CIE IGCSE Chemistry Term Definitions

all important term definitions from all chapters in the igcse syllabus

Condensation

State change of a gas into a liquid.

Evaporation

A process occurring at the surface of a liquid involving the state change of a liquid into a gas at a temperature below the boiling point.

Freezing point

The temperature at which a liquid change into a solid. This temperature is equivalent to the melting point.

Melting point

The temperature at which a solid change into a liquid. Pure substances exhibit a distinct melting point.

Boiling point

The temperature at which a substance changes from a liquid to a gas. Pure substances exhibit a distinct boiling point.

Diffusion

The spreading out of particles (movement from an area of high concentration to an area of low concentration) due to random movement of particles

Elements

A substance that cannot be further divided into simpler substances by chemical methods. An element is made of only one type of atom which contain the same number of protons.

Compounds

Substances formed by the chemical bonding of two or more different elements in fixed proportions.

Mixtures

Combinations of two or more substances that are not chemically bonded together and can be separated by physical means.

Atom

The smallest unit of an element. It has a central nucleus containing neutrons and protons surrounded by electrons in shells. An atom contains equal numbers of protons and electrons.

Proton number (atomic number)

The number of protons in the nucleus of an atom

Mass number (nucleon number)

The total number of protons and neutrons found in the nucleus of an atom

Isotopes

Different atoms of the same element that have the same number of protons but different numbers of neutrons.

Ion

An electrically charged atom or group of atoms formed by the loss or gain of electrons: cation (+), anion (-)

Lattice

A regular arrangement (repeating pattern) of atoms, molecules, or ions in a solid.

Ionic bond

A strong electrostatic force of attraction between oppositely charged ions.

Giant ionic lattice

A regular arrangement of alternating positive and negative ions

Molecule

A group of atoms, whether of the same or different elements, covalently bonded together.

Covalent bond

A chemical bond formed by the sharing of one or more pairs of electrons between two atoms.

Intermolecular force

A weak force of attraction between simple molecules.

Metallic bonding

The electrostatic attraction between the positive ions in a giant metallic lattice and a 'sea' of delocalised electrons.

Delocalised electrons

Electrons that are free to move within a metal structure. The electrons are not bound to individual metal ions and contribute to the metal's electrical conductivity.

Molecular formula

A formula showing the actual number and type of different atoms of each element present in one molecule of a compound or element.

Empirical formula

A formula showing the simplest whole number ratio of atoms or ions present in a compound.

Relative atomic mass, Ar

The average mass of the isotopes of an element compared to 1/12thof the mass of an atom of 12C.

Relative molecular mass, Mr

The sum of the relative atomic masses in a covalent molecule

Relative formula mass, Mr

The sum of the relative atomic masses in an ionic compound

Mole

The amount of substance which contains 6.02 × 1023 atoms, ions, or molecules.

Avogadro constant

The number of atoms, ions, or molecules in one mole of a substance. (6.02 × 1023)

Molar mass

The mass of one mole of a compound. It has units of g/mol and is numerically equivalent to Mr

Molar gas volume

The volume occupied by one mole of any gas at room temperature and pressure. (24 dm3/mol)

Concentration

Amount of substance per dm3 of solution (Unit: g/dm3 or mol/dm3)

Electrolysis

The decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.

Electrode

The conducting rods by which electric current flows into or out the electrolyte.

Inert electrode

An electrode that does not react with the electrolyte or the products of electrolysis. E.g. carbon (graphite) and platinum.

Anode

The positive (+) electrode where oxidation takes place.

Cathode

The negative (−) electrode where reduction takes place.

Electrolyte

A molten or aqueous substance which conducts electricity and is chemically changed by it.

Bauxite

An aluminium ore

Cryolite

A substance used in the electrolysis of alumina, function: (a) Acts as a solvent for alumina (b) Improves electrical conductivity (c) Lowers operating temperature

Hydrogen-oxygen fuel cell

A fuel cell that uses hydrogen and oxygen to produce electricity with water as the only chemical product.

Exothermic reaction

An exothermic reaction transfers thermal energy to the surroundings leading to an increase in temperature of the surroundings.

Endothermic reaction

An endothermic reaction absorbs thermal energy from the surroundings leading to a decrease in temperature of the surroundings.

Enthalpy change

The transfer of thermal energy during a reaction is called the enthalpy change, ΔH, for the reaction. ΔH is negative for exothermic reactions and positive for endothermic reactions.

Activation energy

The activation energy, Ea, is the minimum energy that colliding particles must have to react.

Rate of reaction

Product formation or reactant consumption per unit time.

Catalyst

A substance which increases the rate of a chemical reaction by providing an alternative reaction path which has a lower activation energy, Ea. A catalyst is chemically unchanged at the end of the reaction.

Enzyme

Enzymes are biological catalysts made from protein molecules

Reversible reaction

A chemical reaction that can go both directions - the forward reaction (which forms the products) and the reverse reaction (which forms the reactants).

Equilibrium

Takes place when a reversible reaction is carried out in a closed system, dynamic equilibrium occurs when: (a) both forward and reverse reactions occur at the same rate (b) concentrations of reactants and products are no longer changing

Haber process

The production of ammonia at 450°C, 20000 kPa (200 atm) and an iron catalyst. Equation: N2(g) + 3H2(g) ⇌ 2NH3(g)

Contact process

Involving the conversion of sulfur dioxide to sulfur trioxide at 450°C, 200 kPa (2 atm) and a vanadium(V) oxide catalyst. Equation: 2SO2(g) + O2(g) ⇌ 2SO3(g)

Redox reaction

A reaction which involves simultaneous oxidation and reduction.

Oxidation

Gain of oxygen OR loss of electrons OR increase in oxidation number.

Reduction

Loss of oxygen OR gain of electrons OR decrease in oxidation number.

Oxidising agent

A substance that oxidises another substance and is itself reduced.

Reducing agent

A substance that reduces another substance and is itself oxidised.

Acid

A proton (H+) donor. Acids can dissolve in water and produce H+(aq) ions.

Strong acid

An acid that is completely dissociated in aqueous solution, resulting in a high concentration of H+ions.

Weak acid

An acid that is partially dissociated in aqueous solution, resulting in a relatively low concentration of H+ions.

Base

A proton (H+) acceptor. E.g. oxides or hydroxides of metals and ammonia.

Alkali

A soluble base which produces OH-(aq) ions in water.

Neutralisation

The process in which an acid reacts with a base to form water.

Hydrated substance

A substance that is chemically combined with water.

Anhydrous substance

A substance containing no water.

Water of crystallisation

Water molecules present in hydrated crystals. E.g. CuSO4•5H2O and CoCl2•6H2O

Periodic Table

A table of elements arranged in order of increasing proton number.

Period

The horizontal rows of elements in the Periodic Table. The atoms of elements in a period have the same number of occupied shells.

Group

A vertical column of elements in the Periodic Table containing elements with the same number of electrons in their outer shell.

Electronic Configuration

A shorthand method of describing the arrangement of electrons within the electron shells of an atom.

Alkali Metals

The six metallic elements in Group I of the Periodic Table.

Halogens

The non-metallic elements found in Group VII of the Periodic Table.

Transition Metals

The elements found in the centre of the Periodic Table, between Groups II and III. Transition elements are known for their high densities, melting points, and their tendency to form coloured compounds; they also frequently serve as catalysts, and their ions exhibit variable oxidation numbers.

Noble Gases

The elements found in Group VIII of the Periodic Table. They are unreactive, monoatomic gases.

Metal

A class of chemical elements characterized by their shiny appearance, malleability, ductility, and ability to conduct heat and electricity effectively. Metals typically have high melting points and boiling points. They can be shaped into various forms without breaking or cracking.

Alloy

A mixture of two or more metals or of a metal with a non-metal. They are harder and stronger than pure metals.

Reactivity series

An order of reactivity of elements, giving the most reactive elements first - potassium, sodium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold

Corrosion

Chemical degradation of metals and alloys due to exposure to oxygen, water, or environmental substances.

Rust

Orange-brown hydrated iron (III) oxide layer formed on iron and steel surfaces due to corrosion.

Sacrificial protection

Rust prevention method using a more reactive metal layer (e.g. Zinc) to shield iron or steel surfaces from corrosion.

Hematite

An iron ore

Air

Approximately 78% nitrogen, N2, 21% oxygen, O2, and the remainder as a mixture of noble gases and carbon dioxide, CO2, and trace amounts of other gases

Catalytic converter

A device for converting pollutant exhaust gases (e.g. oxides of nitrogen and carbon monoxide) from cars into less harmful emissions (nitrogen and carbon dioxide). e.g. 2CO + 2NO → 2CO2 + N2

Fertiliser

A substance applied to soil to provide essential nutrients that support plant growth and health. Fertilizers help replace mineral salts depleted from the soil. E.g. Ammonium, nitrates, and NPK fertilisers.

Photosynthesis

The reaction between carbon dioxide and water to produce glucose and oxygen in the presence of chlorophyll and using energy from light, equation: 6CO2 + 6H2O → C6H12O6 + 6O2 carbon dioxide + water → glucose + oxygen

Displayed formula

A formula showing all the atoms and bonds in one molecule

Molecular formula

A formula showing the actual number of atoms of each element in one molecule of a substance.

Structural formula

A formula showing how groups of atoms are arranged in a molecule.

Structural isomers

Compounds that have the same molecular formula but different structural formulae

Homologous series

A family of similar compounds with similar chemical properties due to the presence of the same functional group. Members of the homologous series possess the same general formula that display a trend in physical properties. Each member differs from one member to the next by a -CH2- unit.

Functional group

The atom or group of atoms responsible for the chemical properties of a homologous series

Hydrocarbon

A compound made of carbon atoms and hydrogen atoms only.

Alkane

A type of hydrocarbon characterized by single carbon-carbon bonds, making it a saturated hydrocarbon. Alkanes have the general formula CnH2n+2. They are generally unreactive, except for combustion and substitution reactions.

Saturated compound

A compound, such as an alkane, in which all carbon-carbon bonds are single bonds.

Substitution reaction

A reaction in which an atom or group of atoms is replaced by another atom or group of atoms.

Alkene

A type of hydrocarbon characterized by carbon-carbon double bond, making it an unsaturated hydrocarbon. Alkenes have the general formula CnH2n and are more reactive than alkanes due to the presence of the double bond, allowing them to undergo addition reactions.

Unsaturated compound

A compound, such as an alkene, in which one or more carbon- carbon bonds are not single bonds.

Addition reaction

A reaction in which reactants combine to form only one product.

Alcohol

Organic compounds characterized by the presence hydroxyl (-OH) groups attached to a carbon atom. The general formula for alcohols is CnH2n+1OH.

Carboxylic acid

Organic compounds characterized by the presence of a carboxyl group (-COOH). The general formula for carboxylic acids is CnH2n+1COOH.