KS3 Science key terms

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

Key terms 120

Acid: A substance with a pH below 7. Acids turn universal indicator red, orange or yellow and turn blue litmus red.
Adaptation: A feature that helps an organism survive and reproduce in its environment, such as the large surface area of a leaf.
Aerobic respiration: Respiration that uses oxygen: glucose + oxygen → carbon dioxide + water. It happens in mitochondria and releases a lot of energy.
Alkali: A base that dissolves in water. Alkalis have a pH above 7 and turn universal indicator blue or purple.
Alveoli: Tiny air sacs in the lungs where oxygen diffuses into the blood and carbon dioxide diffuses out.
Ammeter: A meter that measures current in amps. It is always connected in series.
Amplitude: The size of a vibration or the height of a wave. A bigger amplitude means a louder sound.
Anaerobic respiration: Respiration without oxygen. In muscles it makes lactic acid; in yeast it makes ethanol and carbon dioxide. It releases less energy.
Anomalous result: A result that does not fit the pattern of the others. Check it, and leave it out when you calculate a mean.
Antagonistic muscles: A pair of muscles that work against each other: when one contracts, the other relaxes, like the biceps and triceps.
Atmospheric pressure: The pressure caused by the weight of the air above us. It decreases as you go higher.
Atom: The smallest particle of an element that can exist.
Balanced forces: Forces that cancel out, so the resultant force is zero. The object stays still or keeps moving at the same speed in the same direction.
Biodiversity: The variety of different species living in an area.
Catalyst: A substance that speeds up a chemical reaction without being used up.
Cell membrane: The thin layer around a cell that controls what goes in and out.
Cell wall: A strong layer made of cellulose around plant cells that supports the cell. Animal cells do not have one.
Ceramic: A hard, brittle material made by heating substances such as clay or sand, for example pottery, brick and glass.
Chemical change: A change that makes new substances, such as burning. It is usually hard to reverse.
Chloroplast: The part of a plant cell where photosynthesis happens. It contains green chlorophyll.
Chromatography: A way of separating dissolved substances, such as the dyes in ink, as a solvent moves up paper.
Chromosome: A long molecule of DNA found in the nucleus. Human body cells have 46 chromosomes in 23 pairs.
Combustion: Burning: a reaction in which a fuel reacts with oxygen and releases energy.
Composite: A material made from two or more materials combined, such as reinforced concrete or carbon fibre.
Compound: A substance made of two or more different elements chemically joined, such as water (H_{2}O).
Condensing: The change of state from a gas to a liquid.
Conduction: Heating through a solid, passed on by particles in direct contact. Metals are good thermal conductors.
Conservation of mass: In a physical or chemical change, the total mass stays the same, because atoms are not created or destroyed.
Consumer: An organism that gets its energy by eating other organisms.
Continuous variation: Variation that can take any value in a range, such as height. Shown on a histogram.
Control variable: A variable you keep the same so that a test is fair.
Convection: Heating in a liquid or gas, as warmer fluid rises and cooler fluid sinks.
Current: The flow of electric charge round a circuit, measured in amperes (A).
Density: How much mass is packed into a given volume: density = mass ÷ volume, in g/cm³ or kg/m³.
Dependent variable: The variable you measure in an investigation.
Depressant: A drug that slows down the nervous system, such as alcohol.
Diffusion: The spreading out of particles from where there are lots of them (high concentration) to where there are fewer (low concentration).
Discontinuous variation: Variation that falls into separate groups, such as blood group. Shown on a bar chart.
Displacement reaction: A reaction in which a more reactive metal pushes a less reactive metal out of its compound.
Distillation: Separating a liquid from a solution by boiling it and then condensing the vapour.
DNA: The molecule that carries genetic information. It has a double helix shape.
Electromagnet: A coil of wire carrying a current, which makes a magnetic field that can be switched off.
Element: A substance made of only one type of atom. Elements are listed in the periodic table.
Endothermic: A change that takes in energy from the surroundings, so the temperature falls.
Enzyme: A biological catalyst. Digestive enzymes break large food molecules into small ones.
Evaporation: A liquid turning into a gas at its surface. Also a way to get a dissolved solid back from a solution.
Exothermic: A change that gives out energy to the surroundings, so the temperature rises.
Extinct: When no individuals of a species are left alive.
Fair test: A test in which only the independent variable is changed and all the control variables are kept the same.
Fermentation: Anaerobic respiration in yeast: glucose → ethanol + carbon dioxide.
Fertilisation: When the nucleus of a male sex cell joins with the nucleus of a female sex cell.
Filtration: Separating an insoluble solid from a liquid using filter paper.
Food web: A diagram of linked food chains. The arrows show the direction energy is transferred.
Force: A push or a pull, measured in newtons (N). It is an interaction between two objects.
Frequency: The number of vibrations or waves each second, measured in hertz (Hz). Higher frequency means higher pitch.
Gamete: A sex cell, such as a sperm cell, egg cell or the male cell in pollen.
Gene: A short section of DNA that carries the instructions for a characteristic.
Global warming: The rise in Earth's average temperature, caused mainly by extra greenhouse gases such as carbon dioxide.
Group: A column in the periodic table. Elements in the same group have similar chemical properties.
Hooke's law: The extension of a spring is directly proportional to the force on it, up to the limit of proportionality.
Igneous rock: Rock formed when molten rock (magma or lava) cools and becomes solid, such as granite and basalt.
Independent variable: The variable you choose to change in an investigation.
Insulator: A material that does not let electricity or heat pass through easily, such as plastic or wool.
Interdependence: The way organisms in an ecosystem depend on each other, for food, pollination or shelter.
Kilowatt-hour (kWh): The energy a 1 kW appliance transfers in 1 hour. Electricity bills charge per kWh.
Light-year: The distance light travels in one year. It is a unit of distance, not time.
Longitudinal wave: A wave in which the particles vibrate along the direction the wave travels, such as sound.
Mass: The amount of matter in an object, measured in kilograms (kg). It is the same everywhere.
Mean: An average: add up the readings and divide by how many there are.
Metamorphic rock: Rock formed when existing rock is changed by heat and pressure without melting, such as marble and slate.
Mitochondria: The parts of a cell where aerobic respiration releases energy.
Mixture: Two or more substances mixed but not chemically joined, so they can be separated.
Moment: The turning effect of a force: force × perpendicular distance from the pivot, in newton metres (N m).
Natural selection: Individuals best adapted to their environment are more likely to survive, reproduce and pass on their genes.
Neutralisation: The reaction between an acid and an alkali (or base) that makes a salt and water.
Nucleus: The part of a cell that controls it and contains the chromosomes.
Organ: A structure made of several tissues working together, such as the heart or a leaf.
Oxidation: A reaction in which a substance gains oxygen, such as rusting.
Parallel circuit: A circuit with branches. The current splits between the branches and each branch gets the full potential difference.
Particle model: The idea that all matter is made of tiny particles, which explains solids, liquids and gases.
pH scale: A scale from 0 to 14 that shows how acidic or alkaline a solution is. 7 is neutral.
Photosynthesis: How plants make glucose using light: carbon dioxide + water → glucose + oxygen.
Pollination: The transfer of pollen from an anther to a stigma.
Polymer: A substance made of very long chain molecules. Most plastics are polymers.
Potential difference: The push that drives current round a circuit, measured in volts (V) with a voltmeter in parallel.
Power: How quickly energy is transferred, measured in watts (W). 1 kW = 1000 W.
Pressure: Force spread over an area: pressure = force ÷ area, in pascals (Pa).
Producer: An organism that makes its own food by photosynthesis, such as a green plant.
Pure substance: A single element or compound with nothing else mixed in. It melts and boils at fixed temperatures.
Radiation (heating): Transfer of energy by infrared waves, which can travel through a vacuum.
Random error: An unpredictable error that makes repeat readings spread out. Repeating and taking a mean reduces its effect.
Reactivity series: A list of metals in order of how vigorously they react, from most reactive to least reactive.
Reflection: When light or sound bounces off a surface. The angle of incidence equals the angle of reflection.
Refraction: The bending of light as it passes from one material into another, because its speed changes.
Repeatable: Results are repeatable if the same person using the same method and kit gets similar results again.
Reproducible: Results are reproducible if other people, or different methods or kit, give similar results.
Resistance: How hard it is for current to flow through a component, measured in ohms (Ω). R = V ÷ I.
Resultant force: The single force that has the same effect as all the forces acting on an object.
Risk assessment: A plan that lists the hazards in an experiment and how to reduce the risk of harm.
Sedimentary rock: Rock formed from layers of sediment that are compacted and cemented, such as sandstone and limestone.
Series circuit: A circuit with one loop. The current is the same everywhere and the potential difference is shared.
SI units: The standard units scientists use everywhere, such as the metre, kilogram, second and ampere.
Solute: The substance that dissolves in a solvent.
Solvent: The liquid that a solute dissolves in.
Species: A group of organisms that can breed together to produce fertile offspring.
Speed: How far something travels each second: speed = distance ÷ time, often in m/s.
Stimulant: A drug that speeds up the nervous system, such as caffeine or nicotine.
Stomata: Tiny holes, mostly on the underside of a leaf, that let gases in and out. Guard cells open and close them.
Systematic error: An error that makes every reading wrong by the same amount, such as a balance that is not zeroed. Repeating does not remove it.
Thermal decomposition: A reaction in which heat breaks one substance down into two or more new substances.
Tissue: A group of similar cells working together, such as muscle tissue.
Unbalanced forces: Forces with a resultant that is not zero, so the object speeds up, slows down or changes direction.
Unicellular organism: A living thing made of just one cell, such as amoeba or Euglena.
Upthrust: The upward push of a liquid or gas on an object in it. An object floats when upthrust balances its weight.
Vacuum: A space with no particles at all. Sound cannot travel through a vacuum, but light can.
Variation: The differences between individuals of the same species, caused by genes, the environment or both.
Villi: Tiny finger-like folds lining the small intestine that give a huge surface area for absorbing food.
Voltmeter: A meter that measures potential difference in volts. It is connected in parallel across a component.
Weight: The force of gravity on a mass, measured in newtons (N). Weight = mass × gravitational field strength.
Work done: The energy transferred when a force moves an object: work done = force × distance, in joules (J).

Equations and units 19

Conservation of mass: total mass of reactants = total mass of products. Also: mass of solution = mass of solute + mass of solvent. Example: 2.4 g of magnesium makes 4.0 g of magnesium oxide, so 1.6 g of oxygen reacted.
Cost of electricity: cost = energy (kWh) × price per kWh. Example: 6 kWh at 25p per kWh costs 150p, which is £1.50.
Counting atoms in a formula: A small number after a symbol counts the atoms just before it. Example: H_{2}SO_{4} has 2 H, 1 S and 4 O, which is 7 atoms in total.
Density = mass ÷ volume: density (g/cm³) = mass (g) ÷ volume (cm³), or kg/m³ with kg and m³. Water is 1 g/cm³: less dense objects float in it. Example: 540 g with a volume of 200 cm³ gives 2.7 g/cm³, so it sinks.
Energy = power × time: energy (J) = power (W) × time (s). Example: a 2000 W kettle running for 90 s transfers 180 000 J (180 kJ). 1 kJ = 1000 J and 1 kW = 1000 W.
Gradient of a distance-time graph: The gradient (slope) is the speed: distance up ÷ time across. A flat line means stopped. Example: a line rising 150 m in 30 s shows a speed of 5 m/s.
Hooke's law and extension: extension = stretched length − original length. Extension is directly proportional to force up to the limit of proportionality. Example: a 12 cm spring stretches to 15 cm with 2 N, an extension of 3 cm, so 4 N gives 6 cm.
Kilowatt-hours: energy (kWh) = power (kW) × time (h). Example: a 3 kW heater on for 2 hours uses 6 kWh. 1 kWh = 3 600 000 J (1000 W × 3600 s).
Law of reflection: angle of incidence = angle of reflection, both measured from the normal. Example: a ray at 30° to the normal reflects at 30°, which is 60° from the mirror surface.
Mean: mean = total of the readings ÷ number of readings, leaving out anomalies. Example: 4.2 s, 4.4 s and 4.3 s give a mean of 4.3 s.
Moment = force × distance: moment (N m) = force (N) × perpendicular distance from the pivot (m). Balanced when clockwise moments = anticlockwise moments. Example: 300 N at 2 m gives 600 N m; it balances 400 N at 1.5 m on the other side.
Pressure = force ÷ area: pressure (Pa) = force (N) ÷ area (m²). 1 Pa = 1 N/m². You may also see N/cm². Example: 600 N on 0.04 m² gives 15 000 Pa. Smaller area, bigger pressure.
Relative speed: Two objects moving the same way: subtract their speeds. Moving towards each other: add them. Example: trains at 30 m/s and 25 m/s. Same way: 5 m/s; towards each other: 55 m/s.
Resistance = potential difference ÷ current: resistance (Ω) = potential difference (V) ÷ current (A), R = V ÷ I. Example: 12 V across a lamp with 0.5 A through it gives 24 Ω.
Simple machines (levers): If no energy is wasted, force × distance is the same at both ends. Example: 100 N pushed through 0.6 m lifts a load through 0.2 m with a force of 300 N. You never get extra energy.
Speed = distance ÷ time: speed (m/s) = distance (m) ÷ time (s). Rearranged: distance = speed × time; time = distance ÷ speed. Example: 400 m in 80 s gives 5 m/s. This is the average speed over the journey.
Total magnification: total magnification = eyepiece magnification × objective magnification. Example: a ×10 eyepiece with a ×40 objective gives ×400.
Weight: W = m × g: weight (N) = mass (kg) × gravitational field strength (N/kg). On Earth g = 10 N/kg. Example: a 60 kg student weighs 600 N on Earth and 96 N on the Moon (g = 1.6 N/kg). Mass stays the same.
Work done = force × distance: work done (J) = force (N) × distance moved (m). Work done is energy transferred. Example: pushing with 50 N for 8 m does 400 J of work.

Word equations and facts to know 20

Aerobic respiration: glucose + oxygen → carbon dioxide + water (energy released, in mitochondria)
Anaerobic respiration in muscles: glucose → lactic acid (less energy released)
Chromosomes: Human body cells have 46 chromosomes (23 pairs); sperm and egg cells have 23.
Combustion of methane: methane + oxygen → carbon dioxide + water
Composition of dry air: About 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide (the remaining 1% is mostly argon).
Displacement: iron + copper sulfate → iron sulfate + copper (iron is more reactive than copper)
Extracting iron: iron oxide + carbon → iron + carbon dioxide (carbon removes the oxygen)
Fermentation (yeast): glucose → ethanol + carbon dioxide
Gravitational field strength: On Earth g = 10 N/kg; on the Moon about 1.6 N/kg.
Human hearing: About 20 Hz to 20 000 Hz. Sound above the human range is ultrasound.
Metal + acid: metal + acid → salt + hydrogen. Test for hydrogen: a lit splint gives a squeaky pop.
Names of salts: hydrochloric acid makes chlorides; sulfuric acid makes sulfates; nitric acid makes nitrates.
Neutralisation: acid + alkali → salt + water. Example: hydrochloric acid + sodium hydroxide → sodium chloride + water.
pH and indicators: Below 7 acidic, 7 neutral, above 7 alkaline. Universal indicator: red or orange (acid), green (neutral), blue or purple (alkali). Litmus: red in acid, blue in alkali.
Photosynthesis: carbon dioxide + water → glucose + oxygen (light energy, absorbed by chlorophyll in chloroplasts)
Reactivity series: potassium, sodium, calcium, magnesium, aluminium, (carbon), zinc, iron, tin, lead, (hydrogen), copper, silver, gold: most reactive first.
Speed of sound and light: Sound: about 340 m/s in air, faster in water and most solids, and it cannot travel through a vacuum. Light: about 300 000 km/s (3 × 10^{8} m/s) in a vacuum.
Thermal decomposition: copper carbonate → copper oxide + carbon dioxide
Units to know: Force N, mass kg, length m, time s, current A, potential difference V, resistance Ω, energy J, power W, pressure Pa, frequency Hz.
Water: Pure water melts at 0 °C and boils at 100 °C. Its density is 1 g/cm³. Ice is less dense than water, so it floats.

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