KS3 Combined Science Key Terms & Vocabulary

Alveoli

adapted for efficient gas exchange by having a large surface area, very thin (one-cell-thick) walls, and a rich blood supply.

Alveoli

the tiny air sacs at the ends of the airways in the lungs where gas exchange takes place.

Amylase

the enzyme that breaks down starch (a carbohydrate) into smaller sugars such as glucose.

Bacteria

living cells that can reproduce on their own and can be killed by antibiotics; viruses are not cells, use the host's cells to replicate, and cannot be treated with antibiotics.

New blood cells

made in the red bone marrow found inside some bones, such as the ribs and the femur.

Carbohydrates

the body's main source of energy; they are broken down to glucose for use in respiration.

Cartilage

a smooth, tough tissue that covers the ends of bones at a joint, reducing friction and absorbing shock.

Cranium (skull)

the bony case that protects the brain.

Iron-deficiency anaemia

caused by insufficient iron; the body cannot make enough haemoglobin, leading to tiredness and breathlessness.

Scurvy

caused by a deficiency of vitamin C; symptoms include bleeding gums and slow wound healing.

Diaphragm

the dome-shaped muscle beneath the lungs that contracts and relaxes to help draw air in and push it out.

Drug

any substance that changes the chemical processes in the body or brain; drugs include medicines, alcohol, tobacco, and illegal substances.

Fats

a concentrated energy store and are needed for insulation, cell membranes, and fat-soluble vitamins; excess fat is stored under the skin.

Femur

the thigh bone and is the longest bone in the human body.

Fertilisation

the fusion of a male sperm cell nucleus with a female egg cell (ovum) nucleus to form a single cell called a zygote.

Dietary fibre (roughage)

not digested but helps move food through the digestive system, preventing constipation.

Carpel

the female reproductive part of a flower, made up of the stigma, style, and ovary (which contains ovules).

Stamen

the male reproductive part of a flower, made up of the anther (which makes pollen) and the filament that holds it up.

Haemoglobin

the red protein in red blood cells that binds to oxygen in the lungs and carries it around the body.

Ligaments

tough bands that connect bone to bone and hold a joint together.

Pathogen

a microorganism that causes disease; pathogens include bacteria, viruses, fungi, and parasites.

Pepsin

a protease enzyme in the stomach that breaks down protein into smaller chains of amino acids; it works best in the acidic conditions of the stomach.

Pollination

the transfer of pollen from the anther of one flower to the stigma of another (or the same) flower.

Protein

needed for growth and repair of body tissues, and for making enzymes, hormones, and antibodies.

Protein

the food group needed for the growth and repair of body tissues.

Puberty

triggered by sex hormones (oestrogen in females, testosterone in males) and brings physical and reproductive changes that prepare the body for potential reproduction.

Tendons

tough cords that attach muscle to bone so that the muscle can pull on the bone.

Trachea (windpipe)

the tube that carries air from the throat down towards the lungs.

Water

essential for all chemical reactions in the body, for transporting substances in the blood, and for regulating body temperature through sweating.

Aerobic exercise (e.g. running, swimming) uses oxygen and can be sustained for long periods; it improves cardiovascular fitness.

Showing 30 of 78. Practise the full Health set →

Acid

a substance that dissolves in water to produce hydrogen ions (H⁺); examples include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and ethanoic acid (vinegar).

Alkali

a base that dissolves in water to produce hydroxide ions (OH⁻); examples include sodium hydroxide (NaOH) and potassium hydroxide (KOH).

Carbon monoxide

a colourless, odourless, toxic gas; it binds to haemoglobin in red blood cells more strongly than oxygen, preventing oxygen transport.

Catalyst

a substance that speeds up a chemical reaction without being used up itself; it provides an alternative reaction pathway with lower activation energy.

Combustion

an exothermic reaction between a fuel and oxygen that releases thermal energy and light.

Combustion

an oxidation reaction — the fuel gains oxygen; the word equation for burning carbon: carbon + oxygen → carbon dioxide.

Galvanising

coating iron with zinc to protect it from rusting; zinc is more reactive than iron and reacts with oxygen and water first (sacrificial protection).

Gold

very unreactive and does not corrode or tarnish; this makes it valuable for jewellery and electrical contacts.

Indicator

a substance that changes colour depending on whether the solution is acidic, neutral, or alkaline; examples include litmus (red in acid, blue in alkali) and universal indicator.

Neutralisation

the reaction between an acid and a base (or alkali) to form a salt and water: acid + base → salt + water.

Oxidation

the reaction of a substance with oxygen; when metals oxidise they form metal oxides (e.g. copper → copper oxide).

Phenolphthalein indicator

colourless in acid and neutral solutions and turns pink in alkaline solutions.

Photosynthesis

a chemical reaction: carbon dioxide and water are converted into glucose and oxygen using light energy.

Precipitate

an insoluble solid that forms when two solutions are mixed and their ions combine to make an insoluble compound.

Rusting

the oxidation of iron: iron + water + oxygen → hydrated iron(III) oxide (rust).

Salts

named from the acid used: hydrochloric acid forms chloride salts, sulfuric acid forms sulfate salts, nitric acid forms nitrate salts.

Thermal decomposition

not a redox reaction — heating breaks a compound into simpler products without oxygen transfer (e.g. copper carbonate → copper oxide + carbon dioxide).

Thermometer

used to measure the temperature change during a reaction, showing whether it is exothermic (temperature rises) or endothermic (temperature falls).

Acids react with metal carbonates to produce a salt, water, and carbon dioxide gas; the CO₂ turns limewater milky.

Acids react with metals (above hydrogen in the reactivity series) to produce a salt and hydrogen gas; the hydrogen gas 'squeaky pop' test confirms its presence.

Burning fossil fuels containing sulfur releases sulfur dioxide (SO₂), which dissolves in rainwater to form sulfuric acid — a contributor to acid rain.

Aluminium reacts with oxygen to form a thin, protective layer of aluminium oxide that prevents further corrosion — this is why aluminium does not appear to corrode despite being high in the reactivity series.

Indigestion remedies (antacids) contain bases (e.g. magnesium hydroxide, calcium carbonate) that neutralise excess hydrochloric acid in the stomach.

In a chemical reaction the atoms of the reactants are rearranged into new combinations to form the products; no atoms are created or destroyed.

Breaking chemical bonds takes in energy (endothermic) while making new bonds releases energy (exothermic); the overall energy change decides whether a reaction is exothermic or endothermic.

Carbon acts as a reducing agent when used to extract metals from metal oxides; carbon is oxidised to carbon dioxide while the metal oxide is reduced to the metal.

In a chemical reaction, reactants are converted into products; new substances are formed with different properties from the starting materials.

Reactions occur when particles collide with sufficient energy (the activation energy); increasing temperature increases collision frequency and energy, raising the reaction rate.

Complete combustion (plenty of oxygen): hydrocarbon fuel + oxygen → carbon dioxide + water.

In a chemical reaction, no atoms are created or destroyed — the total mass of reactants equals the total mass of products.

Showing 30 of 67. Practise the full Reactions set →

Acceleration

the rate of change of velocity; it is measured in metres per second squared (m/s²).

Acceleration

measured in metres per second squared (m/s²) and is the rate of change of velocity.

Air resistance (drag)

a friction force between an object and the air through which it moves; it increases as speed increases.

Distance

a scalar (size only); its vector partner is displacement, which is distance in a stated direction.

Elastic limit

the point beyond which a spring is permanently deformed and will no longer return to its original length when the force is removed.

Force

a push or pull that acts on an object; forces are measured in newtons (N) and are represented by arrows showing size and direction.

Friction

a contact force that opposes the relative motion of two surfaces in contact; it can be useful (braking) or unhelpful (wear).

Gas pressure

caused by gas particles colliding with the walls of their container.

Inertia

the tendency of an object to resist a change in its motion; more massive objects have greater inertia.

Force

measured using a newton meter (a calibrated spring balance), which reads the force in newtons.

Resultant force

the single force that has the same effect as all the individual forces acting on an object combined — it is found by adding forces in the same direction and subtracting opposing forces.

Speed

measured in metres per second (m/s) in SI units.

Spring constant k

measured in newtons per metre (N/m); it describes how stiff a spring is.

Upthrust

the upward force a fluid exerts on an object placed in it, caused by the pressure being greater on the bottom of the object than the top.

Speed

how fast an object moves; velocity is speed in a stated direction. An object moving in a circle at constant speed has a changing velocity.

Weight

the gravitational force acting on a mass, measured in newtons (N); mass is the amount of matter in an object, measured in kilograms (kg). Weight = mass × gravitational field strength (W = mg).

An object is accelerating whenever its speed or its direction of motion changes.

Acceleration = change in velocity ÷ time taken.

The atmosphere exerts pressure on all surfaces due to the weight of air above; at sea level this is approximately 101,000 Pa (101 kPa).

Atmospheric pressure decreases with altitude because there is less air above; this is why aircraft cabins are pressurised and why it is harder to breathe at high altitude.

Average speed = total distance ÷ total time; this may differ from instantaneous speed if the object speeds up or slows down during the journey.

When forces are balanced (resultant = 0 N), a stationary object stays still and a moving object continues at constant speed in a straight line.

Contact forces require physical contact between objects; examples include friction, air resistance, normal (reaction) force, and tension.

A horizontal (flat) line on a distance-time graph means the object is stationary — no change in distance over time.

On a distance-time graph, the gradient (slope) represents speed; a steeper gradient means a higher speed.

The elastic potential energy stored in a stretched spring is E = ½ke² (half the spring constant times the extension squared).

A stretched or compressed spring stores energy in its elastic potential store, which is released when the spring returns to its original shape.

From F = ma: acceleration = F ÷ m; the greater the force for a fixed mass, the greater the acceleration; the greater the mass for a fixed force, the smaller the acceleration.

A falling object accelerates due to gravity (10 m/s²) until air resistance equals its weight; then it falls at constant terminal velocity with zero acceleration.

An object floats when the upthrust acting on it is equal to (or greater than) its weight; it sinks when its weight is greater than the upthrust.

Showing 30 of 63. Practise the full Forces and motion set →

Air

a mixture of gases — mainly nitrogen (~78%), oxygen (~21%), argon, and carbon dioxide — not a compound.

Atom

the smallest particle of an element that still has the chemical properties of that element.

Compound

a substance formed when two or more different elements are chemically joined together.

Crystallisation

a more controlled version of evaporation — the solution is heated until saturated, then cooled slowly so that large, pure crystals form.

Element

a substance made of only one type of atom and cannot be broken down into simpler substances by chemical means.

Group 1 elements

called the alkali metals (e.g. lithium, sodium, potassium); they react vigorously with water to produce hydrogen gas and an alkaline solution.

Group 7 elements

called the halogens (e.g. fluorine, chlorine, bromine, iodine); they are reactive non-metals that form salts when they react with metals.

Group

a vertical column in the periodic table; elements in the same group have similar chemical properties because they have the same number of outer electrons.

Metals

on the left and centre of the periodic table; non-metals are on the right.

Molecule

two or more atoms bonded together — it can be made of the same element (e.g. O₂) or different elements (e.g. H₂O).

Period

a horizontal row in the periodic table; elements in the same period have the same number of electron shells.

Pure substance

made of only one type of element or compound and has a definite, sharp melting point and boiling point.

Rf value

the distance travelled by a substance divided by the distance travelled by the solvent front; it can be used to identify substances.

Saturated solution

one in which no more solute can dissolve at that temperature; any extra solute stays undissolved.

Seawater

a mixture of water and dissolved salts (mainly sodium chloride) — the salt can be recovered by evaporation.

Solubility

the maximum mass of a solute that will dissolve in a given amount of solvent at a particular temperature.

Solution

a mixture in which one substance (the solute) dissolves in another (the solvent) to form a clear liquid.

There are 118 confirmed elements in the modern periodic table.

An alloy (e.g. bronze, stainless steel) is a mixture of metals; mixing gives different properties without chemical bonding.

Centrifugation can separate a mixture of solid and liquid by spinning it rapidly so the denser solid sinks to the bottom.

Chromatography can test purity — a pure substance produces a single spot or band, while a mixture separates into multiple spots.

Paper chromatography separates dissolved substances (e.g. food colourings, inks) based on how far they travel up the paper with the solvent.

A compound has different properties from the elements it is made from — for example, water (H₂O) is a liquid at room temperature while hydrogen and oxygen are gases.

In distillation, a condenser cools the vapour back into liquid; the liquid collected is called the distillate.

Distillation separates a soluble solid from a liquid, or two liquids with different boiling points, by boiling and condensing the vapour.

Evaporation separates a dissolved solid from its solvent by heating the solution so the liquid evaporates, leaving the solid behind.

In filtration, the liquid that passes through the filter paper is called the filtrate, and the solid trapped on the paper is the residue.

Filtration works because the insoluble solid particles are too large to pass through the tiny holes in the filter paper, while the smaller dissolved or liquid particles pass through.

Filtration separates an insoluble solid from a liquid (e.g. sand from water) — the liquid passes through the filter paper but the solid stays behind.

The chemical formula for carbon dioxide is CO₂, showing one carbon atom bonded to two oxygen atoms.

Showing 30 of 53. Practise the full Atoms and elements set →

Amplitude

the maximum height of a wave measured from its rest (undisturbed) position.

Echo

the reflection of a sound wave from a surface; echoes are used in sonar to measure distances underwater.

Frequency

measured in hertz (Hz); one hertz is one wave per second.

Infrared radiation

emitted by warm objects; uses include thermal imaging cameras, remote controls, and toasters. Excessive exposure can cause skin burns.

Microwaves

used for cooking (water molecules absorb them and heat up) and for satellite and mobile phone communication.

Normal

an imaginary line drawn perpendicular (at 90°) to a surface at the point where a ray hits it; angles of incidence and reflection are measured from the normal.

Reflection

when a light ray bounces off a surface rather than passing through it.

Refraction

the bending of light as it passes from one medium to another (e.g. air to glass) due to a change in speed.

Sound

produced when an object vibrates; the vibrations pass through the medium and cause the eardrum to vibrate, which is detected as sound.

Sound

a longitudinal wave.

Sound

a longitudinal wave — particles in the medium vibrate back and forth in the same direction as the wave travels, creating compressions (high pressure) and rarefactions (low pressure).

Ultrasound (above 20 kHz)

used in medical imaging (e.g. prenatal scans) and industrial testing because it penetrates the body and reflects off internal boundaries.

Wavelength

the distance between two adjacent peaks (or two adjacent points in the same position) on a wave.

The amplitude of a sound wave (the size of the compression) determines its loudness — greater amplitude means a louder sound.

A black object absorbs all colours of light and reflects none; a white object reflects all colours equally.

An object appears a certain colour because it reflects that colour of light and absorbs all others; a red object reflects red and absorbs all other colours.

A convex (converging) lens focuses parallel rays of light to a point called the focal point; it is used in magnifying glasses, cameras, and the eye.

White light can be dispersed into a spectrum of colours (red, orange, yellow, green, blue, indigo, violet) by a prism because different colours (frequencies) refract by different amounts.

All electromagnetic waves travel at the same speed in a vacuum: 3 × 10⁸ m/s (the speed of light).

The EM spectrum in order of increasing frequency (decreasing wavelength): radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.

All electromagnetic waves are transverse waves — the oscillation is perpendicular to the direction of travel; they can travel through a vacuum.

The frequency of a wave is the number of complete waves passing a point each second.

The frequency of a sound wave (measured in hertz, Hz) determines its pitch — higher frequency means higher pitch.

Gamma rays have the shortest wavelength and highest frequency; uses include killing cancer cells (radiotherapy) and sterilising medical equipment. They are highly ionising and dangerous in large doses.

Humans can typically hear sounds with frequencies between 20 Hz and 20,000 Hz (20 kHz); sounds below 20 Hz are called infrasound and above 20 kHz are ultrasound.

Light (and all electromagnetic waves) are transverse waves.

Light travels at approximately 3 × 10⁸ m/s (300,000 km/s) in a vacuum — the fastest speed possible.

Light travels in straight lines (rays); shadows form because light cannot bend around opaque objects.

In a longitudinal wave the oscillations are parallel to (along) the direction the wave travels, producing compressions and rarefactions.

Prolonged exposure to loud sounds (high amplitude) damages the hair cells in the cochlea, causing permanent hearing loss.

Showing 30 of 45. Practise the full Waves set →

Coal

a non-renewable fossil fuel; burning it releases carbon dioxide and it cannot be replaced on a human timescale.

Conduction

the transfer of thermal energy through a material by vibrating particles passing energy to neighbouring particles, without the particles themselves moving far.

Convection

the transfer of thermal energy through a fluid (liquid or gas) by the movement of the fluid itself — hot fluid rises, cool fluid sinks, forming a convection current.

Efficiency

the fraction of input energy that is usefully transferred; efficiency = useful energy output ÷ total energy input (× 100 for a percentage).

Energy

transferred between stores by four pathways: mechanically (by forces), electrically (by current), by heating, or by radiation (light or sound).

Fossil fuel

a fuel formed over millions of years from the remains of ancient living organisms; coal, oil and natural gas are fossil fuels.

Hydroelectric power

a renewable resource that uses the energy of moving (falling) water to generate electricity.

Energy

measured in joules (J); one kilojoule (kJ) = 1,000 J.

Metals

good conductors of heat because they have free electrons that can carry energy quickly through the material.

Radiation (thermal radiation)

the transfer of energy as infrared waves, which can travel through a vacuum and do not require a medium.

Renewable energy resource

one that is replenished naturally and will not run out; examples include wind, solar, hydroelectric, tidal, geothermal and biomass.

Sankey diagram

used to show energy transfers; the width of the arrows represents the amount of energy flowing in each direction.

Wind power

a renewable resource that uses moving air to turn turbines and generate electricity with no carbon dioxide emissions during use.

Biomass (e.g. wood, crops) is classed as renewable because the plants used can be regrown, reabsorbing carbon dioxide as they grow.

Dark, matt surfaces are the best emitters and absorbers of thermal radiation; light, shiny surfaces are the worst emitters and best reflectors.

The chemical energy store holds energy in the bonds of chemicals such as food, fuel, and batteries, released by chemical reactions.

Energy cannot be created or destroyed — it can only be transferred from one store to another. The total energy before a process always equals the total energy after.

Convection occurs because heating a fluid makes it expand, become less dense, and rise; the cooler, denser fluid sinks to replace it.

In any energy transfer, some energy is always dissipated (spread out) to the surroundings as thermal energy (heat) — this is called wasted energy.

The main energy stores are: kinetic, gravitational potential, elastic potential, thermal, chemical, nuclear, magnetic, and electrostatic.

Burning fossil fuels releases carbon dioxide, a greenhouse gas that contributes to climate change.

The gravitational potential energy store holds energy in an object due to its height above a reference point — the higher the object, the more energy it has.

Thermal energy always flows from a region of higher temperature to a region of lower temperature, never the reverse.

Loft insulation, double-glazing, and cavity wall insulation all reduce heat loss by trapping air (a poor conductor) to reduce conduction and convection.

Insulators (e.g. wood, rubber, wool, air) are poor thermal conductors because their particles are not free to move, so energy transfers slowly.

The kinetic energy store holds energy in a moving object — the faster the object moves or the greater its mass, the more kinetic energy it has.

No device is 100% efficient — some energy is always wasted, usually as heat or sound.

A non-renewable energy resource exists in a fixed amount and will eventually run out; examples include the fossil fuels (coal, oil, natural gas) and nuclear fuel (uranium).

Nuclear fuels such as uranium are non-renewable because the supply of uranium is finite and will eventually run out.

Renewable resources such as wind, solar and hydroelectric have the lowest carbon emissions per unit of electricity generated.

Showing 30 of 34. Practise the full Energy set →

Cell

the smallest unit of life and the basic building block of every living organism.

Plant cell wall

made mostly of cellulose, a tough carbohydrate fibre.

Chlorophyll

the green pigment inside chloroplasts that absorbs light energy for photosynthesis.

Chloroplasts

found in plant cells but not animal cells; they carry out photosynthesis.

Cytoplasm

the jelly-like substance inside the cell where most chemical reactions take place.

Differentiation

the process by which a cell becomes specialised for a particular function.

Heart

an organ made of muscle, nerve, and blood-vessel tissues working together to pump blood.

Mitochondria

the site of aerobic respiration, releasing energy for the cell.

Organ

made up of several different tissues working together.

Organ system

a group of organs that work together to carry out one particular function.

Organism

the highest level of organisation — a complete living thing made up of one or more organ systems.

Red blood cells

biconcave discs with no nucleus, packed with haemoglobin to carry oxygen efficiently.

Tissue

a group of similar cells that work together to perform a particular function.

In a bacterial cell, the genetic material sits free in the cytoplasm as a single loop of DNA.

Bacterial cells do not have a true nucleus — their DNA is not enclosed in a nuclear membrane.

The cell membrane controls what enters and leaves the cell.

Plant cells have a strong outer cell wall that gives the cell its shape and supports the plant.

The circulatory system pumps blood around the body, delivering oxygen and nutrients and removing wastes.

The digestive system breaks food down into small molecules the body can absorb.

An electron microscope uses beams of electrons instead of light, magnifying samples hundreds of thousands of times — enough to see inside organelles.

The endocrine system controls body processes by releasing hormones into the bloodstream.

Levels of organisation, smallest to largest: cells → tissues → organs → organ systems → organism.

A light microscope uses visible light and glass lenses to magnify a sample several hundred times.

Muscle cells contain many mitochondria so they can release enough energy for contraction.

Nerve cells (neurones) carry electrical signals around the body and have long extensions to connect distant parts.

The nucleus contains the cell's DNA and controls cell activity.

The respiratory system takes in oxygen from the air and removes carbon dioxide from the body.

Root hair cells have a long thin extension that gives them a large surface area for absorbing water and minerals from the soil.

Sperm cells have a long tail so they can swim toward the egg cell during fertilisation.

Stains (e.g. iodine, methylene blue) are added to make cell parts easier to see under a microscope.

Showing 30 of 32. Practise the full Cells set →

Ammeter

placed in series in a circuit (in line) so all the current flows through it; a voltmeter is placed in parallel (across a component).

Copper

used for electrical wires because it is an excellent conductor and is ductile (it can be drawn into thin wires).

Electric current

the flow of electric charge (electrons) around a circuit, measured in amperes (A) using an ammeter.

Electric current

the rate of flow of electric charge around a circuit, measured in amperes (A).

Electromagnet

a coil of wire (solenoid) that becomes magnetic when an electric current flows through it; its strength can be increased by increasing the current, adding more coil turns, or adding an iron core.

Electromagnets

used in electric motors, generators, MRI scanners, and cranes for lifting scrap metal — their magnetism can be switched on and off.

Electromagnets

useful in scrapyards because their magnetism can be switched on to pick up iron and steel and switched off to drop it.

Induced magnet

a magnetic material that becomes a temporary magnet when placed in a magnetic field, and loses its magnetism when the field is removed.

Magnetic field

the region around a magnet where a force is exerted on magnetic materials or other magnets.

Resistance

the opposition to the flow of current in a circuit, measured in ohms (Ω).

UK mains electricity

supplied at approximately 230 volts.

Voltage (potential difference)

the push that drives current around a circuit, measured in volts (V) using a voltmeter.

Standard circuit symbols include: cell, battery, switch, bulb, resistor, ammeter, voltmeter, LED, and motor.

A compass contains a small magnetised needle that lines up with a magnetic field, so it is used to detect the direction of a magnetic field.

Conductors (e.g. metals) allow current to flow easily; insulators (e.g. plastic, rubber) have very high resistance and do not conduct electricity well.

In a direct current (DC) circuit the current always flows in the same direction; batteries provide DC.

The Earth acts as a giant magnet with a magnetic field that causes compass needles to point toward magnetic north.

Magnetic field lines run from the north pole to the south pole outside the magnet; where the lines are closer together, the field is stronger.

Outside a magnet, magnetic field lines always point from the north pole to the south pole.

The magnetic field around a bar magnet is strongest at its poles, where the field lines are closest together.

When a wire is moved through a magnetic field (or a magnetic field moves near a wire), a voltage is induced in the wire and electricity can be generated — this is the generator effect.

Magnetic materials (iron, nickel, cobalt, and some alloys) are attracted to magnets; most other materials are not magnetic.

Every magnet has a north pole and a south pole; like poles repel each other, and unlike poles attract.

Ohm's law: voltage = current × resistance (V = I × R); this holds for conductors at constant temperature.

In a parallel circuit there are multiple branches; each branch has the same voltage across it, and the total current is split between branches.

In a parallel circuit the voltage across each branch is the same as the supply voltage.

In a series circuit there is only one path for the current; the same current flows through every component, but the voltage is shared.

In a series circuit the current is the same at every point in the circuit.

In a parallel circuit, adding more bulbs does not make existing bulbs dimmer; in a series circuit, adding bulbs reduces the brightness of all.

Static electricity builds up when insulators gain or lose electrons by friction; like charges repel, unlike charges attract.

Fossils

only found in sedimentary rocks because the layering process can preserve the remains or traces of organisms.

Methane

a powerful greenhouse gas; major human-linked sources include cattle and other livestock (produced during digestion), landfill, and rice paddies.

Acid rain forms when sulfur dioxide (mainly from burning fossil fuels containing sulfur) dissolves in rainwater to make a weak acid; nitrogen oxides also contribute.

The remaining ~1% of the atmosphere is mostly argon, with a small amount of carbon dioxide and other gases.

Nitrogen makes up approximately 78% of the Earth's atmosphere.

Oxygen makes up approximately 21% of the Earth's atmosphere.

In the carbon cycle, photosynthesis removes CO₂ from the atmosphere and stores carbon in living organisms; respiration and combustion return CO₂ to the atmosphere.

Evidence for tectonic plate movement includes matching fossils and rock types on different continents, the fit of coastlines (e.g. Africa and South America), and earthquake/volcano patterns.

The Earth's core (both outer liquid and inner solid parts) is made mainly of the metals iron and nickel.

Igneous rocks that cool slowly (underground) form large crystals; those that cool quickly (at the surface) form small crystals.

Earth's early atmosphere (about 4.5 billion years ago) consisted mainly of carbon dioxide and water vapour, released by volcanic activity — very little oxygen was present.

The Earth has four main layers: crust (thin, outer rock), mantle (solid and semi-molten rock), outer core (liquid metal), and inner core (solid metal).

Human activities (burning fossil fuels, deforestation, agriculture) increase greenhouse gas concentrations, intensifying the greenhouse effect and causing global warming.

Global warming raises sea levels in two ways: melting land ice (glaciers and ice sheets) adds water, and warmer seawater expands (thermal expansion).

The natural greenhouse effect keeps Earth about 33 °C warmer than it would otherwise be — without it, average surface temperature would be around −18 °C.

The main greenhouse gases are water vapour, carbon dioxide, and methane; they absorb infrared radiation from Earth's surface and re-emit it, warming the atmosphere.

Igneous rocks form when magma (molten rock) cools and solidifies — inside Earth (intrusive, e.g. granite) or at the surface (extrusive, e.g. basalt).

Metamorphic rocks form when existing rocks are changed by extreme heat and pressure deep in the Earth, e.g. marble (from limestone), slate (from mudstone).

Oxygen built up in the atmosphere when early photosynthetic organisms (cyanobacteria/algae) evolved and released oxygen as a by-product.

The ozone layer in the stratosphere absorbs most of the Sun's harmful ultraviolet (UV) radiation, protecting living organisms on Earth's surface.

Where tectonic plates meet, they cause earthquakes, volcanoes, and the formation of mountain ranges.

Switching from fossil fuels to renewable energy sources (wind, solar, hydro) reduces greenhouse gas emissions and helps limit climate change.

The rock cycle shows how rocks are continuously transformed from one type to another through processes of melting, cooling, erosion, deposition, and metamorphism.

Sedimentary rocks form when layers of sediment (fragments of rock, shells, minerals) are compressed and cemented over millions of years, e.g. sandstone, limestone.

The Earth's crust and upper mantle are broken into large sections called tectonic plates that move very slowly (a few centimetres per year).

Chlorophyll

the green pigment in chloroplasts that absorbs light energy (mainly red and blue wavelengths) for photosynthesis.

Fermentation

used in baking (CO₂ makes bread rise) and in brewing (ethanol production).

Leaves

adapted for photosynthesis: large flat surface (more light), thin (CO₂ diffuses quickly), many chloroplasts, and stomata for gas exchange.

Carbon dioxide concentration

a limiting factor — increasing CO₂ up to a point raises the photosynthesis rate.

Light intensity

a limiting factor for photosynthesis — doubling light intensity increases the rate of photosynthesis until another factor becomes limiting.

Temperature

a limiting factor because photosynthesis involves enzymes; too cold slows reactions, too hot denatures the enzymes.

Photosynthesis

an endothermic reaction: it takes in (absorbs) energy from light, storing it as chemical energy in glucose.

Oxygen

a by-product of photosynthesis and is released through the stomata into the air.

Plants

called producers because they make their own food using photosynthesis; all other organisms in a food chain ultimately depend on this.

Stomata

tiny pores, mostly on the underside of a leaf, through which carbon dioxide enters and oxygen leaves during photosynthesis.

Aerobic respiration: glucose + oxygen → carbon dioxide + water (+ energy released).

Aerobic respiration takes place in the mitochondria of cells.

Aerobic respiration requires oxygen and releases far more energy per glucose molecule than anaerobic respiration.

All living organisms — including plants, fungi, and bacteria — carry out cellular respiration; it is not unique to animals.

In animals, anaerobic respiration: glucose → lactic acid (+ a small amount of energy).

In yeast (and some plant cells), anaerobic respiration: glucose → ethanol + carbon dioxide (+ a small amount of energy). This is called fermentation.

Plants use the glucose made in photosynthesis for respiration (energy), growth, making cellulose for cell walls, and storing as starch.

The iodine test detects starch in a leaf: iodine solution turns blue-black in the presence of starch, confirming that photosynthesis has occurred.

Lactic acid builds up in muscles during intense exercise when oxygen supply is insufficient, causing muscle fatigue and soreness.

After anaerobic exercise, extra oxygen is consumed to break down lactic acid — this is sometimes called the oxygen debt.

Photosynthesis takes place in the chloroplasts of plant cells, mainly in the leaves.

Photosynthesis: carbon dioxide + water → glucose + oxygen (using light energy).

Respiration releases energy that the cell uses for movement, growth, reproduction, and maintaining body temperature.

Glucose made in photosynthesis is converted to starch for storage because starch is insoluble and does not affect osmosis.

Adaptation

a characteristic that makes an organism better suited to its environment, increasing its chance of survival and reproduction.

Antibiotic resistance

an example of natural selection in action: bacteria with a random mutation that allows them to survive antibiotics reproduce, passing on resistance — a major public health threat.

Chromosomes

long, coiled-up molecules of DNA found in the nucleus of cells; humans normally have 46 chromosomes (23 pairs).

DNA (deoxyribonucleic acid)

a double helix molecule made of two strands of nucleotides; it carries the genetic instructions for building and running an organism.

Environmental variation

caused by factors such as diet, exercise, sunlight, and experience; examples include language spoken, body mass, and scars.

Gene

a section of DNA that carries the instructions for making a specific protein, which determines a characteristic of the organism.

Inherited variation

caused by differences in genes passed from parents to offspring; examples include eye colour, blood type, and genetic disorders.

Gregor Mendel

regarded as the founder of genetics; in the 1860s he worked out the basic rules of inheritance by breeding pea plants.

Mutation

a random change in the DNA sequence; most mutations have no effect, some are harmful, and a very few may be beneficial.

Mutualism

a close relationship between two different species in which both organisms benefit, e.g. bees getting nectar while pollinating flowers.

Selective breeding (artificial selection) is when humans choose which organisms reproduce to pass on desired characteristics, e.g. breeding cattle for more milk or dogs for temperament.

Asexual reproduction produces genetically identical offspring (clones) because there is only one parent and no mixing of genetic material.

Some characteristics are influenced by both genes and environment — for example, height has a strong genetic basis but is also affected by nutrition.

All living organisms share common ancestors; the diversity of life arose through millions of years of evolution from those shared ancestors.

Darwin gathered key evidence for evolution on the Galápagos Islands, where the variation among finches (especially their beak shapes) — collected during the 1831–36 voyage of HMS Beagle — helped inspire his theory of natural selection.

Charles Darwin and Alfred Russel Wallace independently developed the theory of evolution by natural selection; Darwin published On the Origin of Species in 1859.

Extinction occurs when every member of a species dies and no more can reproduce; it can be caused by environmental change, disease, competition, or predation.

The fossil record shows that organisms have changed over time — transitional fossils provide evidence that species evolved gradually from common ancestors.

Humans have 23 pairs of chromosomes — 22 matched pairs (autosomes) and one pair of sex chromosomes (XX for female, XY for male).

Random mutations in DNA are the ultimate source of new genetic variation; natural selection then acts on this variation.

Natural selection works because: (1) individuals in a population vary; (2) some variation is inherited; (3) more offspring are produced than survive; (4) individuals with favourable traits are more likely to survive and reproduce.

Sexual reproduction produces genetic variation in offspring because each inherits a unique combination of chromosomes from both parents.

When populations of the same species become isolated and evolve separately for long enough, they can become so different they can no longer interbreed — a new species has formed.

"Survival of the fittest" means survival of those best adapted to their environment, not necessarily the physically strongest.

Boiling point

the temperature at which a liquid changes to a gas throughout the liquid; evaporation can also occur below the boiling point at the surface.

Diffusion

the net movement of particles from a region of higher concentration to a region of lower concentration, due to their random motion.

Diffusion

faster at higher temperatures (particles move faster) and over shorter distances; lighter particles diffuse faster than heavier ones.

Gases

easily compressed because there is a large amount of space between the particles; solids and liquids are almost incompressible.

Gas pressure

caused by particles colliding with the walls of a container; increasing temperature or decreasing volume increases pressure because there are more frequent and harder collisions.

Melting point

the temperature at which a solid changes to a liquid; at this temperature, energy is used to overcome the attractive forces between particles without raising the temperature.

Brownian motion — the random jiggling of small particles (e.g. pollen grains in water) — is direct evidence for the particle model; it is caused by random collisions with smaller, invisible fluid particles.

Changes of state are physical changes, not chemical changes: no new substance is made and the change can be reversed.

Changes of state: solid → liquid (melting), liquid → solid (freezing/solidifying), liquid → gas (evaporation/boiling), gas → liquid (condensation), solid → gas (sublimation).

Cooling a gas removes energy from the particles, reducing their speed until the attractive forces pull them together and the gas condenses to a liquid.

Density depends on how closely packed the particles are; solids are densest (particles closest), gases least dense (particles furthest apart).

Heating a solid adds energy to the particles, increasing their vibration until they overcome the forces holding them in place and the solid melts.

Gases have no fixed shape or volume and expand to fill any container; their particles are far apart, move rapidly, and in random directions.

Liquids have a fixed volume but take the shape of their container; their particles are close together but free to move past each other.

The particle model assumes that matter is made of tiny particles in constant random motion, with attractions between particles, and that particles themselves are not destroyed or created.

The simple particle model has limitations: it treats particles as solid spheres with no volume of their own and ignores the forces within particles themselves (atoms and bonds).

The higher the temperature, the faster the particles move; temperature is a measure of the average kinetic energy of the particles in a substance.

Particles in all three states are always moving — vibrating in fixed positions in a solid, moving past each other in a liquid, and moving rapidly and randomly in a gas.

A pure substance has a sharp, definite melting point; a mixture melts over a range of temperatures, which can be used as a test of purity.

Solids have a fixed shape and volume; their particles are closely packed in a regular arrangement and vibrate in fixed positions.

Sublimation occurs when a solid changes directly to a gas without passing through the liquid state; dry ice (solid CO₂) sublimes at room temperature.

When a substance is heated its particles move faster and further apart, causing the substance to expand; this is why thermometers and railway tracks are designed with expansion gaps.

At standard atmospheric pressure pure water freezes (and ice melts) at 0 °C and boils at 100 °C.

Carbon fibre

a strong, lightweight composite used in racing cars and aircraft.

Ceramic

a hard, brittle, heat-resistant material usually made by heating clay or other minerals (e.g. pottery, mugs, tiles, brick).

Composite

a material made from two or more different materials combined to give better properties than either material alone.

Concrete

a composite made from cement and aggregate (sand and stones), combining good compressive strength with low cost.

Material selection

based on matching properties (strength, conductivity, density, cost) to the requirements of the application.

Metals

generally shiny, strong, good conductors of heat and electricity, malleable (can be shaped), and have high melting points.

Non-metals

generally poor conductors of heat and electricity, brittle (when solid), and have lower melting points than metals.

PET (polyethylene terephthalate)

the polymer most commonly used to make clear plastic drinks bottles.

Polymer

a very long molecule made of many small repeating units called monomers joined in a chain.

Recycling polymers

important because it saves finite raw materials (crude oil) and reduces the amount of plastic waste sent to landfill.

Polymers (plastics)

made from long chains of repeating units; they are lightweight, flexible, durable, and poor conductors — properties that make them widely useful but also environmentally persistent.

Ceramics (e.g. pottery, glass, brick) are hard, brittle, and resistant to heat — they are good insulators but crack rather than bend under stress.

Composites combine two or more materials to get the best properties of each (e.g. fibreglass = glass fibres in plastic resin — strong and lightweight).

Most materials come from finite Earth resources (ores, fossil fuels); recycling reduces resource depletion and the energy cost of extraction.

Polymers such as polyethylene are useful because they are strong, light, cheap and easily moulded into shape.

Kitchen tiles and mugs are made from clay-based pottery, a common ceramic.

Smart materials change their properties in response to stimuli — for example, shape-memory alloys return to their original shape when heated; thermochromic pigments change colour with temperature.

Biodiversity

the variety of different species in an ecosystem; high biodiversity makes ecosystems more stable and resilient to change.

Consumers

organisms that eat other organisms to obtain energy; primary consumers eat producers, secondary consumers eat primary consumers, and so on.

Ecosystem

all the living organisms in an area plus their non-living environment, and the interactions between them.

Habitat

the place where an organism lives, providing the conditions and resources it needs to survive.

Habitat destruction

the biggest single cause of species extinction; it reduces biodiversity and disrupts ecosystem services.

Herbivore

an animal that eats only plants; a carnivore is an animal that eats only other animals (meat); an omnivore eats both.

Producers

organisms (plants, algae) that make their own food through photosynthesis; they are the start of every food chain.

Quadrat

a square frame placed on the ground to sample and count the plants or slow-moving animals in a fixed area, allowing populations to be estimated.

Decomposers return carbon to the atmosphere as CO₂ by respiring during the breakdown of dead organisms.

Decomposers (bacteria and fungi) break down dead organisms and waste, recycling nutrients back into the soil.

Only about 10% of the energy at one trophic level is transferred to the next; most is lost as heat through respiration or in undigested waste.

In a food chain, an arrow shows the direction energy flows — it points from the organism being eaten to the organism doing the eating.

A food web shows the feeding relationships between many species in an ecosystem; it is made up of many overlapping food chains.

Invasive (non-native) species can damage ecosystems by outcompeting native species for food and space, reducing biodiversity.

In a predator–prey relationship, increases in prey population lead to increases in predator population, which then reduce prey numbers — creating a cyclical pattern.

Galaxy

a huge collection of stars, gas, and dust held together by gravity; our galaxy is the Milky Way.

Gravity

the force that keeps planets in orbit around the Sun and moons in orbit around planets; it acts at a distance and between all objects with mass.

Light-year

the distance light travels in one year (about 9.5 trillion km); it is used to express the vast distances between stars and galaxies.

Mars

known as the "red planet" because its surface is covered in iron oxide (rust) dust, giving it a reddish colour.

Milky Way

the galaxy that contains our solar system; it is a spiral galaxy containing hundreds of billions of stars.

Seasons

caused by Earth's tilted axis (about 23.5°) as it orbits the Sun; when a hemisphere tilts toward the Sun it receives more direct sunlight and experiences summer.

Sun

a star — a massive ball of hot plasma undergoing nuclear fusion — at the centre of our solar system; it provides the light and heat energy for life on Earth.

The Big Bang theory proposes that the universe began about 13.8 billion years ago from an extremely hot, dense point and has been expanding ever since.

Day and night are caused by the Earth rotating on its own axis once every 24 hours; the side facing the Sun experiences day, the side facing away experiences night.

The Earth takes about one year (365.25 days) to complete one orbit around the Sun; the extra quarter-day is why a leap year adds a day every four years.

The eight planets in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

The Moon orbits the Earth roughly every 27–28 days; its gravitational pull causes the ocean tides.

The solar system consists of the Sun and everything that orbits it — eight planets, their moons, dwarf planets (e.g. Pluto), asteroids, comets, and dust.

Stars form from clouds of gas and dust (nebulae), spend most of their lives fusing hydrogen in their cores (main sequence), and eventually expand into red giants before ending as white dwarfs, neutron stars, or black holes, depending on their mass.

The observable universe contains hundreds of billions of galaxies, each containing billions of stars; the distances involved are almost incomprehensibly large.