BGCSE Chemistry 0570 key terms

A very small piece of matter, such as an atom, ion or molecule. The way particles are arranged and how they move explains the properties and behaviour of solids, liquids and gases.

The spreading of particles from a region of higher concentration to a region of lower concentration because of their random motion. It happens fastest in gases and slowest in solids.

A model that describes matter as made of tiny particles in constant random motion. The energy of the motion increases with temperature, which explains changes of state and gas pressure.

The temperature at which a solid changes into a liquid. At this temperature the particles gain enough energy to break free from their fixed positions, and the value is a useful test of purity.

The change of a liquid into a gas at temperatures below its boiling point. The fastest particles escape from the surface, so the remaining liquid cools down.

The number of protons in the nucleus of an atom. It identifies the element and equals the number of electrons in a neutral atom.

The total number of protons and neutrons in the nucleus of an atom. It is always a whole number and is written above the chemical symbol.

One of two or more atoms of the same element that have the same number of protons but different numbers of neutrons. Isotopes have identical chemical properties but different masses.

An atom or group of atoms that has gained or lost electrons, giving it a positive or negative electrical charge. Ions form when metals lose electrons and non-metals gain them.

The strong electrostatic attraction between oppositely charged ions. It usually forms when a metal transfers electrons to a non-metal, producing a giant lattice of ions.

A bond formed when two atoms share one or more pairs of electrons. It usually occurs between non-metal atoms and produces molecules or giant covalent structures.

The smallest particle of a covalent substance that can exist on its own, made of two or more atoms joined by covalent bonds, such as water or oxygen.

The average mass of the atoms of an element compared with one-twelfth of the mass of a carbon-12 atom. It takes the natural mixture of isotopes into account.

The sum of the relative atomic masses of all the atoms shown in the formula of a molecule or compound. It is used to convert between masses and amounts in moles.

The amount of a substance that contains the same number of particles as there are atoms in 12 g of carbon-12. One mole contains about 6.02 x 10^23 particles.

A measure of how much solute is dissolved in a given volume of solution, usually expressed in moles per cubic decimetre or grams per cubic decimetre.

The reaction between an acid and a base that produces a salt and water. The hydrogen ions from the acid combine with hydroxide ions from the base.

A scale from 0 to 14 that measures how acidic or alkaline a solution is. Values below 7 are acidic, 7 is neutral and values above 7 are alkaline.

A compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion. Common examples include sodium chloride and copper sulfate.

A substance that increases the rate of a reaction by providing an alternative pathway of lower activation energy, without being used up or permanently changed itself.

How quickly reactants are used up or products are formed, usually measured as a change in amount or mass per unit time. It is affected by concentration, temperature, surface area and catalysts.

The breakdown of an ionic compound, either molten or dissolved in water, by passing an electric current through it. Positive ions move to the cathode and negative ions to the anode.

A reaction in which a substance loses electrons, or gains oxygen, or loses hydrogen. A substance that is oxidised has its oxidation state increased.

A reaction in which a substance gains electrons, or loses oxygen, or gains hydrogen. A substance that is reduced has its oxidation state decreased.

A reaction in which oxidation and reduction happen at the same time. One substance loses electrons while another gains them, so the total number of electrons is unchanged.

Describing a reaction that releases energy to the surroundings, usually as heat. The temperature of the surroundings rises and the products have less energy than the reactants.

Describing a reaction that takes in energy from the surroundings, usually as heat. The temperature of the surroundings falls and the products have more energy than the reactants.

The minimum energy that colliding particles must have for a reaction to happen. It acts as an energy barrier, and a catalyst lowers it.

The state reached in a reversible reaction when the forward and reverse reactions occur at equal rates, so the amounts of reactants and products stay constant.

A reaction in which the products can react together to reform the original reactants. Such reactions can reach a balance called equilibrium in a closed system.

A reaction in which a more reactive element takes the place of a less reactive element in a compound. It is common in the reactions of metals with salts or acids.

A list of metals arranged in order of how readily they react, with potassium most reactive and gold least. It predicts reactions with water, acids and metal salts.

A rock or mineral from which a metal can be extracted profitably. Ores often contain metal oxides or sulfides mixed with unwanted material called gangue.

A mixture of a metal with other elements, usually other metals, made to improve properties such as strength or resistance to corrosion. Brass and steel are common examples.

A compound containing only carbon and hydrogen atoms. Hydrocarbons such as methane and ethene are the main components of fuels and are widely used in industry.

A hydrocarbon with only single bonds between its carbon atoms, forming a family with the general formula CnH2n+2. Alkanes are saturated and burn to give carbon dioxide and water.

A very large molecule made by joining many small units called monomers together. Examples include polythene, made from ethene, and proteins made from amino acids.

A gas in the atmosphere that absorbs and re-emits infrared radiation, helping to keep the Earth warm. Carbon dioxide and methane are important examples linked to climate change.

A practical method of finding the concentration of a solution by adding a solution of known concentration until the reaction is just complete, using an indicator to show the end point.

A substance that changes colour depending on whether a solution is acidic or alkaline. Litmus and methyl orange are common examples used in titrations.