Separation, Purity and Chromatography
Pure Substances and Mixtures
In chemistry, a pure substance consists of a single element or compound. A pure substance melts and boils at a precise, fixed temperature. A mixture contains two or more substances not chemically bonded together. Mixtures melt and boil over a range of temperatures, not at a fixed point — and this difference is the basis of purity testing.
To test whether a sample is pure, measure its melting point and compare it to the accepted value for that substance. If the melting point is lower than expected and occurs over a range of temperatures, the sample contains impurities. This technique is used routinely in pharmaceutical quality control to check that a medicine contains the correct active compound.
A pure substance has a sharp melting point at a fixed temperature. A mixture melts over a range of temperatures. Impurities lower the melting point and broaden the melting range.
Separation Techniques
Because mixtures are not chemically combined, their components can be separated using physical methods. The correct technique depends on the nature of the mixture and the properties of its components.
Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel; the solid remains as the residue and the liquid passes through as the filtrate. For example, sand is separated from salt solution by filtration.
Crystallisation recovers a dissolved solid from solution. The solution is heated to evaporate most of the solvent, then left to cool slowly. As the solution cools, the dissolved substance becomes less soluble and forms crystals. The crystals can be filtered and dried. This is used to purify soluble salts such as copper sulfate.
Simple distillation works when the substances in a mixture have very different boiling points (usually more than 25°C apart). The mixture is heated; the more volatile liquid evaporates first, the vapour is cooled in a condenser, and the liquid is collected. For example, distilled water is produced from salt solution by simple distillation — water evaporates and is condensed, leaving salts behind.
Fractional distillation is used when liquids have similar boiling points. A fractionating column is packed with glass beads or plates that provide surfaces for condensation. Liquids with lower boiling points travel further up the column and are collected first. This is the method used to separate crude oil into fractions such as petrol, kerosene, and diesel.
Chromatography
Chromatography separates substances in a mixture based on how strongly they are attracted to the stationary phase (e.g. paper) compared to how soluble they are in the moving phase (the solvent). In paper chromatography, a pencil line is drawn near the bottom of the paper, spots of mixture are placed on the line, and the paper is placed in solvent. As the solvent moves up, it carries substances with it — those that are more soluble travel further.
The position of each spot is described by its Rf value:
$$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}$$
Each substance has a unique Rf value in a given solvent at a given temperature. A pure substance gives a single spot; a mixture gives multiple spots at different heights. By comparing Rf values with reference values, or by running known substances alongside the unknown, substances can be identified.
Rf = distance moved by substance ÷ distance moved by solvent. In a given solvent, each substance has a unique Rf value. A pure substance produces one spot; a mixture produces multiple spots.
This course has no ‘required practicals’. It has eight practical activity groups, and chromatography sits in the separation group. To carry out a paper chromatography investigation: draw a pencil baseline (not pen — ink would also travel) near the bottom of the chromatography paper. Spot the sample mixture and reference substances on the baseline. Place the paper in a small amount of appropriate solvent — the solvent level must be below the pencil line so the spots do not dissolve directly into the solvent pot. Allow the solvent to rise until it nears the top of the paper, then remove and immediately mark the solvent front with pencil. Measure distances from the pencil line to calculate Rf values. Variables to control include the type of solvent and the length of time the chromatogram is run.
Instrumental Methods of Identification
As well as chromatography, modern laboratories use instrumental techniques that are faster, more sensitive, and capable of analysing very small samples. The instrumental technique named on this course is emission spectroscopy: a hot sample gives out light, each element makes its own pattern of lines, and the lines are matched to reference data for known elements. Mass spectrometry identifies compounds by fragmenting molecules into ions and measuring their mass-to-charge ratios. The output is a mass spectrum: a graph showing the relative abundance of different fragment masses. The largest fragment (the molecular ion) gives the relative molecular mass of the compound. Mass spectrometry is used in forensic science, drug testing, and quality control of pharmaceuticals.
Flame tests are a standard laboratory test, not an instrumental method. They use the colour of light emitted when a compound is heated in a flame to identify metal ions. Lithium gives a crimson flame; sodium gives yellow; potassium gives lilac; calcium gives red-orange; copper gives blue-green. These are qualitative tests — they tell you which metal is present but not how much.