Measurement, Separation and Purity – Quick Review

Physical Quantities and SI Units

In chemistry the International System of Units provides a common language. Time is measured in seconds, temperature in kelvin, length in metres, mass in kilograms and volume in cubic metres. Conversions between larger or smaller multiples of these units are routinely required during experimentation.

Time Measurement

The SI unit is the second. Key conversions are 1 min=60 s1\,\text{min}=60\,\text{s} and 1 h=3600 s1\,\text{h}=3600\,\text{s}. Digital stop-watches usually read to (\pm0.01\,\text{s}), whereas analogue designs give only (\pm0.1\,\text{s}).

Temperature Measurement

Temperature is recorded in kelvin; the relationship with the Celsius scale is T(K)=T(∘!C)+273T(\text{K})=T(^\circ!\text{C})+273. Laboratory measurements use alcohol or mercury thermometers, digital probes or data-logger sensors. Kelvin readings are written without a degree symbol.

Length Measurement

The basic unit is the metre with the equivalences 1 m=10 dm=100 cm=1000 mm1\,\text{m}=10\,\text{dm}=100\,\text{cm}=1000\,\text{mm}. Metre rules and measuring tapes are common; typical precisions are (\pm0.1\,\text{cm}) for a rule and up to (\pm0.5\,\text{cm}) for long tapes.

Mass Measurement

Mass is expressed in kilograms. Relationships include 1 kg=1000 g=1 000 000 mg1\,\text{kg}=1000\,\text{g}=1\,000\,000\,\text{mg} and 1 t=1000 kg1\,\text{t}=1000\,\text{kg}. Electronic balances provide convenient digital readings with about (\pm0.01\,\text{g}) accuracy.

Volume Measurement

The SI unit is m3\text{m}^3 and the scale links are 1 m3=1000 dm3=1 000 000 cm31\,\text{m}^3=1000\,\text{dm}^3=1\,000\,000\,\text{cm}^3. Pipettes and volumetric flasks deliver single accurate volumes, burettes read to (\pm0.05\,\text{cm}^3), and measuring cylinders to (\pm0.5\,\text{cm}^3). A gas syringe (≈100 cm3100\,\text{cm}^3) measures gaseous volumes. Liquid readings must be taken at eye level with the meniscus to prevent parallax error.

Gas Collection Methods

Gases insoluble or slightly soluble in water are collected by displacement of water. Gases denser than air, such as chlorine, are gathered by downward delivery, while lighter gases like ammonia are collected by upward delivery. Gas syringes provide precise small-volume collection regardless of solubility or density.

Separation Techniques

Solid–solid mixtures exploit particle or magnetic differences: magnets remove magnetic components, sieving separates by size, and selective solvents dissolve only one solid.

Solid–liquid mixtures employ filtration for insoluble solids, evaporation to dryness when the solute is heat-stable, crystallisation for heat-sensitive solutes, and simple distillation when the goal is to recover the liquid solvent.

Liquid–liquid mixtures are resolved by chromatography or fractional distillation. Chromatography separates based on different solubilities and produces a chromatogram used for identification by comparison with pure standards. Fractional distillation adds a fractionating column to improve condensation–re-evaporation cycles, enabling separation of miscible liquids with close boiling points; it underpins processes such as crude-oil refining and air liquefaction.

Purity Determination

A pure substance exhibits a sharp melting point and boiling point. Impurities broaden these ranges and shift the values—higher for boiling points and lower for melting points in most cases. Consequently melting/boiling data, alongside chromatographic behaviour, serve to confirm identity and purity.