OCR GCSE Combined Science A key terms

The sum of the relative atomic masses of all the atoms shown in a substance’s chemical formula. It has no unit.

The mass of a particle compared with a reference mass; on the subatomic-particle scale, a proton’s mass is taken as 1.

The average mass of a molecule compared with one-twelfth of the mass of a carbon-12 atom. It is the sum of the relative atomic masses of all atoms in its molecular formula. Its symbol is $M_r$, and it has no units.

A nerve cell within the central nervous system that connects sensory neurones to motor neurones.

The extent to which findings can be depended on, supported by consistent results from repeated measurements or independent investigations.

An energy source that is naturally replenished at least as quickly as it is used.

The extent to which the same person obtains closely agreeing results when repeating an investigation using the same method and equipment under the same conditions.

Carrying out a study again to investigate whether similar results are obtained.

A model that uses a picture, physical object or symbols to show selected features of something.

A sample that reflects the important variation in the larger group or area being investigated.

The extent to which closely agreeing results are obtained when an investigation is carried out by another person, or using different equipment or methods.

A force that pushes objects away from one another.

The insoluble solid left on the filter paper after filtration.

A measure of how strongly a component opposes the flow of current, measured in ohms (Ω).

The smallest change in a quantity that a measuring instrument can distinguish.

Replacing one force vector with two perpendicular component vectors that together have the same effect.

Chemical reactions in cells that release energy from food molecules. Aerobic respiration produces carbon dioxide and water.

An action made after detecting a stimulus, such as closing the fingers or pressing a button.

An enzyme that cuts DNA at specific base sequences.

The overall force acting on an object after all the forces have been combined, taking their directions into account.

The time taken for a substance to pass through a gas chromatography column and reach the detector.

Using a product again without processing its material into a new product.

A method in which high pressure forces water through a partially permeable membrane that allows water molecules through but holds back most dissolved ions.

The reaction that converts products back into reactants, reading a chemical equation from right to left.

A change that can be undone to restore the original material.

A chemical reaction in which the products can react to form the original reactants.

The ratio of the distance moved by a substance to the distance moved by the solvent, both measured from the baseline: $R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$.