GCSE Chemistry Glossary

by Jessica Clifton

Whether you’re revising for your GCSE chemistry exam or writing up an experiment, you’ll need to have a good understanding of some key chemistry terms. But if you don’t know your hydrocarbons from your valence electrons, don’t worry – this handy GCSE chemistry glossary is here to help.

Covering a range of key topics, this page outlines some basic GCSE chemistry terms and their definitions. Although the glossary is far from exhaustive, it should help you brush up on the key phrases you’ll need during your studies. 

And don’t forget to take a look at our chemistry education resources section for more information and advice.

Range of GCSE topics

From atomic structure to chemical change, the GCSE chemistry syllabus covers a wide range of topics. These include the following:

  1. Atomic structure and the periodic table
  2. Bonding, structure, and the properties of matter
  3. Quantitative chemistry
  4. Chemical changes
  5. Energy changes
  6. The rate and extent of chemical change
  7. Organic chemistry
  8. Chemical analysis
  9. Chemistry of the atmosphere
  10. Using resources

The terms listed on this page belong to one or more of the above chemistry topics. Continue reading to find out more.

Acid rainAlcoholsAmino acids
Boiling pointBoyle’s lawCarboxylic acids
CatalystChromatographyCollision theory
CombustionComplete combustionCrystallisation
DecompositionDiatomic moleculeDistillation
Fossil fuelsFractional distillationGreenhouse effect
Greenhouse gasesGrapheneHaber process
HalogensImpure substanceJoule
Molar massNeutralisationOxidation
PhotosynthesisQualitative analysisQuantitative analysis
Reverse osmosisSustainable developmentTitration
Unsaturated hydrocarbonsValence electronsWastewater treatment
Zaitsev rule


Acid rain

Acid rain occurs when pollutants from industrial emissions and exhaust fumes react with water vapour in the air. These pollutants – which include carbon monoxide, carbon dioxide, nitrogen oxides, and sulphur dioxide – form acidic compounds when they gain protons from water. 

Nitric acid and sulphuric acid are the most common byproducts of pollutants and water vapour. Acidic rain can damage statues, concrete structures, and metallic objects.


Classed as derivatives of hydrocarbons, alcohols are organic compounds that have at least one hydroxyl group attached to a saturated carbon. For example, if you add a hydroxyl group to a methyl radical (from methane), you can form methanol, the simplest form of alcohol.

Amino acids

Often regarded as the building blocks of life, amino acids are chains of molecules that combine to form proteins. These proteins can fold and form different three-dimensional structures that have various properties and functions. Some act as enzymes while others provide structural support. 

Many of the organs in multicellular organisms have proteins in them both for functional and structural purposes. All amino acids have two functional groups –  the amine group and the carboxyl group – as well as a unique side chain.

Boiling point

The boiling point of any substance is the temperature at which its vapour pressure is equal to the surrounding pressure when it transitions into vapour. Pressure is a crucial factor in determining the boiling point of a substance.

Boyle’s law

Boyle’s law is an ideal gas law that states the pressure of a gas is inversely proportional to the volume when held at a constant temperature. The theory is named after Robert Boyle, who developed the law in 1662. 

Carboxylic acids

An organic acid that contains a carboxyl functional group. It has a double-bonded oxygen and a hydroxyl group.


A catalyst is a substance that increases the rate of a chemical reaction or reduces the temperature needed to start a chemical reaction – but the catalyst itself isn’t consumed. A catalyst could be a metal, an inorganic salt, or organic compounds like enzymes. 

Although a catalyst doesn’t participate in the chemical reaction per se, it increases the surface area and modifies the activation energy of the reactants.


Chromatography is a technique that separates out the individual components of a mixture or solution based on their different weights. There are various types of chromatography, but one of the simplest is paper chromatography, which allows you to separate colours in a mixture. Chromatography has a wide range of applications both in analytical chemistry and industry.

Collision theory

The idea that the rate of a chemical reaction largely depends on the probability that the molecules or atoms of the reactants will bump into each other. This is the reason why heat can increase the rate of chemical reactions (particles move faster when heated and therefore collide more frequently).


A type of chemical reaction that occurs when oxygen combines with an element or compound to form an oxide. If the compound is organic, the reaction produces water and carbon dioxide. 

Combustion is an exothermic reaction because it generates heat. For combustion to take place there needs to be oxygen, fuel (e.g. wood or gasoline), and a trigger such as a spark or flame.

Complete combustion

Complete combustion is when a fuel, or the reactant, is completely consumed by oxygen. This type of reaction produces carbon dioxide, water and heat, whereas incomplete combustion forms soot, carbon monoxide and other residues.


A physical or chemical change that involves the formation of lattice or crystalline structures. Crystallisation may occur in a supersaturated solution (a solution that contains more than the maximum amount of solute that can be dissolved by the solvent). 

Crystallisation can also occur when there’s a reaction between two substances that produce crystal compounds.


A type of chemical reaction in which the constituent elements or molecular groups of a substance split into simpler forms. One common example is the decomposition of sugar into carbon and water when a catalyst like sulphuric acid is added.

Diatomic molecule

A diatomic molecule is a compound in which each molecule has two atoms of the same element. Some elements, like oxygen and nitrogen, exist as stable substances in nature as diatomic molecules. 


The process of purifying and extracting substances from a mixture. Simple distillation involves heating and separating liquids with different boiling points. As the temperature of the water rises, the water vapour cools, condenses and drips into a beaker. The technique is often used to make pure water from seawater. 


A process in which an electric current is passed through an aqueous solution of electrolyte, splitting the water molecules into elemental hydrogen and oxygen gases. Electrolysis is used to extract hydrogen and oxygen gases on an industrial scale. It’s also useful in fuel-cell electric cars.


The sum of the internal energy in a thermodynamic system and the product of its pressure and volume. Enthalpy is a good stand-in for the energy of a chemical system, representing the bonds, lattice, solvation, and other energies.


A technique used to remove unwanted particles or solid matter from a solution using a filter. Filtration can be performed with a simple medium, like a filtration paper. The fluid passes through the paper and the residue particles are left behind. 

Filtration can also be done in multiple stages, which can involve nano-sized filtration pores and reverse osmosis.

Fossil fuels

The liquid or gaseous organic remains of living organisms such as plants and animals that lived millions of years ago. Fossil fuels are essentially different types of hydrocarbons that can be processed and used as fuels like gasoline and diesel.

Fractional distillation

A form of distillation designed to separate mixtures of liquids. The differences in the boiling points of the mixture’s components make it possible to condense the liquids at various points of the fractionating column

Fractional distillation is commonly used in the petroleum industry to separate the different components of crude oil. 

Greenhouse effect

The greenhouse effect happens when gases like carbon dioxide trap heat from the sun in the earth’s atmosphere. These heat-absorbing gases warm the earth and are consequently one of the biggest drivers of climate change. The phenomenon gets its name because the gases trap heat in a similar way to glass panels in a greenhouse.

Greenhouse gases

A type of gas that absorbs infrared radiation and thereby contributes to the greenhouse effect (see above). Greenhouse gases can either come from natural sources such as volcanic eruptions or they can be produced by human-made industrial processes.

Examples of greenhouse gases include carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, and hydrochlorofluorocarbons. 


One of the toughest synthetic materials, consisting of a single layer of carbon atoms that are arranged in a nano-sized lattice hexagonal structure. Graphene is 200 times stronger than steel but five times lighter than aluminium.

Haber process

The industrial technique of mass-producing ammonia using an iron catalyst, high temperatures, and high pressure. In the Haber process, nitrogen and hydrogen are put under 200 to 400 atmospheres at temperatures ranging from 400° to 650°C.


The halogens are the six gaseous elements in Group 17 of the periodic table of elements. Their name came from the Greek roots hal- and -gen, which means “salt” and “to produce”, respectively. All of these elements produce salt with sodium.

Impure substance

A type of substance that consists of different types of molecules or atoms. Virtually all substances that exist in their natural state are impure. From minerals to air, substances occur as mixtures with other substances or as compounds. 


The Joule is the SI unit of energy, which is equivalent to the amount of work exerted when a force of one Newton is applied to a mass through a distance of one metre. It has the derived unit of kg⋅m2⋅s−2. The Joule is named after the English physicist James Prescott Joule in recognition of his scientific contributions.


The Kelvin is the SI unit used for measuring temperature. Named after the renowned physicist Lord Kelvin, the scale defines 0 K as absolute zero. This is the theoretical temperature at which all energy in the system is removed. Absolute zero is equivalent to -273.15° C.


In chemistry, lattice refers to the orderly arrangement of a series of points in a crystalline structure. It’s used to describe the pattern of ions, atoms, or particles of the crystalline solid.


A liquid is a virtually incompressible fluid that flows, making it able to take the shape of its container. It’s one of the three states of matter and an intermediate between a solid and a gas. A liquid has particles that slide against each other.

Molar mass

The molar mass of any substance is the gram equivalent of its constituent particles, atoms or molecules. It can be calculated by adding the atomic mass of its composition. One mole of a substance is equal to 6.02 x 1023, or Avogadro’s number.


A type of chemical reaction that occurs between an acid and a base. The opposing pH levels of the reactants are neutralised, forming salt and water. There are different types of neutralisation reactions, which can be classified according to the strength of the reactants involved.


Oxidation doesn’t necessarily involve oxygen; it’s simply the process in which electrons are removed from a reducing substance by an oxidising agent. Oxidation is one half of a redox reaction. It can either be spontaneous or nonspontaneous.


Photosynthesis is a biochemical process that’s performed by autotrophic organisms like plants and algae. Essentially, they convert carbon dioxide and water into sugar with the help of sunlight. Photosynthesis occurs in the chloroplast using chlorophyll (the green pigment found in plants).

Qualitative analysis

Qualitative analysis is a type of preliminary analysis that looks at visual characteristics of a chemical reaction, such as the formation of steam or a colour change. 

Qualitative analysis is relatively easy to perform because it doesn’t usually require expensive equipment. Plus, unlike quantitative analysis, it doesn’t involve calculations or precise measurements.

Quantitative analysis

A form of analysis that involves measurable parameters that have physical equivalents like temperature. Scientists use quantitative analysis to measure data sets and make precise calculations.

Reverse osmosis

Reverse osmosis, or RO, is a water purification process in which water is put under pressure and then passed through a semipermeable membrane. Reverse osmosis can remove various microorganisms and mineral impurities from water, including salt.

Sustainable development

The balance between the economic need for progress and the need for environmental protection. Chemistry plays an important role in helping scientists to develop more sustainable products and thereby conserve the Earth’s finite resources.


Titration is a volumetric analytical technique that’s commonly used to determine the molar concentration of an analyte (the solution being identified). 

During a titration experiment, a known concentration of a reactant is prepared and gradually added to the analyte until a neutral threshold is reached. The final volume is compared with the initial volume to calculate the concentration of the analyte.

Unsaturated hydrocarbons

Unsaturated hydrocarbons are hydrocarbons with double or triple bonds with the adjacent carbon atoms. They’re called unsaturated hydrocarbons because not all of the valence electrons of carbon atoms are bonded with hydrogen atoms.

Valence electrons

These are the electrons located in the outermost shell (valence shell) of an atom. Valence electrons are most likely to be involved in chemical reactions and chemical bonding. They follow the octet rule, which means the atom is most stable when there are eight electrons in the valence shell.

Wastewater treatment

Wastewater treatment is a multi-stage process designed to remove potentially harmful impurities and pollutants from water so it can be reused. Wastewater should always be treated before it’s released into the environment. 

Firstly, solid materials are removed from the water. This is followed by flocculation or precipitation. The water then passes through filtration systems before finally being treated with chlorine to kill any microorganisms.


Xenon is a chemical element that belongs to Group 18 in the periodic table, otherwise known as the noble gases. It has the symbol Xe and the atomic number 54. Xenon is often used in arc lamps that produce UV rays.


A high energy ray with wavelengths that range from 0.01 nm up to 10 nm. X-rays can easily pass through objects and are commonly used in medicine to examine possible bone fractures. It’s also used in chemistry for analytical crystallography.


The yield of a chemical reaction refers to the amount of matter produced given a certain amount of reactants. 

Zaitsev rule

A rule in organic chemistry that’s used to predict the formation of alkenes in elimination reactions. According to Zaitsev’s rule, elimination reactions will produce alkenes that are highly substituted.
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