GCSE Combined Science key terms

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

Command words 28

Add / Label: Add something to a diagram, graph or table you are given, such as labels or units.
Assess: Weigh up all the relevant factors and decide which matter most.
Calculate: Work out a numerical answer and show your working. Include the unit if the answer has one.
Comment on: Pull together several pieces of data or information to reach a judgement.
Compare: Give similarities or differences between two or more things.
Compare and contrast: Give both similarities and differences: at least one of each.
Complete: Fill in the missing parts of a table or diagram.
Deduce: Reach a conclusion from the information given.
Describe: Give an account of something, with linked, developed statements. No reasons are needed.
Determine: Your answer must use numbers from the data given, or show how the answer can be worked out from them.
Devise: Plan a procedure (for example an experiment) from scientific ideas you know.
Discuss: Identify the issue, explore all sides of it and reason through it.
Draw: Produce a diagram, with a ruler or freehand.
Estimate: Find an approximate value from a diagram, from data or by a quick calculation.
Evaluate: Review the information (data, methods, evidence), including strengths and weaknesses, and reach a supported conclusion.
Explain: Give a point and the reason or justification for it. Use 'because' or 'so'.
Give / State / Name: Recall one or more facts. No explanation is needed.
Give a reason / reasons: A statement has been made: say only why it is true.
Identify: Pick out key information from the source, graph or diagram given.
Justify: Give evidence that supports a statement, or an earlier answer.
Measure: Find a length or angle from a diagram using an instrument such as a ruler or protractor.
Plot: Mark points accurately on a grid from data and draw a line of best fit.
Predict: Give the result you would expect.
Show that: Prove the value or statement given in the question, showing every step.
Sketch: Draw freehand. A sketch graph needs labelled axes and the right shape, but no scale.
State and explain: Make a point, then link ideas to justify it.
State what is meant by: Give the meaning of a term, in your own words if you like.
Write: Used when you must write an equation (word, symbol or half equation).

Key terms 101

Acceleration: The rate of change of velocity, in m/s^{2}.
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.
Active transport: Moving substances against a concentration gradient (low to high), using energy from respiration.
Aerobic respiration: Respiration using oxygen: glucose + oxygen → carbon dioxide + water, releasing energy.
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}.
Allele: A different version of the same gene.
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.
Alpha particle: A helium nucleus (2 protons and 2 neutrons). Strongly ionising, stopped by paper.
Alternating current (a.c.): Current that keeps changing direction. UK mains is a.c. at 50 Hz and about 230 V.
Amplitude: The maximum displacement of a wave from its rest position.
Anaerobic respiration: Respiration without oxygen. In muscles it makes lactic acid; in yeast it makes ethanol and carbon dioxide.
Anion: A negative ion. It moves to the anode in electrolysis.
Antibiotic: A drug that kills bacteria or stops them growing. Antibiotics do not work on viruses.
Antibody: A protein made by lymphocytes that binds to a specific antigen on a pathogen.
Atomic number: The number of protons in the nucleus of an atom. Elements are arranged in order of atomic number in the periodic table.
Beta particle: A fast electron emitted from the nucleus. Stopped by a few millimetres of aluminium.
Biodiversity: The variety of different species (and their genes and habitats) in an area.
Catalyst: A substance that speeds up a reaction without being used up, by providing a pathway with a lower activation energy.
Cation: A positive ion. It moves to the cathode in electrolysis.
Cell membrane: The partially permeable layer around a cell that controls what enters and leaves.
Chloroplast: The organelle in plant cells where photosynthesis happens. It contains chlorophyll.
Chromatography: A method that separates the substances in a mixture as a solvent (mobile phase) carries them different distances through paper (stationary phase).
Chromosome: A long, coiled DNA molecule carrying many genes. Human body cells have 23 pairs.
Compound: A substance made of two or more elements chemically combined in fixed proportions.
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.
Current: The rate of flow of charge, in amperes (A).
Density: Mass per unit volume, in kg/m^{3} or g/cm^{3}.
Diffusion (biology): The net movement of particles from a higher to a lower concentration. It is passive (no energy needed).
DNA: Deoxyribonucleic acid: a double helix polymer that carries the genetic code.
Dominant: An allele that is expressed even when only one copy is present.
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.
Ecosystem: All the living organisms in an area together with the non-living (abiotic) conditions.
Efficiency: The fraction of the total energy supplied that is usefully transferred.
Electrolysis: Breaking down an ionic compound, molten or in solution, using electricity.
Electromagnetic spectrum: The family of transverse waves that travel at the same speed in a vacuum: radio, microwave, infrared, visible, ultraviolet, X-ray and gamma.
Empirical formula: The simplest whole-number ratio of atoms of each element in a compound.
Endothermic: A reaction that takes in heat energy from the surroundings, so the temperature falls. ΔH is positive.
Enzyme: A biological catalyst made of protein. Its active site fits a specific substrate.
Evolution: The gradual change in the inherited characteristics of a population over time, by natural selection.
Exothermic: A reaction that gives out heat energy to the surroundings, so the temperature rises. ΔH is negative.
Fractional distillation: Separating a mixture of liquids with different boiling points, using a fractionating column. Used for crude oil.
Frequency: The number of waves passing a point each second, in hertz (Hz).
Gamete: A sex cell (sperm or egg) with half the normal number of chromosomes (haploid).
Gamma ray: High-energy electromagnetic radiation from the nucleus. Weakly ionising, reduced by thick lead or concrete.
Gene: A section of DNA that codes for a specific protein.
Genotype: The alleles an organism has for a characteristic, for example Bb.
Greenhouse gas: A gas such as carbon dioxide, methane or water vapour that absorbs heat radiated from the Earth and keeps the atmosphere warm.
Half-life: The time taken for half the unstable nuclei in a sample to decay, or for the activity to halve.
Halogen: An element in Group 7, such as chlorine, bromine and iodine. Halogens are diatomic and get less reactive down the group.
Heterozygous: Having two different alleles of a gene.
Homeostasis: Keeping internal conditions, such as blood glucose and body temperature, constant.
Homozygous: Having two identical alleles of a gene.
Hormone: A chemical messenger made by an endocrine gland and carried in the blood to target organs.
Hydrocarbon: A compound of hydrogen and carbon only.
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.
Isotopes: Atoms of the same element (same number of protons) with different numbers of neutrons.
Kinetic energy: Energy stored in a moving object: ½ × mass × speed^{2}.
Longitudinal wave: A wave whose vibrations are parallel to the direction of energy transfer, such as sound.
Mass: The amount of matter in an object, in kilograms. It does not change with location.
Mass number: The total number of protons and neutrons in the nucleus of an atom.
Meiosis: Cell division that makes four genetically different gametes, each with half the chromosome number.
Mitosis: Cell division that makes two genetically identical diploid cells, for growth and repair.
Mole: The amount of substance containing 6.02 × 10^{23} particles. Its mass in grams equals the A_{r} or M_{r}.
Momentum: Mass × velocity, in kg m/s. It is conserved in collisions.
Natural selection: Individuals with variations better suited to their environment survive and reproduce more, passing on their alleles.
Neutralisation: The reaction between an acid and a base to make a salt and water. H^{+} + OH^{-} → H_{2}O.
Noble gas: An element in Group 0. It has a full outer shell, so it is very unreactive and exists as single atoms.
Osmosis: The net movement of water molecules across a partially permeable membrane, from a dilute solution to a more concentrated one.
Oxidation: Gain of oxygen, or loss of electrons.
Pathogen: A microorganism that causes disease, such as a bacterium, virus, fungus or protist.
Phenotype: The characteristics an organism shows, produced by its genotype and environment.
Photosynthesis: Plants use light energy to make glucose: carbon dioxide + water → glucose + oxygen. It is endothermic.
Potential difference: The energy transferred per unit charge between two points, in volts (V).
Power: The rate of energy transfer, in watts (W). 1 W = 1 J/s.
R_{f} value: Distance moved by a spot ÷ distance moved by the solvent front. It is always less than 1.
Recessive: An allele expressed only when two copies are present.
Reduction: Loss of oxygen, or gain of electrons.
Refraction: The change in direction of a wave when it enters a different medium at an angle, because its speed changes.
Relative formula mass (M_{r}): The sum of the relative atomic masses of all the atoms in a formula.
Resistance: How much a component opposes the current, in ohms (Ω). R = V ÷ I.
Resultant force: The single force that has the same effect as all the forces acting on an object.
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.
Scalar: A quantity with size only, such as speed, mass or energy.
Specific heat capacity: The energy needed to raise the temperature of 1 kg of a substance by 1 °C, in J/kg °C.
Stem cell: An undifferentiated cell that can divide and become other types of cell.
Strong acid: An acid that is completely ionised in water, such as hydrochloric, nitric and sulfuric acid.
Transpiration: The loss of water vapour from a plant, mainly through the stomata of the leaves.
Transverse wave: A wave whose vibrations are at right angles to the direction of energy transfer, such as light.
Vaccination: Giving a harmless form of a pathogen's antigen so the body makes memory cells and becomes immune.
Vector: A quantity with size and direction, such as velocity, force or momentum.
Velocity: Speed in a given direction, in m/s.
Wavelength: The distance from one point on a wave to the same point on the next wave, in metres.
Weight: The force of gravity on an object's mass, in newtons. W = m × g.
Work done: The energy transferred when a force moves an object: force × distance moved in the direction of the force.

Physics equations 25

Acceleration: Recall. acceleration (m/s^{2}) = change in velocity (m/s) ÷ time taken (s). a = (v - u) ÷ t
Charge: Recall. charge (C) = current (A) × time (s). Q = I × t
Density: Recall. density (kg/m^{3}) = mass (kg) ÷ volume (m^{3}). ρ = m ÷ V
Efficiency: Recall. efficiency = useful energy transferred by the device ÷ total energy supplied to the device
Electrical energy: Given on the equation list. energy transferred (J) = current (A) × potential difference (V) × time (s). E = I × V × t
Electrical power: Recall. electrical power (W) = current (A) × potential difference (V), P = I × V; and electrical power = (current)^{2} × resistance, P = I^{2} × R
Energy and charge: Recall. energy transferred (J) = charge moved (C) × potential difference (V). E = Q × V
Energy in a stretched spring: Given on the equation list. energy transferred in stretching (J) = 0.5 × spring constant (N/m) × (extension)^{2} (m). E = ½ × k × x^{2}
Force and acceleration: Recall. force (N) = mass (kg) × acceleration (m/s^{2}). F = m × a
Force and momentum: Given on the equation list. Higher tier only. force (N) = change in momentum (kg m/s) ÷ time (s). F = (mv - mu) ÷ t
Force on a conductor: Given on the equation list. Higher tier only. force on a conductor at right angles to a magnetic field carrying a current (N) = magnetic flux density (T) × current (A) × length (m). F = B × I × l
Gravitational potential energy: Recall. change in GPE (J) = mass (kg) × gravitational field strength (N/kg) × change in vertical height (m). ΔGPE = m × g × Δh
Kinetic energy: Recall. kinetic energy (J) = ½ × mass (kg) × (speed)^{2} (m/s). KE = ½ × m × v^{2}
Momentum: Recall. Higher tier only. momentum (kg m/s) = mass (kg) × velocity (m/s). p = m × v
Motion equation: Given on the equation list. (final velocity)^{2} - (initial velocity)^{2} = 2 × acceleration × distance. v^{2} - u^{2} = 2 × a × x
Potential difference: Recall. potential difference (V) = current (A) × resistance (Ω). V = I × R
Power: Recall. power (W) = work done (J) ÷ time taken (s). P = E ÷ t
Specific heat capacity: Given on the equation list. change in thermal energy (J) = mass (kg) × specific heat capacity (J/kg °C) × change in temperature (°C). ΔQ = m × c × Δθ
Specific latent heat: Given on the equation list. thermal energy for a change of state (J) = mass (kg) × specific latent heat (J/kg). Q = m × L
Speed: Recall. distance travelled (m) = average speed (m/s) × time (s). x = v × t
Spring force: Recall. force exerted on a spring (N) = spring constant (N/m) × extension (m). F = k × x
Transformers: Given on the equation list. for 100% efficiency: potential difference across primary coil × current in primary coil = potential difference across secondary coil × current in secondary coil. V_{p} × I_{p} = V_{s} × I_{s}
Wave speed: Recall. wave speed (m/s) = frequency (Hz) × wavelength (m), v = f × λ; and wave speed = distance (m) ÷ time (s), v = x ÷ t
Weight: Recall. weight (N) = mass (kg) × gravitational field strength (N/kg). W = m × g
Work done: Recall. work done (J) = force (N) × distance moved in the direction of the force (m). E = F × d

Biology and chemistry calculations 18

Avogadro constant: number of particles = moles × 6.02 × 10^{23}.
Bond energy calculation: energy change (kJ/mol) = total energy to break bonds in reactants - total energy released making bonds in products. Negative = exothermic.
Cardiac output: cardiac output (cm^{3}/min) = stroke volume (cm^{3}) × heart rate (beats/min).
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).
Light intensity (Higher): light intensity is inversely proportional to the square of the distance from the lamp: intensity ∝ 1 ÷ d^{2}.
Magnification: magnification = image size ÷ real size (both in the same units).
Mean rate of reaction: mean rate = amount of reactant used or product formed ÷ time, for example in cm^{3}/s or g/s.
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 change in mass (osmosis): % change = (final mass - initial mass) ÷ initial mass × 100.
pH and H^{+} concentration (Higher): 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): The rate at one moment is the gradient of the tangent to the curve at that time: change in y ÷ change in x.
Rate of enzyme activity: rate = 1 ÷ time taken (s^{-1}), or amount of substrate used or product made ÷ time.
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 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.

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