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neurologyYou need to know

• In chemistry, a pure substance is a single element or a single compound that is not mixed with any other substance.
• A pure substance has a sharp melting point and a sharp boiling point at fixed temperatures.
• An impure substance melts or boils over a range of temperatures because different particles disrupt the regular structure.
• A melting point apparatus heats a small sample and records the temperature range over which it melts.
• A formulation is a mixture that has been designed as a useful product with substances present in carefully measured amounts.
• Examples of formulations include fuels, medicines, fertilisers, foods, paints, alloys and cleaning products.
• A soluble substance dissolves in a solvent to form a solution, while an insoluble substance does not dissolve in that solvent.
• Solubility is the maximum mass of solute that can dissolve in a given volume of solvent at a particular temperature.
• Chromatography separates substances because they have different solubilities in the solvent and different attractions to the stationary phase.
• In paper chromatography, the paper is the stationary phase and the solvent is the mobile phase.
• In a chromatogram, a pure substance normally produces one spot, while an impure substance or mixture produces more than one spot.
• The `Rf` value of a substance is calculated using `Rf = {distance moved by the substance}/{distance moved by the solvent front}`.
• Hydrogen gives a squeaky pop when a lit splint is placed at the mouth of a test tube containing the gas.
• Oxygen relights a glowing splint when the splint is placed into the gas.
• Carbon dioxide turns limewater from colourless to milky or cloudy.
• Chlorine bleaches damp litmus paper, turning it white after first turning blue litmus red.
• In flame tests, lithium ions give a crimson flame, sodium ions give a yellow flame, potassium ions give a lilac flame, calcium ions give an orange-red flame, and copper ions give a green flame.
• With sodium hydroxide solution, copper(II) ions form a blue precipitate, iron(II) ions form a green precipitate, iron(III) ions form a brown precipitate, and aluminium ions form a white precipitate that dissolves in excess sodium hydroxide.
• Carbonate ions react with dilute acid to produce carbon dioxide, sulfate ions form a white precipitate with barium chloride solution after acidifying, and halide ions form precipitates with silver nitrate solution after acidifying.
• In halide tests with silver nitrate solution, chloride ions form a white precipitate, bromide ions form a cream precipitate, and iodide ions form a yellow precipitate.

• Use melting point data to classify a substance as pure if it melts sharply at the expected temperature, or impure if it melts over a range of temperatures.
• Identify a formulation from information about a product by recognising that it is a purposeful mixture with measured ingredients.
• Carry out paper chromatography by drawing a pencil baseline, placing the sample on the line, keeping the solvent below the baseline, and marking the solvent front before it dries.
• Interpret a chromatogram by comparing the number, position and colour of spots to decide whether samples are pure, impure or contain the same substances.
• Calculate an `Rf` value by measuring from the baseline to the centre of the spot and from the baseline to the solvent front, then substituting into the `Rf` equation.
• Test for hydrogen using a lit splint and record a squeaky pop as the positive result.
• Test for oxygen using a glowing splint and record relighting as the positive result.
• Test for carbon dioxide by bubbling the gas through limewater and record a milky or cloudy colour change as the positive result.
• Test for chlorine using damp litmus paper and record bleaching to white as the positive result.
• Interpret unknown ion test results by matching flame colours, precipitate colours and gas test observations to the correct positive or negative ions.