Chemical Analysis & Chromatography
Pure Substances and Mixtures
In chemistry, a pure substance is a single element or compound that is not mixed with anything else. A pure substance has a sharp, fixed melting point and boiling point. A mixture contains two or more substances that are not chemically bonded. Mixtures melt and boil over a range of temperatures, which is how they can be distinguished from pure substances.
A pure substance has a sharp, fixed melting point. A mixture melts over a range of temperatures. Melting point data can therefore be used to check the purity of a substance.
Separation Techniques
Different mixtures require different separation methods, depending on the properties of the components. The main techniques are:
- Filtration — separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel; the solid residue stays on the paper while the liquid filtrate passes through.
- Crystallisation — used to obtain a dissolved solid (solute) from a solution. The solution is heated to evaporate some solvent, then left to cool so crystals form.
- Simple distillation — separates a solvent from a solution. The solution is heated until the solvent evaporates, then the vapour is cooled and condensed in a condenser and collected as a pure liquid.
- Fractional distillation — separates a mixture of liquids with different boiling points. A fractionating column provides a temperature gradient so that each liquid condenses at a different height.
Paper Chromatography
Paper chromatography separates mixtures of soluble substances. A spot of the mixture is placed on a pencil line near the bottom of chromatography paper. The paper is placed in a solvent (the mobile phase), which rises up the paper by capillary action. As the solvent moves through the mixture, different substances travel at different rates because they have different attractions to the paper (the stationary phase) and the solvent.
The result of paper chromatography is called a chromatogram. A pure substance produces a single spot. A mixture produces multiple spots, one for each component. You can identify unknown substances by comparing their spots with spots from known reference substances run alongside on the same chromatogram.
Rf Values
Each substance in a chromatogram can be identified by its Rf value. This is calculated using:
$$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}$$
The Rf value is always between 0 and 1. It depends on the solvent used, so Rf values can only be compared when the same solvent and conditions are used. A substance with a higher Rf value is more soluble in the mobile phase and travels further up the paper.
Rf = distance moved by substance ÷ distance moved by solvent front. Rf values are unique to each substance in a given solvent and can be used to identify unknown compounds.
In this core practical, spots of different inks are placed on chromatography paper along a pencil baseline. The paper is lowered into a beaker of solvent so the baseline is just above the solvent level. As the solvent rises, it separates the dyes in each ink. A pencil line (not pen) is used for the baseline because pencil is insoluble and will not dissolve in the solvent. The solvent front is marked with a pencil when the paper is removed, and Rf values are calculated for each spot.
Chemical Tests for Gases
Several important gases can be identified using simple chemical tests:
- Oxygen — relights a glowing splint.
- Hydrogen — burns with a squeaky pop when a lit splint is held near the mouth of a test tube.
- Carbon dioxide — turns limewater milky (cloudy white).
- Chlorine — bleaches damp litmus paper, turning it white.
Oxygen relights a glowing splint. Hydrogen gives a squeaky pop. Carbon dioxide turns limewater milky. Chlorine bleaches damp litmus paper white.
Potable Water
Potable water is water that is safe to drink. It is not the same as pure water in chemistry — it contains low levels of dissolved substances. In the UK, potable water is produced from fresh water sources by sedimentation (letting solids settle), filtration (removing fine particles) and chlorination (killing bacteria). In areas without fresh water, sea water can be made potable by distillation or reverse osmosis, but these are energy-intensive processes.
Water used in chemical analysis must not contain any dissolved salts. This is because dissolved ions could interfere with test results, giving false positives or inaccurate measurements. Deionised or distilled water is used instead of tap water in laboratory analysis.
When answering questions about chromatography, always state both the mobile phase (solvent) and the stationary phase (paper). If asked to calculate Rf values, show your working clearly — divide the distance moved by the substance by the distance moved by the solvent front.