• acid A substance that reacts with bases and some metals to form salts. Hydrogen is given off when acids react with metals. The pH of aqueous solutions of acids is less than 7.
• acidic Describes a solution that has a pH of less than 7. Acids dissolve in water to make acidic solutions. These turn blue litmus paper red; and universal indicator red, orange or yellow.
• anion A negatively charged ion. It is attracted to the positively charged electrode during electrolysis. Non-metals form negatively charged ions when they gain electrons.
• anode The positively charged electrode in an electrochemical cell. During electrolysis, negatively charged ions (anions) lose electrons here and are oxidised to atoms.
• atom The simplest particle of an element that has the element's chemical properties. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons arranged in energy levels or shells.
• atomic model A scientific description of the structure of an atom.
• atomic number The number of protons in the nucleus of an atom. Each element has a unique atomic number. Elements in the periodic table are arranged in order of increasing atomic number.
• base A substance that neutralises acids. Bases that dissolve in water are called alkalis. The pH of an aqueous solution of a base is greater than 7.
• bond energy The average energy needed to break one mole of a particular covalent bond in molecules in the gas state. It is measured in kJ mol⁻¹. Bond-breaking is an endothermic process, so all bond energies have positive values. The stronger the bond, the greater the bond energy.

  Bond	Bond energy /kJ mol–1
  C–C	347
  C–O	358
  N–H	391
  C–H	415
  H–H	436
  O–H	464
  O=O	498
  C=C	612
  C=O	805
  N≡N	945

• Brønsted-Lowry acid A proton donor. For example: HCl(aq) → H⁺(aq) + Cl⁻ (hydrochloric acid is a strong acid) CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq) (ethanoic acid is a weak acid)
• Brønsted-Lowry base A proton acceptor. For example: OH⁻(aq) + H⁺(aq) → H₂O(l) (the hydroxide ion is a strong base) NH₃(aq) + H⁺(aq) ⇌ NH₄⁺(aq) (ammonia is a weak base)
• cathode The negatively charged electrode in an electrochemical cell. During electrolysis, positively charged ions (cations) receive electrons here and are reduced to atoms.
• closed system A system in which substances cannot enter or leave. For example, a chemical reaction that happens in a stoppered flask.
• collision theory A model that explains why chemical reactions happen and how their rate is affected by different factors. For a reaction to happen, reactant particles must collide in the correct orientation and with the activation energy or more for that reaction. Collisions that result in a chemical reaction(...)
• concentration The mass or amount of dissolved solute particles in a given volume of solution. Its units are g dm⁻³ or mol dm⁻³.
• corrosive Describes a substance that can damage metals and clothing, and can cause severe damage to the eyes and skin.
• covalent bond Strong electrostatic force of attraction between the nuclei of two atoms and a shared pair of electrons. Covalent bonds are commonly found between atoms of non-metals.
• displacement reaction A reaction in which a more reactive element replaces a less reactive element in one of its compounds. For example, magnesium is more reactive than copper, so this reaction happens: magnesium + copper(II) oxide → magnesium oxide + copper
• distillate The purified substance collected as a result of simple distillation.
• electrolysis The decomposition of an electrolyte into simpler substances using an electric current. The electrolyte is usually an ionic compound in the liquid state or in solution. Water is a covalent compound but it partially ionises to form hydrogen ions and hydroxide ions. The electrolysis of water(...)
• electron A type of subatomic particle that has a negative charge. Electrons have negligible mass, about 1836 times less than the mass of a proton. Electrons are found in energy levels or shells around nuclei in atoms, and in covalent bonds.
• energy level A region around the nucleus of an atom that electrons can occupy. As the distance from the nucleus increases, the energy of each energy level and the maximum number of electrons it can contain also increase.
• fermentation An anaerobic process that converts sugar into ethanol and carbon dioxide: sugar → ethanol + carbon dioxide C₆H₁₂O₆ → 2CH₃CH₂OH + 2CO₂ Yeast (a single-celled fungus), provides the enzymes needed for fermentation to happen. The process is used to produce alcoholic drinks, bioethanol(...)
• filtrate The liquid or solution that passes through the filter during filtration.
• filtration A method used to separate a mixture of an insoluble solid and a solution or liquid. The insoluble solid is left behind as a residue in the filter, and the liquid or solution passes through the filter as a filtrate.
• fractional distillation A method used to separate individual liquids from a mixture of liquids. When the mixture is gradually heated, substances boil and leave the mixture in the gas state. Each one is then cooled and condensed back to its liquid state. The method relies on the different liquids having different(...)
• gas state A state of matter in which particles are relatively far apart, randomly arranged, and can move quickly in any direction. A substance in the gas state flows, completely fills its container and is easily compressed.
• Haber process A method used by the chemical industry to make ammonia from nitrogen and hydrogen: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) The typical conditions used are 450 °C, 200 atmospheres pressure (20 MPa) and an iron catalyst.
• homologous series A 'family' of compounds which: have the same functional group have the same general formula differ by CH₂ in molecular formulae from neighbouring compounds show a gradual variation in physical properties, such as their boiling points, and have similar chemical properties.
• hydrocarbon An organic compound that consists of hydrogen and carbon only.
• ion A charged particle formed when an atom, or group of atoms, loses or gains electrons. Positively charged ions are called cations and negatively charged ions are called anions. Ions that form from one atom are called monatomic ions. Ions that form from a group of two or more atoms are called(...)
• ionisation The process of forming ions. It happens in reactions between metals and non-metals, in displacement reactions and during electrolysis. Cations (positive ions) form when electrons are lost, and anions (negative ions) form when electrons are gained.
• isotopes Atoms of an element that have the same number of protons but different numbers of neutrons. This means that their atomic number is the same but they have different mass numbers. Isotopes of a particular element have identical chemical properties because they have the same electronic(...)
• joule The SI unit of energy or work. Its symbol is J. To give two examples, 1 kJ (1000 J) is needed to increase the temperature of 50 cm³ of water by just under 5 °C, or is released when about 0.1 g of carbon burns completely.
• kinetic energy An energy store in an object due to its motion. This can be moving from one place to another, spinning or vibrating. Particles (atoms, ions or molecules) have kinetic energy.
• liquid state A state of matter in which particles are relatively close together, randomly arranged, and can move around each other. A substance in the liquid state flows, fills the bottom of its container and is difficult to compress.
• litmus A single pH indicator used as an indicator paper or in solution. It is red in acidic solutions, purple in neutral solutions, and blue in alkaline solutions. The table shows the colours of litmus paper in acidic, neutral and alkaline solutions.

  	acidic solutions	neutral solutions	alkaline solutions
  red litmus	stays red	stays red	turns blue
  blue litmus	turns red	stays blue	stays blue

• mass number The total number of protons and neutrons in the nucleus of an atom. Isotopes of an element have the same atomic number, but different mass numbers. Take care: do not confuse mass number with relative atomic mass.
• measuring cylinder Transparent glass or plastic laboratory apparatus for measuring the volumes of liquids. They come in different capacities, typically 10, 25, 50, 100 and 250 cm³ (ml). To get the best accuracy, choose a capacity that is just greater than the volume you want to measure.
• neutron A type of subatomic particle that has no charge - it is neutral. The mass of a neutron is about the same as the mass of a proton, and about 1836 times the mass of an electron. They are found in the nuclei of atoms.
• nucleus The central part of an atom. It consists of protons and neutrons, and is surrounded by electrons arranged in energy levels or shells.
• organic compound A substance in which one or more carbon atoms are joined to atoms of other elements by covalent bonds. The other elements are usually hydrogen, oxygen, nitrogen or a halogen such as chlorine. Hydrogen carbonates and carbonates are not organic compounds, even though they contain carbon(...)
• oxidation The process of gaining oxygen or losing electrons in a chemical reaction.
• oxidising agent A substance that can oxidise another substance. During an oxidation reaction, an oxidising agent transfers oxygen or gains electrons, so it is reduced when it oxidises the other substance.
• particle model A scientific model intended to explain the physical properties of substances in the three common states of matter. It imagines atoms, ions and molecules as tiny balls that are in constant motion, and which can be arranged and move in different ways. SolidLiquidGas Relative(...)
• pipette filler Laboratory apparatus used to draw liquids into burettes, graduated pipettes and volumetric pipettes. Two types are in common use, a plastic filler and a rubber filler. The plastic ones have a thumb wheel to draw the liquid up, and a valve to let it out again. The rubber ones have a squeezable(...)
• product A substance produced in a chemical reaction. The formula of a product is written on the right-hand side of a chemical equation.
• proton A type of subatomic particle that has a positive charge. The mass of a proton is about the same as the mass of a neutron, and about 1836 times the mass of an electron. They are found in the nuclei of atoms.
• qualitative analysis Identifying a substance without measuring how much there is in the sample. Simple laboratory analyses include chromatography, flame tests, gas tests, and precipitate tests.
• quantitative analysis Measuring how much of a substance there is in a sample. For example, the mass, volume, amount or percentage of the substance can be measured.
• reactant A substance before it takes part in a chemical reaction. The formula of a reactant is written on the left-hand side of a chemical equation.
• redox reaction A reaction in which reduction and oxidation happen at the same time. For example, when zinc reacts with copper(II) oxide, zinc is oxidised to zinc oxide and copper(II) oxide is reduced to copper.
• reducing agent A substance that can reduce another substance. During a reduction reaction, a reducing agent gains oxygen or loses electrons, so it is oxidised when it reduces the other substance.
• reduction The process of losing oxygen or gaining electrons in a chemical reaction.
• relative atomic mass The average mass of the atoms of an element compared to 1⁄12th the mass of a carbon-12 atom. Take care: do not confuse relative atomic mass with mass number.
• relative formula mass The mass of a formula unit of a substance compared to 1⁄12th the mass of a carbon-12 atom. It has the symbol Mr. You can use the term "relative molecular mass" instead if the substance exists as molecules. You calculate the relative formula mass of a substance by adding together all the(...)
• residue The insoluble solid material that does not pass through the filter during filtration.
• reversible reaction A chemical reaction which can go in either direction, depending on the reaction conditions. It will reach equilibrium if it takes place in a closed system.
• simple distillation Method used to separate a solvent from a solution. The solution is heated causing solvent to boil and leave the solution. The solvent vapour is then cooled and condensed, and the pure liquid solvent is collected. The separated solvent is called the distillate.
• distillation Method used to separate a solvent from a solution. The solution is heated causing solvent to boil and leave the solution. The solvent vapour is then cooled and condensed, and the pure liquid solvent is collected. The separated solvent is called the distillate.
• solid state A state of matter in which particles are relatively close together, regularly arranged, and can only vibrate in fixed positions. A substance in the solid state does not flow, keeps its shape and cannot be compressed.
• state of matter One of the physical forms of a substance. The common states of matter are solid, liquid and gas.
• subatomic particle A subatomic particle is a particle smaller than an atom. Protons, neutrons and electrons are subatomic particles that make up atoms. The table shows the properties of these subatomic particles.

  	Proton	Neutron	Electron
  Location	nucleus	nucleus	shells (energy levels)
  Relative charge	+1	0	–1
  Relative mass	1	1	1⁄1836

• toxic Describes a substance that can cause temporary or permanent harm to humans or animals, or can cause death. Toxic substances may be dangerous when breathed in, swallowed, or when in contact with skin or eyes.
• universal indicator A mixture of different pH indicators, chosen to produce a spectrum of colours across the pH scale. It is used as a paper or in solution.
• volumetric pipette Glass laboratory apparatus for adding a set volume of liquid. It is used when you want to add the same accurate volume of liquid over and over again. They come in different sizes, typically 10, 25 and 50 cm³ (ml). You need to use a pipette filler to get liquid into the pipette, but you then(...)
• word equation A way to describe a chemical reaction using the names of reactants and products. For example: aluminium + copper(II) oxide → aluminium oxide + copper The arrow shows the direction of the reaction. Aluminium and copper(II) oxide are the reactants, and aluminium oxide and copper are the(...)
• yeast A single-celled fungus that contains the enzymes needed for fermentation (the process that converts sugar into ethanol and carbon dioxide).
• zeolites Solid compounds of aluminium, silicon and oxygen. They have tiny pores that give them very large surface area to volume ratios. Oil refineries use zeolites in the catalytic cracking of alkanes to form alkenes and hydrogen.