IGCSE Science (Double Award) key terms

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

Command words 25

Add / Label: Add to a diagram or table you are given, for example labels or units.
Analyse the data / graph to explain: Look at the data or graph closely and use it to give an explanation.
Calculate: Work out a numerical answer, showing your working.
Comment on: Combine several pieces of data or information to form a judgement.
Complete: Fill in a table or diagram.
Deduce: Reach a conclusion from the information provided.
Describe: Give an account of something in linked statements. Reasons are not needed.
Design: Plan a procedure, such as an investigation, from scientific ideas.
Determine: Your answer must include a number worked out from the information, or show how it could be worked out.
Discuss: Identify the issue, explore all its aspects and reason it through.
Draw: Produce a diagram, with a ruler or freehand.
Estimate: Find an approximate value from a diagram, data or a calculation.
Evaluate: Review information such as data or methods, including strengths and weaknesses, and come to a supported judgement.
Explain: Give a point with its justification or reasoning (this can include a calculation).
Give / State / Name: Recall one or more pieces of information.
Give a reason / reasons: A statement has been made: give only the reason or reasons why.
Identify: Select key information from the material given.
Justify: Give evidence to support a statement or an earlier answer.
Plot: 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: Give an expected result.
Show that: Prove the statement or value in the question, step by step.
Sketch: A freehand drawing. A sketch graph needs a line, labelled axes and key features, but no scale.
State what is meant by: Give the meaning of a term.
Suggest: Use what you know to propose an answer in an unfamiliar situation.
What / Why / Which: Direct question words used for multiple-choice questions.

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

1.10 Final speed: Use (spec says 'use the relationship'). (final speed)^{2} = (initial speed)^{2} + (2 × acceleration × distance moved). v^{2} = u^{2} + (2 × a × s)
1.17 Force and acceleration: Know and use (learn it). force (N) = mass (kg) × acceleration (m/s^{2}). F = m × a
1.18 Weight: Know and use (learn it). weight (N) = mass (kg) × gravitational field strength (N/kg). W = m × g
1.4 Average speed: Know and use (learn it). average speed (m/s) = distance moved (m) ÷ time taken (s)
1.6 Acceleration: Know and use (learn it). acceleration (m/s^{2}) = change in velocity (m/s) ÷ time taken (s). a = (v - u) ÷ t
2.13 Voltage: Know and use (learn it). voltage (V) = current (A) × resistance (Ω). V = I × R
2.15 Charge: Know and use (learn it). charge (C) = current (A) × time (s). Q = I × t
2.21 Energy and charge: Know and use (learn it). energy transferred (J) = charge (C) × voltage (V). E = Q × V
2.4 Electrical power: Know and use (learn it). power (W) = current (A) × voltage (V). P = I × V
2.5 Electrical energy: Use (spec says 'use the relationship'). energy transferred (J) = current (A) × voltage (V) × time (s). E = I × V × t
3.18 Refractive index: Know and use (learn it). refractive index = sin(angle of incidence) ÷ sin(angle of refraction). n = sin i ÷ sin r
3.22 Critical angle: Know and use (learn it). sin(critical angle) = 1 ÷ refractive index. sin c = 1 ÷ n
3.5 Wave speed: Know and use (learn it). wave speed (m/s) = frequency (Hz) × wavelength (m). v = f × λ
3.6 Frequency and period: Use (spec says 'use the relationship'). frequency (Hz) = 1 ÷ time period (s). f = 1 ÷ T
4.11 Work done: Know and use (learn it). work done (J) = force (N) × distance moved in the direction of the force (m). W = F × d
4.13 Gravitational potential energy: Know and use (learn it). GPE (J) = mass (kg) × gravitational field strength (N/kg) × height (m). GPE = m × g × h
4.14 Kinetic energy: Know and use (learn it). kinetic energy (J) = ½ × mass (kg) × speed^{2} (m/s). KE = ½ × m × v^{2}
4.17 Power: Use (spec says 'use the relationship'). power (W) = work done (J) ÷ time taken (s). P = W ÷ t
4.4 Efficiency: Know and use (learn it). efficiency = useful energy output ÷ total energy output × 100%
5.21 Pressure and temperature: Use (spec says 'use the relationship'). for a fixed mass of gas at constant volume: p_{1} ÷ T_{1} = p_{2} ÷ T_{2} (T in kelvin)
5.22 Pressure and volume: Use (spec says 'use the relationship'). for a fixed mass of gas at constant temperature: p_{1} × V_{1} = p_{2} × V_{2}
5.3 Density: Know and use (learn it). density (kg/m^{3}) = mass (kg) ÷ volume (m^{3}). ρ = m ÷ V
5.5 Pressure: Know and use (learn it). pressure (Pa) = force (N) ÷ area (m^{2}). p = F ÷ A
5.7 Pressure difference: Know and use (learn it). pressure difference (Pa) = height (m) × density (kg/m^{3}) × gravitational field strength (N/kg). p = h × ρ × g
8.6 Orbital speed: Use (spec says 'use the relationship'). orbital speed (m/s) = 2 × π × orbital radius (m) ÷ time period (s). v = 2 × π × r ÷ T

Biology and chemistry calculations 14

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).
Estimating a population (quadrats): population ≈ mean number per quadrat × (total area ÷ area of one quadrat).
Heat energy change (Q): 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): ΔH (kJ/mol) = -Q ÷ moles that reacted, with Q in kJ. Exothermic changes give a negative ΔH.
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: % change = (final mass - initial mass) ÷ initial mass × 100. Used in osmosis experiments.
Percentage of oxygen in air: % oxygen = decrease in gas volume ÷ starting volume of air × 100 (about 21%).
Percentage yield: percentage yield = actual yield ÷ theoretical yield × 100.
R_{f} value: R_{f} = distance moved by the spot ÷ distance moved by the solvent front. No unit, always less than 1.
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: 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.

Practise Science (Double Award) free Revision checklist