GCSE / Chemistry
Chemical Analysis
Learn GCSE Chemistry chemical analysis, including pure substances, formulations, chromatography, Rf values, gas tests, flame tests, ion identification, precipitate tests and instrumental methods such as flame emission spectroscopy.
A pure substance in chemistry is a single element or compound that is not mixed with any other substance. Pure substances have fixed melting and boiling points, so these values can be compared with known data to help identify a substance or assess its purity.
A formulation is a carefully designed mixture made for a particular purpose. Each component is present in a measured proportion so that the final product has specific properties; examples include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.
Chromatography is a separation technique used to separate mixtures and help identify substances. It involves a stationary phase and a mobile phase, and substances separate because they have different distributions between these two phases.
In paper chromatography, components of a mixture may travel different distances with the solvent and form separate spots. A pure substance normally produces a single spot in a given solvent, while a mixture can produce several spots.
The retention factor, or value, is calculated using . values can be compared with reference values obtained under the same conditions to help identify compounds.
Chromatograms must be interpreted carefully because the same compound can have different values in different solvents. Chromatography can therefore be used to distinguish pure substances from mixtures and identify components when suitable reference data are available.
Hydrogen gas is identified using a burning splint at the opening of a test tube. If hydrogen is present, it burns rapidly and produces a characteristic squeaky pop sound.
Oxygen gas is identified using a glowing splint. When the glowing splint is inserted into a test tube containing oxygen, the splint relights because oxygen supports combustion.
Carbon dioxide is tested using limewater, which is an aqueous solution of calcium hydroxide. Carbon dioxide turns limewater milky or cloudy because an insoluble calcium carbonate precipitate forms.
Chlorine gas is identified using damp litmus paper. Chlorine first affects the indicator and then bleaches the damp litmus paper white because of its bleaching action.
Flame tests can identify certain metal ions from their characteristic flame colours: lithium gives crimson, sodium gives yellow, potassium gives lilac, calcium gives orange-red and copper gives green.
Sodium hydroxide solution can identify some metal ions by forming characteristic hydroxide precipitates. Aluminium, calcium and magnesium ions form white precipitates; aluminium hydroxide dissolves in excess sodium hydroxide, copper(II) forms blue, iron(II) green and iron(III) brown precipitates.
Carbonate ions are identified by adding dilute acid, which produces carbon dioxide gas. The carbon dioxide can then be confirmed because it turns limewater milky.
Halide ions are tested using silver nitrate solution in the presence of dilute nitric acid. Chloride ions form a white precipitate, bromide ions form a cream precipitate and iodide ions form a yellow precipitate.
Sulfate ions form a white precipitate with barium chloride solution in the presence of dilute hydrochloric acid. Instrumental methods such as flame emission spectroscopy provide rapid, sensitive and accurate analysis, and flame emission spectra can identify metal ions and measure their concentrations.