SI units

The International System of Units, abbreviated to SI, provides a shared measurement language. Using agreed units allows results collected by different people to be compared and processed without ambiguity. OCR requires SI units to be used where appropriate: the unit should suit both the quantity being measured and the scale of the values.

Examples named in the specification include:

A laboratory mass might sensibly be recorded in grams or milligrams, whereas the mass of a large industrial load might be clearer in kilograms. The purpose is not merely to attach any unit, but to communicate the measurement clearly.

Writing values and unit symbols

Unit symbols follow fixed conventions. Their capitalisation matters: joule is written J, while gram is written g. Prefixes and units are joined into one symbol, such as kg, mg, km, mm and kJ. A space is normally left between the number and the unit, as in $25.0\ \mathrm{g}$ or $120\ \mathrm{kJ}$. Unit symbols are not pluralised, so $6\ \mathrm{kg}$ is correct rather than “6 kgs”.

Units must survive every stage of data processing. Include them in recorded observations, table headings, graph-axis labels and calculated answers. A clear table heading can be written as mass / g or mass (g). Each number in that column is then understood to be a mass measured in grams, so the symbol does not need to be repeated in every cell.

The quantity and unit must agree. A heading such as “mass / cm” is wrong because centimetres measure length, not mass. Likewise, a graph axis labelled only “energy” is incomplete if its scale represents joules or kilojoules.

Chemistry also needs standardised names

Measurements are only useful if it is also clear which substance was measured. Chemists therefore use systematic chemical names and formulae. IUPAC—the International Union of Pure and Applied Chemistry—develops internationally agreed nomenclature so that names communicate chemical information consistently.

Older or informal names can be vivid but ambiguous. A systematic name such as potassium sulfate, together with the formula $\mathrm{K_2SO_4}$, identifies the substance much more precisely. OCR therefore expects IUPAC chemical nomenclature to be used unless it would be inappropriate in the particular context.

A chemical formula is more than an abbreviation. It identifies the elements present and shows their relative amounts. In $\mathrm{K_2SO_4}$, the subscripts show a ratio of two potassium atoms to one sulfur atom to four oxygen atoms. A missing subscript means one. Subscripts must not be changed simply to make an equation balance because changing a subscript changes the identity of the substance.

Capital letters also matter. An element symbol begins with a capital letter and may have a second, lower-case letter. Thus $\mathrm{CO}$ represents carbon monoxide, containing carbon and oxygen, whereas $\mathrm{Co}$ is the symbol for the element cobalt.

Reading and using systematic names

For many compounds containing two non-metals, prefixes help state the relative numbers of atoms. For example, carbon dioxide, $\mathrm{CO_2}$, contains two oxygen atoms for each carbon atom. Prefixes such as mono-, di- and tri- contribute quantitative information to such names.

Ionic compounds are named using the ions present. Familiar polyatomic ions keep their complete names: $\mathrm{MgSO_4}$ is magnesium sulfate, not “magnesium sulfur tetroxide”. Other names commonly encountered in chemistry include chloride, oxide, hydroxide, nitrate and carbonate.

Not every familiar chemical name can be worked out directly from its formula, and chemistry retains some established names. Formulae and systematic naming rules therefore need regular practice rather than guesswork. The qualification “unless inappropriate” also means following the context of the question: if a familiar accepted name is supplied or specifically requested, it should not be replaced merely to make the wording sound more complicated.

Applying both conventions to processed data

Suppose a results table records the mass of potassium sulfate used in several trials. A precise heading would be mass of potassium sulfate / g, and the substance may also be identified as $\mathrm{K_2SO_4}$. This communicates three separate things:

• the measured quantity is mass;
• every value in the column is expressed in grams;
• the substance measured is potassium sulfate.

The same clarity should continue into calculations and conclusions. Quote the processed value with its appropriate unit, identify chemicals by correct names or formulae, and preserve meaningful distinctions such as subscripts, capitalisation and state symbols. For example, $\mathrm{H_2O(l)}$ identifies liquid water, while $\mathrm{H_2O(g)}$ identifies water vapour. Consistent notation prevents uncertainty about what was measured and makes the resulting scientific conclusion easier for others to check.