protons but different numbers of neutrons, so they have different masses but identical chemical behaviour.

The total number of protons and neutrons in the nucleus of an atom. Written as the superscript before the element symbol.

The simplest whole-number ratio of atoms of each element in a compound. Determined from percentage composition or experimental masses.

The actual number of atoms of each element in one molecule of a compound. A whole-number multiple of the empirical formula.

The quantitative relationship between amounts of reactants and products shown by the coefficients in a balanced equation. Basis of reacting-mass and volume calculations.

A chemical equation in which the number of atoms of each element is equal on both sides, achieved by adjusting coefficients only, never subscripts.

A letter in brackets after a formula showing physical state: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water. Included in full balanced equations.

The reactant that is used up first in a reaction, determining the maximum amount of product formed. Other reactants are in excess.

The actual mass of product obtained expressed as a percentage of the theoretical mass predicted from stoichiometry. Always less than or equal to 100%.

The amount of solute dissolved per unit volume of solution, usually expressed in mol L⁻¹. Calculated as moles divided by volume in litres.

A solution of accurately known concentration, prepared by dissolving an exact mass of solute and making up to a known volume in a volumetric flask.

Adding solvent to a solution to lower its concentration. The amount of solute stays constant, so c₁V₁ = c₂V₂.

The volume occupied by one mole of any gas at a stated temperature and pressure, approximately 24.0 L mol⁻¹ at standard conditions.

The relationship pV = nRT linking pressure, volume, amount of gas, and absolute temperature, with R the gas constant.

A volumetric technique in which a solution of known concentration is added from a burette to a measured volume of another until the reaction is just complete.

The point in a titration where the reactants have reacted in the exact mole ratio shown by the equation. Detected experimentally by an indicator colour change.

The observed point in a titration where the indicator changes colour. Ideally as close as possible to the equivalence point.

A weak acid or base that shows a distinct colour change over a narrow pH range, chosen so the change occurs near the equivalence point of a titration.

A measure of hydrogen ion concentration, calculated as the negative logarithm to base 10 of [H₃O⁺]. A decrease of one unit means tenfold more acidic.

A substance that donates a proton (H⁺) to another species. In aqueous solution it increases the hydrogen ion concentration.

A substance that accepts a proton from another species. In aqueous solution it increases hydroxide ion concentration or decreases hydrogen ion concentration.

Two species related by the loss or gain of one proton, such as an acid and the base formed after it donates H⁺.

The reaction between an acid and a base producing water and, in most cases, a salt. The hydrogen ions and hydroxide ions combine to form water.

An ionic compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion, for example sodium chloride or copper sulfate.

An insoluble solid that forms when two solutions are mixed and ions combine to produce a compound that cannot stay dissolved.

The maximum amount of a substance that dissolves in a given volume of solvent at a stated temperature, often expressed in grams per 100 g of water.

An equation showing only the ions and molecules that actually change during a reaction, with spectator ions omitted. Used for precipitation and acid–base reactions.

An ion present in a reaction mixture that does not take part in the chemical change and is left out of the net ionic equation.

An atom or group of atoms that has gained or lost electrons, giving it a positive or negative charge, such as Na⁺ or SO₄²⁻.

The loss of electrons by a species, corresponding to an increase in its oxidation number. Often observed as gain of oxygen or loss of hydrogen.

The gain of electrons by a species, corresponding to a decrease in its oxidation number. Often observed as loss of oxygen or gain of hydrogen.

A reaction in which oxidation and reduction occur together, with electrons transferred from one species to another.

A number assigned to an element in a compound representing the charge it would have if electrons were fully transferred. Used to identify oxidation and reduction.

A species that causes another substance to be oxidised by accepting electrons from it, and is itself reduced in the process.

A species that causes another substance to be reduced by donating electrons to it, and is itself oxidised in the process.

The decomposition of an ionic compound by passing a direct electric current through it, either molten or in aqueous solution.

A conductor through which current enters or leaves an electrolyte. The cathode is where reduction occurs; the anode is where oxidation occurs.

The heat energy exchanged with the surroundings during a reaction at constant pressure, given the symbol ΔH and usually measured in kJ mol⁻¹.

A reaction that releases heat energy to the surroundings, so the temperature rises and the enthalpy change is negative.

A reaction that absorbs heat energy from the surroundings, so the temperature falls and the enthalpy change is positive.

The minimum energy colliding particles need before a reaction can occur. Shown as the height of the barrier on an energy profile.

A substance that increases the rate of a reaction by providing an alternative pathway with lower activation energy, without being consumed overall.

The model explaining reaction rate in terms of particles colliding with sufficient energy and correct orientation. More frequent or more energetic collisions increase rate.

The change in concentration of a reactant or product per unit time. Affected by concentration, temperature, surface area, pressure and catalysts.

The state of a reversible reaction in a closed system where forward and reverse rates are equal, so concentrations remain constant though reactions continue.

When a system at equilibrium is disturbed by a change in conditions, the system shifts to oppose that change and restore equilibrium.

A reaction in which products can react to reform reactants, so the reaction can reach a state of equilibrium rather than going to completion.

A compound based on carbon chains or rings, usually containing hydrogen, and often oxygen, nitrogen or halogens. Excludes simple carbon compounds such as carbonates.

The specific group of atoms within an organic molecule that determines its characteristic chemical reactions and properties, such as –OH or –COOH.

A family of organic compounds with the same functional group, successive members differing by a CH₂ unit and showing a gradual trend in physical properties.

Compounds with the same molecular formula but different structural arrangements of atoms, giving them different physical and sometimes chemical properties.

Describing an organic molecule containing at least one carbon–carbon double or triple bond, which can undergo addition reactions.

A reaction in which two or more reactants combine to form a single product, typical of unsaturated compounds adding hydrogen, halogens or water across a double bond.

A large molecule formed when many small monomer molecules join together, either by addition or by condensation with the loss of a small molecule.

Chemical analysis that determines how much of a substance is present, using measurements such as mass, volume or concentration.

Chemical analysis that identifies which substances are present in a sample, for example by flame tests or precipitation reactions.