IGCSE Chemistry Glossary
Every term below is defined to match Cambridge 0620 mark-scheme language. Use the definition to answer "define" questions directly. Each term links to the full topic notes.
Looking for exam technique words like "describe", "explain" or "deduce"? Those are command words, not chemistry terms. See the Command Words guide for all 14 definitions.
A
- Acid
- A substance that produces hydrogen ions (H+) when dissolved in water, has a pH below 7, and turns litmus red.
- Activation energy
- The minimum energy that colliding particles must have for a reaction to occur.
- Addition polymerisation
- A reaction in which many small unsaturated monomer molecules (alkenes) join together to form a long-chain polymer with no other products.
- Addition reaction
- A reaction in which atoms are added across the C=C double bond of an alkene, converting it to a saturated compound.
- Alkali
- A soluble base that produces hydroxide ions (OH-) in aqueous solution and has a pH above 7.
- Alkali metals
- The elements in Group I of the periodic table (lithium, sodium, potassium, etc.), which are soft metals that react vigorously with water to form alkaline hydroxide solutions and hydrogen.
- Alloy
- A mixture of a metal with one or more other elements (metals or non-metals) to give improved properties such as greater hardness or corrosion resistance.
- Amphoteric oxide
- An oxide that reacts with both acids and bases to form a salt and water only.
- Anode
- The positive electrode in electrolysis, where negatively charged ions (anions) are attracted and oxidation occurs.
- Atom
- The smallest particle of an element that can take part in a chemical reaction, consisting of a nucleus (protons and neutrons) surrounded by electrons.
- Avogadro constant
- The number of particles (atoms, molecules or ions) in one mole of a substance: 6.02 x 10²³ per mol.
B
- Base
- A substance that reacts with an acid to form a salt and water only; bases are metal oxides, metal hydroxides, or ammonia.
C
- Catalyst
- A substance that increases the rate of a reaction without being used up or appearing in the products, by providing an alternative reaction pathway with lower activation energy.
- Cathode
- The negative electrode in electrolysis, where positively charged ions (cations) are attracted and reduction occurs.
- Chromatography
- A technique used to separate and identify mixtures of soluble substances by allowing them to move with a solvent up a stationary phase (such as paper).
- Collision theory
- The theory that a reaction occurs only when particles collide with the correct orientation and with energy equal to or greater than the activation energy.
- Compound
- A substance formed when two or more elements are chemically combined in fixed proportions and cannot be separated by physical means.
- Concentration
- The amount of solute (in mol) dissolved per unit volume of solution, expressed in mol/dm³ (mol per cubic decimetre).
- Condensation polymerisation
- A reaction in which monomers with two functional groups join together repeatedly, with a small molecule (usually water) lost at each step, forming a polymer.
- Covalent bond
- A bond formed by the sharing of a pair of electrons between two atoms, holding the atoms together.
- Crystallisation
- A purification technique in which a dissolved solid is recovered as pure crystals by evaporating the solvent until a saturated solution forms, then cooling to allow crystals to grow.
D
- Diffusion
- The net movement of particles from a region of higher concentration to a region of lower concentration, as a result of their random motion.
E
- Electrode
- A conductor (graphite or metal) through which current enters or leaves an electrolyte during electrolysis.
- Electrolysis
- The decomposition of an ionic compound (when molten or in aqueous solution) by passing a direct electric current through it.
- Electrolyte
- An ionic compound that conducts electricity when molten or dissolved in water, because it contains free-moving ions.
- Electroplating
- The use of electrolysis to deposit a thin layer of one metal onto the surface of another object, for protection or decoration.
- Element
- A substance made of only one type of atom and cannot be broken down into simpler substances by chemical means.
- Empirical formula
- The simplest whole-number ratio of atoms of each element in a compound, for example CH2O for glucose.
- Endothermic
- A reaction that absorbs heat energy from the surroundings, causing the temperature of the surroundings to fall, and where the products have more energy than the reactants.
- Equilibrium
- The state reached in a reversible reaction when the rate of the forward reaction equals the rate of the reverse reaction and the concentrations of reactants and products remain constant.
- Exothermic
- A reaction that releases heat energy to the surroundings, causing the temperature of the surroundings to rise, and where the products have less energy than the reactants.
F
- Filtration
- A separation technique used to separate an insoluble solid from a liquid by passing the mixture through a filter paper, which retains the solid residue.
- Fractional distillation
- A separation technique used to separate a mixture of liquids with different boiling points by heating the mixture and condensing the vapours at different heights in a fractionating column.
G
- Giant covalent structure
- A three-dimensional lattice of atoms held together by strong covalent bonds throughout the structure, giving very high melting points; examples include diamond and silicon dioxide.
- Group
- A vertical column in the periodic table; elements in the same group have the same number of electrons in their outer shell and similar chemical properties.
H
- Halogens
- The reactive non-metal elements in Group VII of the periodic table (fluorine, chlorine, bromine, iodine, astatine), which exist as diatomic molecules and react with metals to form salts.
- Homologous series
- A family of organic compounds with the same general formula and functional group, showing a gradual change in physical properties and similar chemical properties, with each member differing by CH2.
- Hydrocarbon
- An organic compound containing carbon and hydrogen only.
I
- Ion
- A charged particle formed when an atom or group of atoms gains or loses one or more electrons.
- Ionic bond
- The strong electrostatic force of attraction between oppositely charged ions, formed when electrons are transferred from a metal atom to a non-metal atom.
- Isomer
- One of two or more compounds that have the same molecular formula but different structural arrangements of atoms.
- Isotope
- Atoms of the same element that have the same number of protons but different numbers of neutrons, and therefore different mass numbers.
M
- Metallic bonding
- The strong electrostatic attraction between a lattice of positive metal ions and a sea of delocalised (free-moving) electrons surrounding them.
- Mixture
- Two or more substances that are not chemically combined, can be present in any proportion, and can be separated by physical means.
- Mole
- The amount of substance that contains the same number of particles (6.02 x 10²³) as there are atoms in exactly 12 g of carbon-12.
- Molecular formula
- The formula showing the actual number of atoms of each element in one molecule of a compound, for example C6H12O6 for glucose.
- Monomer
- A small molecule that can react with many identical or similar molecules to form a polymer.
N
- Neutralisation
- The reaction between an acid and a base (or alkali) that produces a salt and water only, resulting in a solution of pH 7 when stoichiometric amounts react.
- Noble gases
- The unreactive elements in Group 0 (Group 18) of the periodic table (helium, neon, argon, etc.) that have a full outer electron shell and exist as monatomic gases.
O
- Oxidation
- A reaction in which a substance gains oxygen, loses hydrogen, or loses electrons; in terms of oxidation numbers, oxidation is an increase in oxidation number.
- Oxidising agent
- A substance that oxidises another substance and is itself reduced (gains electrons) during the reaction.
P
- Percentage yield
- The actual yield of a product expressed as a percentage of the theoretical (calculated) yield: (actual yield / theoretical yield) x 100.
- Period
- A horizontal row in the periodic table; elements in the same period have the same number of electron shells, and properties change across a period from metallic to non-metallic.
- pH
- A logarithmic scale from 0 to 14 used to measure the acidity or alkalinity of a solution: pH below 7 is acidic, pH 7 is neutral, pH above 7 is alkaline.
- Polymer
- A long-chain molecule made of many small repeating units (monomers) joined together.
R
- Rate of reaction
- The change in concentration of a reactant or product per unit time; it can be measured by how quickly a reactant is used up or a product is formed.
- Reactivity series
- A list of metals arranged in order of decreasing reactivity, from most reactive (potassium) to least reactive (gold), used to predict displacement reactions and methods of extraction.
- Redox
- A reaction in which both oxidation (loss of electrons) and reduction (gain of electrons) occur simultaneously.
- Reducing agent
- A substance that reduces another substance and is itself oxidised (loses electrons) during the reaction.
- Reduction
- A reaction in which a substance loses oxygen, gains hydrogen, or gains electrons; in terms of oxidation numbers, reduction is a decrease in oxidation number.
- Relative atomic mass
- The average mass of the atoms of an element relative to 1/12 of the mass of a carbon-12 atom, taking into account the relative abundance of each isotope.
- Relative molecular mass
- The sum of the relative atomic masses of all atoms in one molecule of a compound, as shown by its molecular formula.
- Reversible reaction
- A reaction that can proceed in both the forward and reverse directions, shown by the sign of two half-arrows pointing in opposite directions.
- Rf value
- In chromatography, the ratio of the distance travelled by a substance to the distance travelled by the solvent front from the same starting point; used to identify substances.
- Rusting
- The corrosion of iron, forming hydrated iron(III) oxide (rust), which requires both water and oxygen to be present.
S
- Sacrificial protection
- A method of preventing rusting by attaching a more reactive metal (such as zinc or magnesium) to iron; the more reactive metal corrodes preferentially and protects the iron.
- Salt
- A compound formed when the hydrogen ions of an acid are replaced (wholly or partly) by metal ions or ammonium ions, produced in a neutralisation reaction.
- Saturated
- An organic compound (such as an alkane) in which all carbon-carbon bonds are single bonds, so no more hydrogen atoms can be added.
- Simple distillation
- A separation technique used to separate a liquid from dissolved solids (or to separate two liquids with very different boiling points) by heating to vaporise the liquid and then condensing the vapour.
- Substitution reaction
- A reaction in which one atom or group of atoms in a molecule is replaced by a different atom or group of atoms, as occurs with alkanes and halogens under UV light.
T
- Transition elements
- The block of metallic elements in the periodic table between Groups II and III (the d-block), which typically form coloured compounds, act as catalysts, and can have variable oxidation states.
U
- Unsaturated
- An organic compound (such as an alkene) that contains one or more carbon-carbon double bonds (C=C) and can therefore undergo addition reactions.
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