LGCSE Physical Science (0181) key terms

The factor that an experimenter deliberately changes or selects in an investigation so that its effect on the outcome can be observed, while all other relevant conditions are held constant.

The quantity that is measured in an experiment because it is expected to respond to changes in the independent variable; its values form the data plotted on the vertical axis of a graph.

Any factor other than the independent variable that is kept the same throughout an investigation so that it cannot influence the measured outcome and confound the conclusion drawn.

An investigation in which only the independent variable is altered, all other conditions are controlled, and the outcome is measured reliably, so that any change observed can be attributed to the variable tested.

How close a measured or calculated value lies to the true value. Accuracy is improved by using better instruments and careful technique rather than simply by repeating readings.

How closely repeated measurements of the same quantity agree with one another, or how fine the scale of an instrument is. Precise readings may still be inaccurate if there is a systematic fault.

The extent to which an investigation, when repeated, gives consistent results. It is improved by taking repeat readings, averaging them, and using a sufficiently large sample or range of values.

Unpredictable variation in readings caused by limitations of the observer or instrument. It scatters results above and below the true value and is reduced by repeating measurements and calculating a mean.

A consistent offset in all readings caused by a faulty instrument, a zero error, or flawed method. It shifts results in one direction and is not removed by averaging repeats.

The net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion. It happens in gases and liquids and is faster at higher temperature.

The model that all matter is made of tiny particles in constant random motion, with the energy and spacing of the particles determining whether a substance is a solid, liquid or gas.

The change of state from solid to liquid, occurring at a fixed temperature called the melting point as particles gain enough energy to overcome the forces holding them in fixed positions.

The change from liquid to vapour at the surface of a liquid at any temperature, as the fastest-moving surface particles escape; the rate increases with temperature, surface area and air movement.

A separation technique in which a mixture of an insoluble solid and a liquid is poured through filter paper, trapping the solid as residue while the liquid passes through as filtrate.

A separation method in which a liquid is boiled to vapour and then condensed back to liquid, used to separate a solvent from dissolved solids or to separate liquids with different boiling points.

A technique that separates dissolved substances according to how readily they are carried by a moving solvent phase over a stationary phase, identified by comparing the distances travelled.

The number of protons in the nucleus of an atom, which uniquely identifies the element and equals the number of electrons in a neutral atom.

The total number of protons and neutrons in the nucleus of an atom. It is always a whole number and is written as a superscript before the chemical symbol.

One of two or more atoms of the same element that have identical numbers of protons but different numbers of neutrons, and therefore different mass numbers but the same chemical properties.

The strong electrostatic attraction between oppositely charged ions formed when one atom transfers electrons to another, typically between a metal and a non-metal, giving a giant lattice structure.

A bond formed when two atoms share one or more pairs of electrons, typically between non-metal atoms, producing discrete molecules or giant covalent structures such as diamond.

A horizontal row of the Periodic Table in which elements have the same number of electron shells; properties change gradually across a period as the number of outer electrons increases.

A vertical column of the Periodic Table whose elements share the same number of electrons in their outer shell and therefore show similar chemical properties and form ions of the same charge.

The weighted average mass of the naturally occurring isotopes of an element compared with one-twelfth of the mass of a carbon-12 atom; it is used to calculate reacting masses.

The amount of substance containing 6.02 x 10^23 particles. The mass of one mole of a substance in grams equals its relative atomic or relative molecular mass.

The simplest whole-number ratio of atoms of each element in a compound, found from the masses or percentage composition of the elements present in a sample.

The amount of solute dissolved in a given volume of solution, usually expressed in moles per cubic decimetre (mol/dm3) or grams per cubic decimetre.

The reaction between an acid and a base in which hydrogen ions and hydroxide ions combine to form water, producing a salt and water and bringing the pH towards neutral.

A scale from 0 to 14 measuring the concentration of hydrogen ions in a solution: values below 7 are acidic, 7 is neutral and above 7 is alkaline.

The minimum energy colliding particles must possess for a reaction to occur. Reactions proceed faster when temperature, concentration or surface area increase, or when a catalyst lowers this barrier.

The change in amount of a reactant or product per unit time. It increases with higher temperature, higher concentration or pressure, greater surface area and the presence of a catalyst.

A substance that increases the rate of a chemical reaction by providing an alternative pathway of lower activation energy, while remaining chemically unchanged in mass and composition at the end.

When two bodies interact, they exert forces on each other that are equal in magnitude and opposite in direction, acting on different bodies along the same line.

The single force that has the same effect as all the forces acting on a body combined. A non-zero resultant causes acceleration in the direction of that force.

The constant maximum speed reached by a falling object when the upward drag force equals its weight, so the resultant force and acceleration become zero.

The gravitational force acting on a mass, calculated as mass multiplied by gravitational field strength (W = mg) and measured in newtons; it varies with location while mass does not.

The product of an object's mass and velocity, a vector quantity measured in kilogram metres per second. Total momentum is conserved in collisions and explosions in a closed system.

The energy transferred when a force moves an object through a distance in the direction of the force, calculated as force multiplied by distance and measured in joules.

The rate at which energy is transferred or work is done, equal to energy transferred divided by time and measured in watts, where one watt is one joule per second.

The proportion of the total energy supplied to a device or system that is transferred usefully, expressed as useful output energy divided by total input energy, often as a percentage.

The change in direction of a wave as it passes from one medium into another of different optical density, caused by a change in speed at the boundary.

The complete reflection of light back inside a denser medium when it strikes the boundary at an angle greater than the critical angle; the principle behind optical fibres.

The number of complete waves or oscillations passing a point each second, measured in hertz. It is set by the source and does not change when a wave enters a new medium.

The current through a metallic conductor at constant temperature is directly proportional to the potential difference across it, so that V = IR where R is the resistance.

The opposition of a component to the flow of electric current, equal to potential difference divided by current (R = V/I) and measured in ohms.

A circuit in which components are connected one after another so that the same current flows through each, the total potential difference is shared, and total resistance is the sum of the parts.

The generation of a potential difference in a conductor when it experiences a changing magnetic field, for example when a magnet is moved in or out of a coil.

The energy absorbed or released by a substance during a change of state at constant temperature, without any change in temperature, used to overcome or release intermolecular forces.

The energy needed to raise the temperature of one kilogram of a substance by one degree Celsius, measured in joules per kilogram per degree Celsius.

The transfer of thermal energy through a material by collisions between particles or by free electrons, without bulk movement of the material; metals are good conductors.

The transfer of thermal energy by the bulk movement of a fluid, where warmer, less dense regions rise and cooler, denser regions sink, setting up a circulation current.

The transfer of energy by electromagnetic waves, mainly infrared, which needs no medium and can travel through a vacuum; dark, matt surfaces absorb and emit it best.

The warming of a planet's surface because certain atmospheric gases absorb outgoing infrared radiation and re-emit it, reducing the energy lost to space.

The force per unit mass acting on an object at a point in a gravitational field, measured in newtons per kilogram and approximately 9.8 N/kg near the Earth's surface.

The Sun together with the planets, moons, asteroids and comets bound to it by gravity, all orbiting the Sun with periods that increase with distance from it.