AQA Chemical analysis
Pure substances
Purity, formulations and chromatography
Chromatography | Can be used to separate mixtures and help identify substances. | Involves a mobile phase (e.g. water or ethanol) and a stationary phase (e.g. chromatography paper). |
Rf Values | The ratio of the distance moved by a compound to the distance moved by solvent. | Rf = distance moved by substance distance moved by solvent |
Pure substances | The compounds in a mixture separate into different spots. | This depends on the solvent used. A pure substance will produce a single spot in all solvents whereas an impure substance will produce multiple spots. |
Pure substances | A pure substances is a single element or compound, not mixed with any other substance. | Pure substances melt and boil at specific temperatures. Heating graphs can be used to distinguish pure substances from impure. |
Formulations
Formulation | A formulation is a mixture that has been designed as a useful product. |
How are formulations made? | By mixing chemicals that have a particular purpose in careful quantities. |
Examples of formulations. | Fuels, cleaning agents, paints, medicines and fertilisers. |
Gas | Test | Positive result |
Hydrogen | Burning splint | ‘Pop’ sound. |
Oxygen | Glowing splint | Re-lights the splint. |
Chlorine | Litmus paper (damp) | Bleaches the paper white. |
Carbon dioxide | Limewater | Goes cloudy (as a solid calcium carbonate forms). |
Chromatography
Identification of common gases
Identification of ions (CHEMISTRY ONLY)
Flame tests (chem only)
Element | Colour flames |
Lithium | Crimson |
Sodium | Yellow |
Potassium | Lilac |
Calcium | Orange-red |
Copper | Green |
Metal hydroxides (chem only)
Sodium hydroxide | Is added to solutions to identify metal ions. |
White precipitates | Aluminium, calcium and magnesium ions form this with sodium hydroxide solution. |
Coloured precipitates | Copper (II) = blue Iron (II) = green Iron (III) = brown |
Carbonates, halides and sulfates
(chem only)
Carbonates | React with dilute acids to form carbon dioxide. |
Halide ions | When in a solution, they produce precipitates with silver nitrate solution in the presence of nitric acid. |
Sulfate ions | When in a solutions they produce a white precipitate with barium chloride solutions in the presence of hydrochloric acid. |
Instrumental methods
Flame emission spectroscopy
Flame emission spectroscopy | An instrumental method used to analyse metal ions. | The sample solution is put into a flame and the light that is given out is put through a spectroscope. The output line spectrum, can be analysed to identify the metal ions in the solution. It can also be used to measure concentrations. |
Instrumental methods | Methods that rely on machines | Can be used to identify elements and compounds. These methods are accurate, sensitive and rapid. |
Melting point of a pure substance
Melting point of an impure substance
Position solvent reaches
Mixture separated
Mixture
Solvent