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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 or mixture melts and boils over a range of temperatures rather than at one fixed temperature.
• Impurities often lower the melting point of a solid and make its melting range wider.
• A formulation is a mixture designed as a useful product, with components present in carefully measured amounts.
• Examples of formulations include fuels, medicines, paints, cleaning products, fertilisers, alloys and foods.
• A soluble substance dissolves in a solvent to form a solution, and an insoluble substance does not dissolve in that solvent.
• Solubility is the maximum mass of solute that dissolves in a given volume of solvent at a particular temperature.
• 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.
• A pure substance produces one spot on a chromatogram in a particular solvent, while an impure substance or mixture produces more than one spot.
• The Rf value is calculated using `R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}`.
• Hydrogen gives a squeaky pop when tested with a lit splint.
• Oxygen relights a glowing splint.
• Carbon dioxide turns limewater cloudy or milky because calcium carbonate forms.
• Chlorine bleaches damp litmus paper, usually turning blue litmus red and then white.

• Use melting point apparatus by heating a small sample in a capillary tube and recording the temperature range over which it melts.
• Interpret melting point data by identifying a sharp melting point as pure and a broad melting range as impure.
• Carry out paper chromatography by drawing a pencil baseline, placing a sample spot on the line, standing the paper in solvent below the baseline, and marking the solvent front before it dries.
• 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 `R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}`.
• Test for gases safely by using a lit splint for hydrogen, a glowing splint for oxygen, limewater for carbon dioxide, and damp litmus paper for chlorine.