The Ultimate A-Level Chemistry Glossary A-Z

by Kate Onissiphorou

To succeed in your chemistry studies, you’ll need to have a good grasp of the key concepts and principles. That’s why we’ve created this handy glossary of chemical terms used in A-level chemistry

We’re constantly adding more definitions to our chemistry glossary so be sure to visit this page regularly. And if you’re looking for more help and information, don’t forget to take a look at our chemistry education resources

AcidAlkali metalsAvogadro’s number
Baeyer reagentBaseBecquerel
Dehydration reactionDalton’s LawDecantation
Electron affinityElastomerElectrochemical cell
ElectrochemistryFirst law of thermodynamicsFractional distillation
Gay-Lussac’s lawGibbs free energyGravimetric analysis
Haber processHeisenberg uncertaintyHydronium ion
Ideal gas lawIndependent variableJoule
Kelvin temperature scaleLaw of chemical equilibriumMole
Molar massNeutralisationOctet rule
Periodic lawQuantitative analysisReverse osmosis
Saturated hydrocarbonsThermodynamicsUN ID
Valence electronWeak acid or weak baseXenon
YieldZaitsev rule

Glossary of chemical terms for A level students


A substance with a pH level of less than 7. According to the Bronsted-Lowry definition, an acid is a proton donor. This means it readily loses its hydrogen in a neutralisation reaction with a base. The hydrogen is also easily replaced by a metal, causing it to be released as gas. 

Acid-base buffers

Solutions that can resist small pH changes in a system. Acid-base buffers are important for many biological systems, including human blood. They also have a variety of industrial applications and are commonly used to manufacture food and medical devices. Typically, these buffers consist of an acid and its conjugate base. 


The degree to which a measurement is closest to the actual value being measured. For example, if the temperature of ice is zero degrees Celsius and a thermometer indicates the same, the measurement is said to be accurate. For measurements to be accurate, instruments must be calibrated and there must be agreement among several instruments measuring the same parameter. 

Alkali metals

The elements in the first column of the periodic table. Alkali metals form alkaline or base substances such as sodium hydroxide when they react with water. Their reaction with water is extremely exothermic (generates heat) as hydrogen is released.

Avogadro’s number

The number of unit particles in a mole of any substance. It’s experimentally determined to be 6.02 x 1023. The term is named in honour of the Italian count and scientist, Lorenzo Romano Amedeo Carlo Avogadro. 


A substance (typically an aqueous solution) that has a pH value above 7. According to the Bronsted-Lowry definition, a base is a proton acceptor. This means that during a neutralisation reaction, it strips away the hydrogen proton from the acid to form water. Much like an acid, a strong and concentrated base is corrosive because it can break molecular bonds.

Baeyer reagent

A type of reagent that’s prepared by diluting and cooling a potassium permanganate solution. It’s commonly used for the oxidation of alkenes and alkynes.


The SI unit for measuring radioactivity. Named after the French Nobel laureate for physics, Antoine Henri Becquerel, one unit is equivalent to one radioactive decay per second. 

Balanced equation

A written representation of the chemical or ionic reactions using the symbols of the elements and their corresponding subscript numbers. It’s separated by an arrow to denote the different sides of the reaction. In a balanced equation, there are equal numbers of atoms or charged ions on either side of the equation.


A relatively unstable intermediate chemical species that’s composed of a benzene ring with an adjacent chemical bond. It’s created when the sp2 orbitals overlap on the adjacent carbon atoms of benzene.


A transition metal element with the atomic number 48 and the symbol Cd. Cadmium is one of the most important transition metals due to its wide range of applications. As well as being a component in batteries and solar power cells, it’s used in nuclear reactors to absorb neutrons. Cadmium is also used to manufacture plastics and pigments. 


The process of adjusting and evaluating instruments to ensure they provide accurate and precise measurements. This crucial step involves comparing the readings or outputs of an instrument based on certain standards of measurement. 


One of the most common and familiar units of measuring thermal energy. A calorie is the heat energy needed to increase the temperature of a gram of pure water by one degree Celsius under standard conditions. One calorie is equivalent to 4.184 joules. 


The SI unit for measuring light intensity. A candela is defined as the luminosity of a monochromatic spherical source of 540 nm light with the equivalent radiant energy of 1/683 watts per steradian. 


Chain organic compounds that have subunits of sugar. Composed of carbon, hydrogen, and oxygen, carbohydrates can be viewed simply as hydrated carbon. The general formula for carbohydrates is Cm(H2O)n. As carbohydrates are mainly the product of the photosynthesis of autotrophs, they can be stored in various forms such as root crops and grains. 

Dalton’s Law

A gas law that states the total pressure of a gas mixture is equal to the partial pressures of the gases that the mixture is made up of when they’re added together. 


A simple process of separating mixtures either in a laboratory or an industrial setting. During decantation, small particulate matter is separated from the liquid in which it’s suspended. The solids settle at the bottom, and the particulate-free top layer of liquid can be extracted. 


A type of chemical reaction in which carbon dioxide is removed from a substrate. A carboxyl group is removed from a carbon chain, which then releases carbon dioxide. Decarboxylation plays an important role in biology because it converts amino acids into amines and carboxylic acid into ketones.

Dehydration reaction

A chemical reaction in which water molecules are removed from a reactant. When water molecules are removed from certain types of monomers, the monomers form covalent bonds that lead to polymerization.


The removal of ions from a solution (usually water). Deionization is one of the key stages of water purification. Water passes through a series of ion exchange columns, which remove the dissolved salts and minerals. Deionized water is commonly used in laboratories and the pharmaceutical industry to prepare various mixtures and chemicals.


When gas diffuses into another gaseous region or a vacuum by passing through a porous medium or capillaries. This process is important in many industrial and chemical processes, such as in the separation of uranium isotopes.


A type of polymer material that’s stretchy and compressible. When the tensile force or compressive force is removed, an elastomer returns to its original shape. Rubber is an example of an elastomer.

Electrochemical cell

A device that generates electricity through an oxidation-reduction reaction. The reaction occurs when the two electrodes – the anode and cathode (two different metals) – are immersed in an electrolyte solution. A series of electrochemical cells can be connected to form a battery of a particular voltage and electrical capacity.


The scientific study of how different types of electrolyte solutions, chemical reactions, and electrodes can be combined to produce an electrical current or store electrical energy.

Electron affinity

A measure of the change in energy when an electron is added to atoms in a gaseous state. Every atom can accept electrons at varying degrees – the stronger the attraction, the more energy is released.


A group of compounds or elements that have similar properties. For example, methane, ethane, and propane belong to the family of hydrocarbons known as alkanes. ‘Family’ is also used to refer to elements in the periodic table that have the same number of valence electrons. 

First law of thermodynamics

The concept that energy is neither created nor destroyed but merely transformed from one form to another. It’s also known as the law of conservation of energy. For example, the chemical energy in petrol cars is transformed into heat energy. The heat energy then creates pressure that moves the pistons, transforming it into mechanical energy.

Fractional distillation

A separation technique that relies on the different boiling points of the various components of a liquid. Fractional distillation is typically used on an industrial scale, such as in the oil and gas industry when refining crude oil into different types of fuels. 

Gay-Lussac’s law

A version of the ideal gas law that establishes the relationship between the pressure of the gas and the absolute temperature in Kelvin. It states that if the volume is constant, the pressure of an ideal gas is directly proportional to its absolute temperature.

Gibbs free energy

A measure of the maximum work done by a system that is reversible given constant pressure and temperature. 

Gravimetric analysis

A set of analytical techniques based on examining the mass and density of sample substances, reactants, and products. One famous example of gravimetric analysis is Archimedes’ ‘Eureka’ moment. According to legend, Archimedes determined that the King’s crown wasn’t made of pure gold because of the difference in the displaced water.

Haber process

An industrial process that’s used to produce ammonia through the reaction between nitrogen and hydrogen gas. The technique, which is crucial to fertiliser production, involves the use of a metal catalyst under high temperatures and pressures.

Heisenberg uncertainty principle

The principle that it’s impossible to simultaneously measure the position and momentum or velocity of a particle with 100% accuracy. It was named after the German theoretical physicist, Werner Heisenberg.

Hydronium ion

The positively-charged ion or cation that’s represented by the symbol H3O+ in an aqueous solution.

Ideal gas law

A combination of Charles’ law and Boyle’s law that summarises the relationship between volume, pressure and temperature. It can be written as an equation:

PV = nRT


  • P – pressure
  • V – volume
  • n – number of moles
  • R – ideal gas constant
  • T – temperature

Independent variable

The variable in an experiment that determines the outcome of the dependent variables. The independent variable is tweaked to see its degree of effect on the dependent variable. For example, in a gas experiment, you can tweak the temperature or pressure and then observe the effect on the other variable. If all of the variables are held constant, an increase in temperature results in an increase in pressure and vice versa.


An SI unit used for measuring energy. Named after the English physicist, James Prescott Joule, it’s the equivalent of the work done using one Newton of force moving a distance of one metre. 

Kelvin temperature scale

A method of measuring temperature based on the Celsius scale, but recalibrating for absolute zero temperature. Zero degrees Celsius, which is the freezing point of water, is defined as 273.15 K. This means that absolute zero temperature is -273.15 °C. This is the theoretical temperature at which all particle movements cease as the energy of the system reaches zero.

Law of chemical equilibrium

Chemical reactants and products are said to be in equilibrium when they’re completely balanced and the rate of both forward and backward reactions is equal. Chemical equilibrium specifies the concentration of both the reactants and the products. Their stoichiometric coefficients are constant provided other factors remain constant.


An SI unit used to measure large quantities of very small particles or constituents of a substance. It’s the unit equivalent of a particular fixed quantity of a substance, usually in grams. A mole of any substance is always equal to Avogadro’s number, which is 6.02214076 × 1023.

Molar mass

The mass in grams of one mole of a substance. Molar mass is calculated based on the mass of the individual constituents of the fundamental unit of a substance. For example, the molar mass of water is calculated by determining the atomic mass of hydrogen and oxygen. The atomic mass of each element is multiplied by its coefficients and then added together. This means that the molar mass of water is 18 g/mol.


The reaction between an acid and a base. It’s called neutralisation because the resulting products have a neutral pH of 7. Generally, this type of reaction produces salts and water.

Octet rule

The principle that the outermost shell of an atom must be filled with eight electrons for it to be complete. This rule applies to atomic bonding.

Periodic law

A law developed by the Russian scientist, Dmitri Mendeleev, that states the chemical properties of the elements have predictable patterns when they’re arranged in order of increasing atomic mass in a periodic table. This law helped Mendeleev predict the gaps in the periodic table.

Quantitative analysis

A precise type of scientific analysis that aims to measure the percentage or proportion of components in a sample. It uses advanced statistical techniques, such as correlation and regression. Quantitative analysis differs from qualitative analysis, which instead focuses on the visual characteristics of a chemical reaction.

Reverse osmosis

A water purification process in which pressure is applied on one side of a semi-permeable membrane to force the flow of water from a high concentration of solute to a low concentration of solute. Reverse osmosis is effective in removing microorganisms and mineral impurities from water, including salt.

Saturated hydrocarbons

Saturated hydrocarbons have single bonds between their carbon atoms. Hydrocarbon chains are said to be saturated when the maximum number of hydrogen atoms are attached to the carbon atoms. 


The scientific study of how energy, related mechanical systems, and chemical systems work and interact. Energy and its tendency to become less useful is one of the key concepts tackled by thermodynamics. 


A standard, internationally-recognised four-digit code that’s used to identify and classify hazardous chemicals. UN ID stands for United Nations Identifier.

Valence electron

The electrons located at the outermost energy shell of an atom. Valence electrons are the most likely to participate in chemical reactions by forming bonds such as ionic and covalent bonds.

Weak acid or weak base

Both a weak acid and a weak base have ions that partially dissociate in aqueous solutions. This means the acid or base is not completely consumed in a reaction. They may have to go through several steps before they achieve a level of dynamic equilibrium.


One of the seven noble gases that make up group 18 of the periodic table. It has the atomic number 54 and an atomic weight of 131.29. Xenon does not readily react with other elements or compounds, which is one of the reasons why it’s commonly used in cathode ray tubes.


The quantity of products produced by a chemical reaction. This may depend on the amount of the reactants and their respective stoichiometry. In chemistry, yield can refer to experimental yield or the actual yield. It may also refer to the theoretical yield and percentage yield. 

Zaitsev rule

A rule used in organic chemistry to predict the formation of alkenes in elimination reactions. The Zaitsev rule states that elimination reactions will produce alkenes that are highly substituted. The one with the most number of alkyl groups that are attached to the double-bonded carbons is the highly substituted alkene.
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