WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:08.732 Unit S C 2: Methods of separating and purifying substances. 00:00:11.400 --> 00:00:16.987 A chemically pure substance contains a single element or compound, with a fixed composition. 00:00:17.137 --> 00:00:22.141 Everyday “pure” can mean natural or without additives and need not mean chemically pure. 00:00:24.800 --> 00:00:27.616 An element contains only one type of atom. 00:00:27.766 --> 00:00:37.226 A compound contains two or more elements chemically combined in fixed proportions; a mixture has substances together without chemical bonding between its components. 00:00:39.900 --> 00:00:45.914 A molecule contains covalently bonded atoms: O 2 is an element and C O 2 a compound. 00:00:46.064 --> 00:00:51.631 Not all compounds consist of separate molecules; sodium chloride has an ionic lattice. 00:00:54.300 --> 00:00:58.663 Mixtures such as air or salt water can have variable proportions. 00:00:58.813 --> 00:01:03.576 Their components can be separated using physical methods based on different properties. 00:01:06.233 --> 00:01:10.145 A pure substance has a sharp melting point under given conditions. 00:01:10.295 --> 00:01:15.073 Mixtures commonly melt over a range; impurities often lower the melting point. 00:01:15.223 --> 00:01:18.312 Compare measured data with a reliable reference. 00:01:20.967 --> 00:01:25.533 Pure substances have characteristic boiling points at a specified pressure. 00:01:25.683 --> 00:01:31.890 Mixtures often boil over a range; a single measurement alone cannot establish purity with certainty. 00:01:34.567 --> 00:01:37.997 A solute is dissolved in a solvent to form a solution. 00:01:38.147 --> 00:01:43.327 Water is a common solvent; the dissolved substance can be solid, liquid or gas. 00:01:46.000 --> 00:01:49.392 Dissolving distributes solute particles through the solvent. 00:01:49.542 --> 00:01:54.747 It is not necessarily a chemical reaction and does not always break molecules apart. 00:01:57.400 --> 00:02:01.339 Solubility depends on the substance, solvent and temperature. 00:02:01.489 --> 00:02:05.567 A substance insoluble in water might dissolve in another solvent. 00:02:08.233 --> 00:02:17.032 Choose filtration for an insoluble solid and liquid, crystallisation for a dissolved solid, and simple distillation to collect solvent from a solution. 00:02:19.700 --> 00:02:26.216 Use fractional distillation to separate liquids that mix together (miscible liquids) and have different boiling points. 00:02:26.366 --> 00:02:33.533 Use chromatography to separate dissolved substances that move at different rates through a stationary phase such as paper. 00:02:36.200 --> 00:02:43.205 To separate sand and salt, add water to dissolve salt, filter off sand, then crystallise salt from the filtrate. 00:02:43.355 --> 00:02:46.565 No single filtration can remove dissolved salt. 00:02:49.233 --> 00:02:58.331 In filtration, filter paper in a funnel retains insoluble solid as the residue; liquid and dissolved substances pass through as the filtrate. 00:02:58.481 --> 00:03:03.045 Ordinary filter paper retains insoluble solid, not dissolved salt. 00:03:07.700 --> 00:03:12.841 Particles of a dissolved solute are too small to be trapped by ordinary filter paper. 00:03:12.991 --> 00:03:15.966 Filtering salt water will not produce pure water. 00:03:18.633 --> 00:03:24.755 To obtain crystals, gently heat a solution to evaporate some solvent until it is near saturation. 00:03:24.905 --> 00:03:27.337 Then allow it to cool so crystals form. 00:03:30.000 --> 00:03:37.061 Filter the cooled crystals, wash with a little suitable cold solvent if appropriate, and dry between filter papers. 00:03:37.211 --> 00:03:39.856 Extensive washing can dissolve the product. 00:03:40.006 --> 00:03:43.659 Keep crystals; avoid heating hydrated salts to dryness. 00:03:48.333 --> 00:03:54.252 Do not boil every solution completely dry: this can cause spitting or damage hydrated crystals. 00:03:54.402 --> 00:03:57.419 Use suitable controlled heating and eye protection. 00:04:00.100 --> 00:04:04.272 Evaporation obtains a dissolved solid but loses the solvent. 00:04:04.422 --> 00:04:08.499 If the solvent is needed, collect it using distillation instead. 00:04:11.167 --> 00:04:15.401 In simple distillation, heat a solution so its solvent evaporates. 00:04:15.551 --> 00:04:19.749 Cool the vapour in a condenser and collect the liquid, called the distillate. 00:04:19.899 --> 00:04:24.506 Dissolved salts that do not evaporate (non-volatile salts) stay in the flask. 00:04:27.167 --> 00:04:30.801 A thermometer should measure vapour temperature near the side arm. 00:04:30.951 --> 00:04:36.848 Cooling water enters the lower condenser inlet and leaves the upper outlet, keeping the jacket full. 00:04:36.998 --> 00:04:42.628 Schematic: condenser water flows from lower inlet to upper outlet; system is not sealed. 00:04:47.300 --> 00:04:52.547 Keep the apparatus open to the atmosphere: never heat a sealed distillation system. 00:04:52.697 --> 00:04:57.390 Use an appropriate heat source; avoid naked flames with flammable solvents. 00:05:00.067 --> 00:05:03.298 Fractional distillation uses a fractionating column. 00:05:03.448 --> 00:05:10.721 As vapour rises, repeated evaporation and condensation mean that more of it is the liquid with the lower boiling point. 00:05:10.871 --> 00:05:12.791 This helps separate the liquids. 00:05:12.941 --> 00:05:16.033 A column improves separation of miscible liquids. 00:05:20.700 --> 00:05:24.813 Different liquid fractions are collected over suitable temperature ranges. 00:05:24.963 --> 00:05:29.945 Distillation changes state; it does not turn one substance chemically into another. 00:05:32.600 --> 00:05:39.531 The ink core practical uses distillation to recover solvent and chromatography to investigate dissolved dyes. 00:05:39.681 --> 00:05:42.911 The processes answer different questions about the mixture. 00:05:45.567 --> 00:05:51.389 Draw a pencil baseline near the bottom of the paper and add small spots of sample and known references. 00:05:51.539 --> 00:05:55.747 Pencil graphite does not dissolve into the solvent as ordinary ink may. 00:05:58.400 --> 00:06:02.661 Place the paper in a little solvent with the baseline above the solvent level. 00:06:02.811 --> 00:06:07.878 Cover the container to reduce evaporation and allow solvent to rise up the paper. 00:06:10.533 --> 00:06:14.195 The solvent moves up the paper, so it is the mobile phase. 00:06:14.345 --> 00:06:17.417 The stationary phase stays in place in the paper. 00:06:17.567 --> 00:06:24.147 Substances separate because they dissolve differently in the solvent and are attracted differently to the stationary phase. 00:06:26.800 --> 00:06:32.612 Remove the paper before solvent reaches the top and immediately mark the solvent front in pencil. 00:06:32.762 --> 00:06:36.002 The resulting separated pattern is a chromatogram. 00:06:38.667 --> 00:06:44.639 R F equals distance travelled by component divided by distance travelled by solvent front. 00:06:44.789 --> 00:06:49.536 Measure both from the baseline; component distance is to the centre of its spot. 00:06:49.686 --> 00:06:51.457 R F has no unit. 00:06:54.133 --> 00:07:02.347 If a spot moves 3 centimetres and the solvent front 6 centimetres, R F equals 3 divided by 6 equals 0.5. 00:07:02.497 --> 00:07:06.903 A valid spot on this chromatogram has R F between 0 and 1. 00:07:07.053 --> 00:07:13.235 Distances are measured from the baseline to spot centre and solvent front; schematic, not a measuring scale. 00:07:17.900 --> 00:07:20.183 Several spots indicate a mixture. 00:07:20.333 --> 00:07:28.204 One spot is consistent with purity under those conditions, but overlapping spots or insoluble components can limit the conclusion. 00:07:30.867 --> 00:07:35.109 Compare known and unknown spots using the same solvent and conditions. 00:07:35.259 --> 00:07:42.960 Matching positions or R F values support identity but are not absolute proof; changing conditions can change R F. 00:07:45.633 --> 00:07:49.615 Potable water is safe to drink; it need not be chemically pure. 00:07:49.765 --> 00:07:52.707 Dissolved minerals can remain in drinking water. 00:07:55.367 --> 00:08:05.339 Fresh water treatment can involve sedimentation to settle suspended solids, filtration through sand and gravel, and chlorination to kill harmful microorganisms. 00:08:05.489 --> 00:08:08.793 Potable water can still contain dissolved minerals. 00:08:13.467 --> 00:08:18.778 Sedimentation and ordinary filtration do not reliably remove dissolved salts. 00:08:18.928 --> 00:08:24.133 Disinfection targets microorganisms rather than removing all dissolved chemicals. 00:08:26.800 --> 00:08:34.255 Sea water can be desalinated by distillation: evaporate water, condense the vapour and collect water while salts remain. 00:08:34.405 --> 00:08:36.740 This requires substantial energy. 00:08:39.400 --> 00:08:46.812 Water for chemical analysis should not contain dissolved salts because they can contaminate samples or interfere with tests. 00:08:46.962 --> 00:08:50.364 Distilled or appropriately deionised water is used. 00:08:53.033 --> 00:09:01.203 Select treatment based on the contaminant: insoluble particles, microorganisms and dissolved salts require different approaches. 00:09:03.867 --> 00:09:07.599 That completes Methods of separating and purifying substances. 00:09:07.749 --> 00:09:11.094 Revisit the notes and test yourself on the revision website.