Mixtures - AQA GCSE Chemistry

• In chemistry, the term 'pure' is used in a different way from its everyday meaning.
• Shops sell cartons labelled as 'pure' orange juice, meaning that the contents are just orange juice, with no other substances added.
• The juice is not pure in the chemical sense, as it contains different substances mixed together.
• A pure substance in chemistry consists only of one element or one compound, while a mixture consists of two or more different substances, not chemically joined together.
• The substances in a mixture can be elements, compounds, or both.
• Being part of a mixture does not change the chemical properties of the substances that are in it.
• Mixtures can be separated by physical processes, which do not involve chemical reactions and no new substances are made.
• Filtration is used to separate an insoluble solid from a liquid.
• Filtration is useful for separating sand from a mixture of sand and water, or excess reactant from a reaction mixture.
• Filtration works because the filter paper has tiny holes or pores in it.
• These pores in the filter paper are large enough to let small molecules and dissolved ions through, but not the much larger particles of undissolved solid.
• Crystallisation is used to produce solid crystals from a solution.
• When the solution is warmed, some of the solvent evaporates leaving crystals behind.
• Crystallisation is used to obtain copper sulfate crystals from copper sulfate solution.
• To obtain large, regularly shaped crystals from crystallisation, the solution can be put in an evaporating basin, warmed by placing the evaporating basin over a boiling water bath and stopped heating when crystals begin to form around the edge of the basin.
• After the remaining solution has cooled down, the excess liquid can be poured away or filtered and the crystals can be dried using a warm oven or by patting them with filter paper.
• Simple distillation is used to separate a solvent from a solution.
• Simple distillation is useful for producing pure water from seawater.
• Simple distillation works because the dissolved solute has a much higher boiling point than the solvent.
• When the solution is heated, solvent vapour leaves the solution.
• The vapour moves away and is cooled and condensed.
• The remaining solution becomes more concentrated as the amount of solvent in it decreases.
• Fractional distillation is used to separate different liquids from a mixture of liquids.
• Fractional distillation is useful for separating ethanol from a mixture of ethanol and water, and for separating different fractions from crude oil.
• Fractional distillation works because the different liquids have different boiling points.
• When the mixture is heated vapours rise through a column which is hot at the bottom, and cooler at the top then the vapours condense when they reach a part of the column that is below the temperature of their boiling point and each liquid is led away from the column.
• There are two ways of obtaining different liquids from the column you can collect different liquids from different parts of the column as the substance with the lowest boiling point is collected at the top of the column.
• By continuing to heat the mixture to increase the temperatures in the column, the substance with the lowest boiling point is collected first.
• Paper chromatography is used to separate mixtures of soluble substances, often coloured substances such as food colourings, inks, dyes or plant pigments.
• Chromatography relies on two different 'phases', the stationary phase, which in paper chromatography is very uniform, absorbent paper, and the mobile phase, which is the solvent that moves through the paper, carrying different substances with it.
• In paper chromatography, the different dissolved substances in a mixture are attracted to the two phases in different proportions, causing them to move at different rates through the paper.
• Separation by chromatography produces a chromatogram.
• A paper chromatogram can be used to distinguish between pure and impure substances, as a pure substance produces one spot on the chromatogram whereas an impure substance, or mixture, produces two or more spots.
• A paper chromatogram can also be used to identify substances by comparing them with known substances, as two substances are likely to be the same if they produce the same number of spots, and these match in colour or the spots travel the same distance up the paper.