GCSE & IGCSE Chemistry key terms

189 terms with short definitions you can learn for the exam. Type to filter the list.

Command words 52

Add / Label GCSE: Add something to a diagram, graph or table you are given, such as labels or units.
Add / Label IGCSE: Add to a diagram or table you are given, for example labels or units.
Analyse the data / graph to explain IGCSE: Look at the data or graph closely and use it to give an explanation.
Assess GCSE: Weigh up all the relevant factors and decide which matter most.
Calculate GCSE: Work out a numerical answer and show your working. Include the unit if the answer has one.
Calculate IGCSE: Work out a numerical answer, showing your working.
Comment on GCSE: Pull together several pieces of data or information to reach a judgement.
Comment on IGCSE: Combine several pieces of data or information to form a judgement.
Compare GCSE: Give similarities or differences between two or more things.
Compare and contrast GCSE: Give both similarities and differences: at least one of each.
Complete GCSE: Fill in the missing parts of a table or diagram.
Complete IGCSE: Fill in a table or diagram.
Deduce GCSE: Reach a conclusion from the information given.
Deduce IGCSE: Reach a conclusion from the information provided.
Describe GCSE: Give an account of something, with linked, developed statements. No reasons are needed.
Describe IGCSE: Give an account of something in linked statements. Reasons are not needed.
Design IGCSE: Plan a procedure, such as an investigation, from scientific ideas.
Determine GCSE: Your answer must use numbers from the data given, or show how the answer can be worked out from them.
Determine IGCSE: Your answer must include a number worked out from the information, or show how it could be worked out.
Devise GCSE: Plan a procedure (for example an experiment) from scientific ideas you know.
Discuss GCSE: Identify the issue, explore all sides of it and reason through it.
Discuss IGCSE: Identify the issue, explore all its aspects and reason it through.
Draw: Produce a diagram, with a ruler or freehand.
Estimate GCSE: Find an approximate value from a diagram, from data or by a quick calculation.
Estimate IGCSE: Find an approximate value from a diagram, data or a calculation.
Evaluate GCSE: Review the information (data, methods, evidence), including strengths and weaknesses, and reach a supported conclusion.
Evaluate IGCSE: Review information such as data or methods, including strengths and weaknesses, and come to a supported judgement.
Explain GCSE: Give a point and the reason or justification for it. Use 'because' or 'so'.
Explain IGCSE: Give a point with its justification or reasoning (this can include a calculation).
Give / State / Name GCSE: Recall one or more facts. No explanation is needed.
Give / State / Name IGCSE: Recall one or more pieces of information.
Give a reason / reasons GCSE: A statement has been made: say only why it is true.
Give a reason / reasons IGCSE: A statement has been made: give only the reason or reasons why.
Identify GCSE: Pick out key information from the source, graph or diagram given.
Identify IGCSE: Select key information from the material given.
Justify GCSE: Give evidence that supports a statement, or an earlier answer.
Justify IGCSE: Give evidence to support a statement or an earlier answer.
Measure GCSE: Find a length or angle from a diagram using an instrument such as a ruler or protractor.
Plot GCSE: Mark points accurately on a grid from data and draw a line of best fit.
Plot IGCSE: Mark points accurately on a grid and draw a line of best fit, adding a scale and labelled axes if they are not given.
Predict GCSE: Give the result you would expect.
Predict IGCSE: Give an expected result.
Show that GCSE: Prove the value or statement given in the question, showing every step.
Show that IGCSE: Prove the statement or value in the question, step by step.
Sketch GCSE: Draw freehand. A sketch graph needs labelled axes and the right shape, but no scale.
Sketch IGCSE: A freehand drawing. A sketch graph needs a line, labelled axes and key features, but no scale.
State and explain GCSE: Make a point, then link ideas to justify it.
State what is meant by GCSE: Give the meaning of a term, in your own words if you like.
State what is meant by IGCSE: Give the meaning of a term.
Suggest IGCSE: Use what you know to propose an answer in an unfamiliar situation.
What / Why / Which IGCSE: Direct question words used for multiple-choice questions.
Write GCSE: Used when you must write an equation (word, symbol or half equation).

Key terms 112

Acid: A substance that releases hydrogen ions, H^{+}, in water (a proton donor). Its solution has a pH below 7.
Activation energy: The minimum energy colliding particles need for a reaction to happen.
Addition polymer: A polymer made when many small alkene monomers join together by opening their C=C double bonds. No other product forms.
Addition reaction: A reaction where atoms add across a C=C double bond, so two molecules make one product, for example an alkene with bromine.
Alkali: A soluble base. It releases hydroxide ions, OH^{-}, in water, giving a pH above 7.
Alkane: A saturated hydrocarbon with only C-C single bonds. General formula C_{n}H_{2n+2}.
Alkene: An unsaturated hydrocarbon containing a C=C double bond. General formula C_{n}H_{2n}.
Alloy: A mixture of a metal with one or more other elements, usually other metals. Alloys are harder than the pure metal because different-sized atoms stop the layers sliding.
Anhydrous: Containing no water of crystallisation, for example white anhydrous copper(II) sulfate.
Anion: A negative ion. It moves to the anode in electrolysis.
Anode: The positive electrode in electrolysis. Anions lose electrons there (oxidation).
Atom: The smallest particle of an element that can take part in a chemical reaction.
Atom economy GCSE: The percentage of the total mass of reactants that ends up in the wanted product. Atom economy = M_{r} of wanted product ÷ total M_{r} of reactants × 100.
Atomic number: The number of protons in the nucleus of an atom. Elements are arranged in order of atomic number in the periodic table.
Avogadro constant GCSE: The number of particles in one mole of a substance: 6.02 × 10^{23}.
Base: A substance that neutralises an acid to form a salt and water (a proton acceptor), such as a metal oxide or hydroxide.
Bond energy: The energy needed to break one mole of a particular covalent bond, in kJ/mol. The same amount is released when the bond forms.
Carboxylic acid: An organic compound with the functional group -COOH, for example ethanoic acid. It is a weak acid.
Catalyst: A substance that speeds up a reaction without being used up, by providing a pathway with a lower activation energy.
Cathode: The negative electrode in electrolysis. Cations gain electrons there (reduction).
Cation: A positive ion. It moves to the cathode in electrolysis.
Chromatography: A method that separates the substances in a mixture as a solvent (mobile phase) carries them different distances through paper (stationary phase).
Collision theory: Particles react only when they collide with at least the activation energy. More frequent or more energetic collisions give a faster rate.
Complete combustion: Burning in plenty of oxygen. A hydrocarbon forms only carbon dioxide and water.
Compound: A substance made of two or more elements chemically combined in fixed proportions.
Concentration: The amount of solute in a given volume of solution, in g dm^{-3} or mol dm^{-3}.
Concordant results: Titres that agree closely, usually within 0.10 cm^{3} of each other. Only these are used to work out the mean.
Condensation polymer GCSE: A polymer made from monomers with two functional groups each, where a small molecule such as water is lost each time a link forms.
Corrosion: The gradual destruction of a metal by reaction with substances around it, such as oxygen and water. Rusting is the corrosion of iron.
Covalent bond: A shared pair of electrons between two non-metal atoms, held by the attraction of both nuclei.
Cracking: Breaking long-chain alkanes into shorter, more useful alkanes and alkenes, using heat and a catalyst.
Crude oil: A finite mixture of hydrocarbons formed over millions of years from the remains of ancient sea organisms.
Crystallisation: Getting crystals of a soluble solid by evaporating some solvent from its solution and leaving it to cool.
Delocalised electrons: Electrons not tied to one atom that are free to move, as in metals and graphite. They carry charge.
Diatomic: Made of molecules containing two atoms, for example H_{2}, O_{2}, N_{2} and the halogens.
Diffusion: The random movement of particles from where they are more concentrated to where they are less concentrated.
Displacement reaction: A reaction where a more reactive element takes the place of a less reactive one in a compound.
Displayed formula: A formula that shows every atom and every bond in a molecule.
Dynamic equilibrium: In a reversible reaction in a closed system, the point where the forward and backward reactions happen at the same rate, so amounts stay constant.
Electrolysis: Breaking down an ionic compound, molten or in solution, using electricity.
Electrolyte: An ionic compound that is molten or dissolved, so its ions can move and carry a current.
Electron: A tiny negative particle (relative charge -1) found in shells around the nucleus.
Electronic configuration: The arrangement of electrons in shells, for example 2.8.1 for sodium.
Element: A substance made of only one type of atom (all atoms with the same atomic number).
Empirical formula: The simplest whole-number ratio of atoms of each element in a compound.
End point: The point in a titration where the indicator just changes colour, showing the reaction is exactly complete.
Endothermic: A reaction that takes in heat energy from the surroundings, so the temperature falls. ΔH is positive.
Enthalpy change (ΔH): The heat energy change in a reaction, usually in kJ/mol. Negative for exothermic, positive for endothermic.
Ester: An organic compound made from a carboxylic acid and an alcohol, with a fruity smell, for example ethyl ethanoate.
Exothermic: A reaction that gives out heat energy to the surroundings, so the temperature rises. ΔH is negative.
Fermentation: Making ethanol from sugar (glucose) using yeast, without oxygen, at about 30 to 40 °C.
Filtration: Separating an insoluble solid from a liquid using filter paper.
Fraction: A group of hydrocarbons with similar boiling points, collected together in fractional distillation.
Fractional distillation: Separating a mixture of liquids with different boiling points, using a fractionating column. Used for crude oil.
Fuel cell GCSE: A cell that makes electricity from a fuel such as hydrogen reacting with oxygen. A hydrogen fuel cell makes only water.
Functional group: The group of atoms that gives a homologous series its typical reactions, such as C=C, -OH or -COOH.
Greenhouse gas: A gas such as carbon dioxide, methane or water vapour that absorbs heat radiated from the Earth and keeps the atmosphere warm.
Group: A vertical column of the periodic table. Elements in a group have the same number of outer electrons and similar reactions.
Half equation: An equation showing the electrons gained or lost at one electrode, for example Cu^{2+} + 2e^{-} → Cu.
Halogen: An element in Group 7, such as chlorine, bromine and iodine. Halogens are diatomic and get less reactive down the group.
Homologous series: A family of organic compounds with the same general formula and functional group, similar chemical properties and a trend in physical properties.
Hydrated: Containing water of crystallisation, for example blue hydrated copper(II) sulfate, CuSO_{4}·5H_{2}O.
Hydrocarbon: A compound of hydrogen and carbon only.
Incomplete combustion: Burning in too little oxygen. It makes carbon monoxide (toxic) and soot (carbon) as well as water.
Indicator: A dye that changes colour depending on pH, such as litmus, methyl orange or phenolphthalein.
Intermolecular forces: Weak forces between molecules. They are overcome when a simple molecular substance melts or boils.
Ion: An atom or group of atoms with a charge, because it has lost or gained electrons.
Ionic bond: The strong electrostatic attraction between oppositely charged ions, formed when a metal transfers electrons to a non-metal.
Isomers: Compounds with the same molecular formula but different structures.
Isotopes: Atoms of the same element (same number of protons) with different numbers of neutrons.
Mass number: The total number of protons and neutrons in the nucleus of an atom.
Metallic bonding: The attraction between positive metal ions and a sea of delocalised electrons.
Mixture: Two or more substances not chemically combined. They can be separated by physical methods.
Molar volume: The volume of one mole of any gas at room temperature and pressure: 24 dm^{3} (24 000 cm^{3}).
Mole: The amount of substance containing 6.02 × 10^{23} particles. Its mass in grams equals the A_{r} or M_{r}.
Molecular formula: The actual number of atoms of each element in one molecule.
Monomer: A small molecule that joins with many others to form a polymer.
Nanoparticle GCSE: A particle 1 to 100 nm across, with a very large surface area to volume ratio.
Neutralisation: The reaction between an acid and a base to make a salt and water. H^{+} + OH^{-} → H_{2}O.
Neutron: A neutral particle (relative mass 1) in the nucleus.
Noble gas: An element in Group 0. It has a full outer shell, so it is very unreactive and exists as single atoms.
Ore: A rock containing enough of a metal or metal compound to make extraction worthwhile.
Oxidation: Gain of oxygen, or loss of electrons.
Percentage yield: The actual yield as a percentage of the theoretical yield. Percentage yield = actual ÷ theoretical × 100.
Period: A horizontal row of the periodic table. The period number equals the number of occupied electron shells.
pH: A scale from 0 to 14 measuring how acidic or alkaline a solution is. Below 7 acidic, 7 neutral, above 7 alkaline.
Polymer: A very large molecule made of many repeating units joined together.
Potable water GCSE: Water that is safe to drink. It is not pure, because it contains dissolved substances.
Precipitate: An insoluble solid that forms when two solutions react.
Proton: A positive particle (relative mass 1, relative charge +1) in the nucleus.
Pure substance: In chemistry, a single element or compound with nothing mixed in. It melts and boils at fixed temperatures.
R_{f} value: Distance moved by a spot ÷ distance moved by the solvent front. It is always less than 1.
Rate of reaction: How fast reactants are used up or products are formed. Mean rate = amount ÷ time.
Redox reaction: A reaction in which oxidation and reduction happen together.
Reduction: Loss of oxygen, or gain of electrons.
Relative atomic mass (A_{r}): The weighted mean mass of the isotopes of an element, compared with 1/12 of the mass of a carbon-12 atom.
Relative formula mass (M_{r}): The sum of the relative atomic masses of all the atoms in a formula.
Reversible reaction: A reaction that can go both forwards and backwards, shown by ⇌.
Salt: The compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion.
Saturated: Organic: having only single C-C bonds. Solutions: holding the maximum solute that can dissolve at that temperature.
Simple distillation: Separating a solvent from a solution by boiling it and condensing the vapour.
Solubility IGCSE: The mass of solute that dissolves in 100 g of solvent at a given temperature, in g per 100 g of solvent.
Solute: The substance that dissolves in a solvent.
Solution: A mixture formed when a solute dissolves in a solvent.
Solvent: The liquid in which a solute dissolves.
Strong acid: An acid that is completely ionised in water, such as hydrochloric, nitric and sulfuric acid.
Thermal decomposition: Breaking down a compound using heat, for example a metal carbonate into a metal oxide and carbon dioxide.
Titration: A method to find the exact volume of one solution that reacts with a measured volume of another, using a burette, pipette and indicator.
Transition metal GCSE: A metal from the central block of the periodic table. Typically high melting point and density, coloured compounds and useful as catalysts.
Unsaturated: Containing at least one C=C double bond, such as an alkene.
Water of crystallisation: Water molecules held within the crystal structure of a hydrated salt.
Weak acid: An acid that is only partially ionised in water, such as ethanoic, citric or carbonic acid.

Key calculations 25

Atom economy GCSE: atom economy = total M_{r} of wanted product ÷ total M_{r} of all reactants × 100 (use the numbers from the balanced equation).
Avogadro constant GCSE: number of particles = moles × 6.02 × 10^{23}.
Bond energy calculation GCSE: energy change (kJ/mol) = total energy to break bonds in reactants - total energy released making bonds in products. Negative = exothermic.
Bond energy calculation IGCSE: ΔH = energy needed to break bonds in reactants - energy released making bonds in products.
Concentration in g dm^{-3}: concentration (g dm^{-3}) = mass of solute (g) ÷ volume of solution (dm^{3}). To change cm^{3} to dm^{3}, divide by 1000.
Concentration in mol dm^{-3}: concentration (mol dm^{-3}) = moles of solute ÷ volume (dm^{3}). g dm^{-3} = mol dm^{-3} × M_{r}.
Empirical formula: For each element: mass (or %) ÷ A_{r}; divide every answer by the smallest; round to a whole-number ratio (double if you get .5).
Heat energy change (Q) IGCSE: Q (J) = m × c × ΔT, where m is the mass of water or solution in g, c = 4.2 J/g/°C and ΔT is the temperature change in °C.
Mean rate of reaction: mean rate = amount of reactant used or product formed ÷ time, for example in cm^{3}/s or g/s.
Molar enthalpy change (ΔH) IGCSE: ΔH (kJ/mol) = -Q ÷ moles that reacted, with Q in kJ. Exothermic changes give a negative ΔH.
Molar gas volume GCSE: volume of gas (dm^{3}) = moles × 24 at room temperature and pressure. The value 24 dm^{3} (24 000 cm^{3}) is given when needed.
Molar gas volume IGCSE: volume of gas (dm^{3}) = moles × 24 at room temperature and pressure (24 dm^{3} or 24 000 cm^{3} per mole).
Moles from mass: moles (mol) = mass (g) ÷ M_{r}. Rearranged: mass = moles × M_{r}.
Percentage by mass of an element: % = (A_{r} × number of atoms of that element) ÷ M_{r} × 100.
Percentage of oxygen in air IGCSE: % oxygen = decrease in gas volume ÷ starting volume of air × 100 (about 21%).
Percentage yield: percentage yield = actual yield ÷ theoretical yield × 100.
pH and H^{+} concentration (Higher) GCSE: A decrease of 1 pH unit means the hydrogen ion concentration is 10 times greater.
R_{f} value: R_{f} = distance moved by the spot ÷ distance moved by the solvent front. No unit, always less than 1.
Rate from a curve (Higher) GCSE: The rate at one moment is the gradient of the tangent to the curve at that time: change in y ÷ change in x.
Reacting masses: 1. Find moles of the known substance. 2. Use the balanced equation ratio to find moles of the other. 3. mass = moles × M_{r}.
Relative atomic mass from isotopes IGCSE: A_{r} = sum of (isotope mass × % abundance) ÷ 100.
Relative formula mass: M_{r} = sum of the A_{r} values of every atom in the formula. Example: M_{r} of CO_{2} = 12 + 16 + 16 = 44. No unit.
Solubility from an experiment IGCSE: solubility (g per 100 g water) = mass of dissolved solid ÷ mass of water × 100.
Titration calculation GCSE: 1. moles of known solution = concentration × volume (dm^{3}). 2. Use the equation ratio. 3. unknown concentration = moles ÷ volume (dm^{3}).
Titration calculation IGCSE: moles = concentration (mol dm^{-3}) × volume (dm^{3}); use the equation ratio; concentration = moles ÷ volume.

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