A
- Acid
- A substance that releases hydrogen ions (H⁺) when dissolved in water. Acids have a pH below 7.
- Activation energy
- The minimum energy that colliding particles need for a reaction to happen.
- Alkali
- A soluble base. It releases hydroxide ions (OH⁻) in water and has a pH above 7.
- Alkane
- A saturated hydrocarbon, meaning every carbon-carbon bond is single. Alkanes fit the formula CₙH₂ₙ₊₂.
- Alkene
- An unsaturated hydrocarbon containing a carbon-carbon double bond. Alkenes fit the formula CₙH₂ₙ.
- Anode
- The positive electrode in electrolysis. Negative ions move towards it and lose electrons there.
- Atom
- The smallest part of an element that can still be that element, made of a nucleus with electrons around it.
- Atomic number
- The number of protons in the nucleus of an atom. It identifies the element.
C
- Catalyst
- A substance that speeds up a reaction by giving it a route with lower activation energy. A catalyst is not used up.
- Cathode
- The negative electrode in electrolysis. Positive ions move towards it and gain electrons there.
- Compound
- A substance made of two or more elements chemically bonded together in a fixed ratio.
- Concentration
- How much solute is dissolved in a given volume of solution, often measured in g/dm³ or mol/dm³.
- Covalent bond
- A bond formed when two atoms share a pair of electrons.
- Cracking
- Breaking long-chain hydrocarbon molecules into shorter, more useful ones, using heat with a catalyst or steam.
D
- Displacement reaction
- A reaction in which a more reactive element takes the place of a less reactive one in a compound.
E
- Electrolysis
- Using an electric current to break down an ionic compound that is molten or dissolved in water.
- Electron
- A tiny negatively charged particle that moves around the nucleus of an atom.
- Element
- A substance made of only one type of atom, with a single atomic number.
- Endothermic reaction
- A reaction that takes in energy from its surroundings, so the surroundings get colder.
- Exothermic reaction
- A reaction that transfers energy to its surroundings, so the surroundings get warmer.
F
- Fractional distillation
- Separating a mixture such as crude oil into fractions by their different boiling points, in a column that is hotter at the bottom.
- Functional group
- An atom or group of atoms that gives a family of compounds its characteristic reactions, such as C=C in alkenes.
G
- Group
- A vertical column of the periodic table. Elements in a group have the same number of outer-shell electrons.
H
- Halogen
- An element in Group 7, such as chlorine or bromine. Halogens are non-metals that exist as pairs of atoms.
- Homologous series
- A family of organic compounds with the same functional group and general formula, each member differing from the next by a CH₂ unit.
- Hydrocarbon
- A compound containing only hydrogen and carbon atoms.
I
- Ion
- An atom or group of atoms that has gained or lost electrons, giving it an electric charge.
- Ionic bond
- The strong attraction between oppositely charged ions, usually formed when a metal transfers electrons to a non-metal.
- Isotope
- One of two or more forms of the same element with the same number of protons but different numbers of neutrons.
L
- Limiting reactant
- The reactant that is used up first in a reaction, which sets the maximum amount of product.
M
- Mass number
- The total number of protons and neutrons in the nucleus of an atom.
- Mixture
- Two or more substances that are mixed but not chemically joined, so they can be separated by physical methods.
- Mole
- The unit for amount of substance. One mole contains 6.02 × 10²³ particles.
N
- Neutralisation
- A reaction between an acid and a base or alkali that forms a salt and water.
- Noble gas
- An element in Group 0. Noble gases are very unreactive because their outer shells are full.
O
- Oxidation
- Gain of oxygen or loss of electrons.
P
- Percentage yield
- The actual mass of product obtained divided by the maximum possible mass, multiplied by 100.
- Period
- A horizontal row of the periodic table. Elements in a period have the same number of electron shells.
- pH
- A scale from 0 to 14 that shows how acidic or alkaline a solution is. Below 7 is acidic, 7 is neutral and above 7 is alkaline.
- Polymer
- A very large molecule made of many small repeating units called monomers joined together.
- Product
- A substance made in a chemical reaction. Products appear on the right-hand side of an equation.
R
- Reactant
- A substance that takes part in and is used up in a chemical reaction. Reactants appear on the left-hand side of an equation.
- Reactivity series
- A list of metals in order of how readily they react, used to predict displacement and extraction methods.
- Reduction
- Loss of oxygen or gain of electrons.
- Relative atomic mass
- The average mass of an atom of an element, taking account of how common each isotope is, on a scale where a carbon-12 atom has a mass of 12.
- Relative formula mass
- The sum of the relative atomic masses of all the atoms shown in a chemical formula.
- Reversible reaction
- A reaction in which products can react to re-form the reactants, shown with the ⇌ symbol.
- Rf value
- In chromatography, the distance moved by a substance divided by the distance moved by the solvent.
S
- Salt
- A compound formed when the hydrogen of an acid is replaced by a metal or by ammonium.
T
- Thermal decomposition
- Breaking a compound down into simpler substances by heating it.
- Titration
- A method that finds the volume of one solution needed to react exactly with a known volume of another, using an indicator to show the end point.
- Transition metal
- A metal from the central block of the periodic table. Transition metals often form coloured compounds and act as catalysts.
See also
Wording here is general. Use the definition your own board and mark scheme expect when you write exam answers.
Part of the GCSE Chemistry revision set: the revision guide and self-marking quiz.