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neurologyYou need to know

• A pure substance in chemistry is a single element or a single compound that is not mixed with any other substance.
• A pure substance has a sharp melting point or boiling point, but an impure substance melts or boils over a range of temperatures.
• Impurities usually lower the melting point of a substance and make melting occur over a wider temperature range.
• A formulation is a useful mixture made by following a precise recipe so that each component is present in a carefully measured amount.
• Examples of formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers, foods and drinks.
• A soluble substance dissolves in a solvent, an insoluble substance does not dissolve in a solvent, and solubility is the amount of solute that dissolves in a given amount of solvent.
• Chromatography separates substances because different substances have different solubilities in the solvent and different attractions to the stationary phase.
• In paper chromatography, the mobile phase is the solvent and the stationary phase is the paper.
• In chromatography, a pure substance normally produces one spot, while an impure substance or mixture produces more than one spot.
• The `R_f` value is calculated using `R_f = \frac{\text{distance moved by substance}}{\text{distance moved by 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 in a test tube containing the gas.
• Carbon dioxide turns limewater milky because calcium carbonate forms.
• Chlorine bleaches damp litmus paper white, usually after turning it red first.

• Use melting point apparatus to test a small dry sample, recording the temperature range over which it melts and judging purity from how sharp or broad the melting range is.
• Interpret melting point, boiling point or chromatography data to decide whether a substance is pure or impure, using one spot or a sharp fixed point as evidence of purity.
• Set up paper chromatography by drawing a pencil baseline, placing the sample on the line, keeping the baseline above the solvent, and allowing the solvent front to rise before marking it in pencil.
• Calculate an `R_f` value by measuring from the baseline to the centre of the spot and from the baseline to the solvent front, then dividing the two distances.
• Test for hydrogen by placing a lit splint at the mouth of the test tube and identifying a squeaky pop as a positive result.
• Test for oxygen by placing a glowing splint into the gas and identifying relighting as a positive result.
• Test for carbon dioxide by bubbling the gas through limewater and identifying a milky white colour as a positive result.
• Test for chlorine using damp litmus paper and identifying bleaching to white as a positive result.