WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:08.765 Unit C C 2: Methods of separating and purifying substances. 00:00:11.433 --> 00:00:17.021 A chemically pure substance contains a single element or compound, with a fixed composition. 00:00:17.171 --> 00:00:22.175 Everyday “pure” can mean natural or without additives and need not mean chemically pure. 00:00:24.833 --> 00:00:27.649 An element contains only one type of atom. 00:00:27.799 --> 00:00:37.259 A compound contains two or more elements chemically combined in fixed proportions; a mixture has substances together without chemical bonding between its components. 00:00:39.933 --> 00:00:45.947 A molecule contains covalently bonded atoms: O 2 is an element and C O 2 a compound. 00:00:46.097 --> 00:00:51.665 Not all compounds consist of separate molecules; sodium chloride has an ionic lattice. 00:00:54.333 --> 00:00:58.696 Mixtures such as air or salt water can have variable proportions. 00:00:58.846 --> 00:01:03.609 Their components can be separated using physical methods based on different properties. 00:01:06.267 --> 00:01:10.178 A pure substance has a sharp melting point under given conditions. 00:01:10.328 --> 00:01:15.106 Mixtures commonly melt over a range; impurities often lower the melting point. 00:01:15.256 --> 00:01:18.345 Compare measured data with a reliable reference. 00:01:21.000 --> 00:01:25.566 Pure substances have characteristic boiling points at a specified pressure. 00:01:25.716 --> 00:01:31.923 Mixtures often boil over a range; a single measurement alone cannot establish purity with certainty. 00:01:34.600 --> 00:01:38.031 A solute is dissolved in a solvent to form a solution. 00:01:38.181 --> 00:01:43.360 Water is a common solvent; the dissolved substance can be solid, liquid or gas. 00:01:46.033 --> 00:01:49.425 Dissolving distributes solute particles through the solvent. 00:01:49.575 --> 00:01:54.780 It is not necessarily a chemical reaction and does not always break molecules apart. 00:01:57.433 --> 00:02:01.372 Solubility depends on the substance, solvent and temperature. 00:02:01.522 --> 00:02:05.600 A substance insoluble in water might dissolve in another solvent. 00:02:08.267 --> 00:02:17.065 Choose filtration for an insoluble solid and liquid, crystallisation for a dissolved solid, and simple distillation to collect solvent from a solution. 00:02:19.733 --> 00:02:26.249 Use fractional distillation to separate liquids that mix together (miscible liquids) and have different boiling points. 00:02:26.399 --> 00:02:33.567 Use chromatography to separate dissolved substances that move at different rates through a stationary phase such as paper. 00:02:36.233 --> 00:02:43.239 To separate sand and salt, add water to dissolve salt, filter off sand, then crystallise salt from the filtrate. 00:02:43.389 --> 00:02:46.599 No single filtration can remove dissolved salt. 00:02:49.267 --> 00:02:58.364 In filtration, filter paper in a funnel retains insoluble solid as the residue; liquid and dissolved substances pass through as the filtrate. 00:02:58.514 --> 00:03:03.079 Ordinary filter paper retains insoluble solid, not dissolved salt. 00:03:07.733 --> 00:03:12.874 Particles of a dissolved solute are too small to be trapped by ordinary filter paper. 00:03:13.024 --> 00:03:15.999 Filtering salt water will not produce pure water. 00:03:18.667 --> 00:03:24.789 To obtain crystals, gently heat a solution to evaporate some solvent until it is near saturation. 00:03:24.939 --> 00:03:27.371 Then allow it to cool so crystals form. 00:03:30.033 --> 00:03:37.094 Filter the cooled crystals, wash with a little suitable cold solvent if appropriate, and dry between filter papers. 00:03:37.244 --> 00:03:39.889 Extensive washing can dissolve the product. 00:03:40.039 --> 00:03:43.692 Keep crystals; avoid heating hydrated salts to dryness. 00:03:48.367 --> 00:03:54.285 Do not boil every solution completely dry: this can cause spitting or damage hydrated crystals. 00:03:54.435 --> 00:03:57.452 Use suitable controlled heating and eye protection. 00:04:00.133 --> 00:04:04.305 Evaporation obtains a dissolved solid but loses the solvent. 00:04:04.455 --> 00:04:08.532 If the solvent is needed, collect it using distillation instead. 00:04:11.200 --> 00:04:15.435 In simple distillation, heat a solution so its solvent evaporates. 00:04:15.585 --> 00:04:19.782 Cool the vapour in a condenser and collect the liquid, called the distillate. 00:04:19.932 --> 00:04:24.539 Dissolved salts that do not evaporate (non-volatile salts) stay in the flask. 00:04:27.200 --> 00:04:30.834 A thermometer should measure vapour temperature near the side arm. 00:04:30.984 --> 00:04:36.881 Cooling water enters the lower condenser inlet and leaves the upper outlet, keeping the jacket full. 00:04:37.031 --> 00:04:42.661 Schematic: condenser water flows from lower inlet to upper outlet; system is not sealed. 00:04:47.333 --> 00:04:52.581 Keep the apparatus open to the atmosphere: never heat a sealed distillation system. 00:04:52.731 --> 00:04:57.423 Use an appropriate heat source; avoid naked flames with flammable solvents. 00:05:00.100 --> 00:05:03.331 Fractional distillation uses a fractionating column. 00:05:03.481 --> 00:05:10.754 As vapour rises, repeated evaporation and condensation mean that more of it is the liquid with the lower boiling point. 00:05:10.904 --> 00:05:12.824 This helps separate the liquids. 00:05:12.974 --> 00:05:16.066 A column improves separation of miscible liquids. 00:05:20.733 --> 00:05:24.847 Different liquid fractions are collected over suitable temperature ranges. 00:05:24.997 --> 00:05:29.978 Distillation changes state; it does not turn one substance chemically into another. 00:05:32.633 --> 00:05:39.565 The ink core practical uses distillation to recover solvent and chromatography to investigate dissolved dyes. 00:05:39.715 --> 00:05:42.945 The processes answer different questions about the mixture. 00:05:45.600 --> 00:05:51.422 Draw a pencil baseline near the bottom of the paper and add small spots of sample and known references. 00:05:51.572 --> 00:05:55.781 Pencil graphite does not dissolve into the solvent as ordinary ink may. 00:05:58.433 --> 00:06:02.695 Place the paper in a little solvent with the baseline above the solvent level. 00:06:02.845 --> 00:06:07.911 Cover the container to reduce evaporation and allow solvent to rise up the paper. 00:06:10.567 --> 00:06:14.228 The solvent moves up the paper, so it is the mobile phase. 00:06:14.378 --> 00:06:17.450 The stationary phase stays in place in the paper. 00:06:17.600 --> 00:06:24.180 Substances separate because they dissolve differently in the solvent and are attracted differently to the stationary phase. 00:06:26.833 --> 00:06:32.645 Remove the paper before solvent reaches the top and immediately mark the solvent front in pencil. 00:06:32.795 --> 00:06:36.035 The resulting separated pattern is a chromatogram. 00:06:38.700 --> 00:06:44.673 R F equals distance travelled by component divided by distance travelled by solvent front. 00:06:44.823 --> 00:06:49.569 Measure both from the baseline; component distance is to the centre of its spot. 00:06:49.719 --> 00:06:51.490 R F has no unit. 00:06:54.167 --> 00:07:02.380 If a spot moves 3 centimetres and the solvent front 6 centimetres, R F equals 3 divided by 6 equals 0.5. 00:07:02.530 --> 00:07:06.937 A valid spot on this chromatogram has R F between 0 and 1. 00:07:07.087 --> 00:07:13.269 Distances are measured from the baseline to spot centre and solvent front; schematic, not a measuring scale. 00:07:17.933 --> 00:07:20.216 Several spots indicate a mixture. 00:07:20.366 --> 00:07:28.237 One spot is consistent with purity under those conditions, but overlapping spots or insoluble components can limit the conclusion. 00:07:30.900 --> 00:07:35.143 Compare known and unknown spots using the same solvent and conditions. 00:07:35.293 --> 00:07:42.993 Matching positions or R F values support identity but are not absolute proof; changing conditions can change R F. 00:07:45.667 --> 00:07:49.648 Potable water is safe to drink; it need not be chemically pure. 00:07:49.798 --> 00:07:52.741 Dissolved minerals can remain in drinking water. 00:07:55.400 --> 00:08:05.372 Fresh water treatment can involve sedimentation to settle suspended solids, filtration through sand and gravel, and chlorination to kill harmful microorganisms. 00:08:05.522 --> 00:08:08.826 Potable water can still contain dissolved minerals. 00:08:13.500 --> 00:08:18.811 Sedimentation and ordinary filtration do not reliably remove dissolved salts. 00:08:18.961 --> 00:08:24.166 Disinfection targets microorganisms rather than removing all dissolved chemicals. 00:08:26.833 --> 00:08:34.288 Sea water can be desalinated by distillation: evaporate water, condense the vapour and collect water while salts remain. 00:08:34.438 --> 00:08:36.773 This requires substantial energy. 00:08:39.433 --> 00:08:46.845 Water for chemical analysis should not contain dissolved salts because they can contaminate samples or interfere with tests. 00:08:46.995 --> 00:08:50.397 Distilled or appropriately deionised water is used. 00:08:53.067 --> 00:09:01.237 Select treatment based on the contaminant: insoluble particles, microorganisms and dissolved salts require different approaches. 00:09:03.900 --> 00:09:07.632 That completes Methods of separating and purifying substances. 00:09:07.782 --> 00:09:11.127 Revisit the notes and test yourself on the revision website.