WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.073 Unit S C 8: Acids and alkalis. 00:00:09.733 --> 00:00:15.843 Acids dissolved in water (in aqueous solution) provide hydrogen ions, H, charge plus,. 00:00:15.993 --> 00:00:21.874 Hydrochloric acid produces H, charge plus, and chloride ions, C L, charge minus,. 00:00:22.024 --> 00:00:25.752 Chloride ions are different from chlorine atoms or chlorine gas. 00:00:28.433 --> 00:00:33.759 Alkalis provide hydroxide ions, O H, charge minus, in aqueous solution. 00:00:33.909 --> 00:00:39.448 Sodium hydroxide solution contains N A, charge plus, and O H, charge minus,. 00:00:42.100 --> 00:00:49.107 A base neutralises an acid to produce salt and water; metal oxides and metal hydroxides are common examples. 00:00:49.257 --> 00:00:52.094 An alkali is a base that dissolves in water. 00:00:54.767 --> 00:00:58.617 All alkalis are bases, but not all bases are alkalis. 00:00:58.767 --> 00:01:04.565 Copper oxide is an insoluble base; sodium hydroxide is a soluble base, an alkali. 00:01:07.233 --> 00:01:16.223 For ordinary GCSE aqueous solutions, acidic means P H below 7, neutral P H 7, and alkaline P H above 7. 00:01:16.373 --> 00:01:21.744 The familiar 0, 14 scale is useful but not an absolute limit for every solution. 00:01:24.400 --> 00:01:33.753 Wear eye protection and use the specified dilute reagents in school practicals; hazard and risk depend on both the substance and its concentration. 00:01:36.433 --> 00:01:44.260 Universal indicator gives an approximate P H by comparison with a colour chart: red and orange and yellow for acidic solutions, 00:01:44.410 --> 00:01:45.925 green near neutral, 00:01:46.075 --> 00:01:48.635 blue and purple for alkaline solutions. 00:01:51.300 --> 00:01:54.371 Litmus is red in acid and blue in alkali. 00:01:54.521 --> 00:01:59.788 It indicates acidity or alkalinity rather than providing an exact P H. 00:02:02.467 --> 00:02:10.645 Methyl orange is red in sufficiently acidic solution and yellow in neutral or alkaline solution, with orange in its transition range. 00:02:13.300 --> 00:02:19.475 Phenolphthalein is colourless in acid and neutral solution and pink in sufficiently alkaline solution. 00:02:19.625 --> 00:02:23.003 Use a suitable indicator for the expected endpoint. 00:02:23.153 --> 00:02:28.794 Colours in suitably acidic and alkaline solutions; indicator transition ranges differ. 00:02:33.467 --> 00:02:39.172 A calibrated P H probe can measure P H more precisely than matching an indicator colour. 00:02:39.322 --> 00:02:42.454 Rinse it between samples to reduce contamination. 00:02:45.133 --> 00:02:48.661 Lower P H means higher hydrogen-ion concentration. 00:02:48.811 --> 00:02:57.146 A decrease of one P H unit corresponds to ten times the H, charge plus, concentration; a decrease of two means a hundred times. 00:02:59.800 --> 00:03:05.901 A strong acid ionises almost completely in water: almost all its acid molecules form ions. 00:03:06.051 --> 00:03:08.675 A weak acid ionises only partly. 00:03:08.825 --> 00:03:11.639 Its molecules and ions are in equilibrium. 00:03:11.789 --> 00:03:15.275 This splitting into ions is also called dissociation. 00:03:17.933 --> 00:03:25.015 Concentrated means a large amount of dissolved acid per volume of solution; dilute means a smaller amount per volume. 00:03:25.165 --> 00:03:27.041 This is different from strength. 00:03:29.700 --> 00:03:34.203 A dilute strong acid and a concentrated weak acid are both possible. 00:03:34.353 --> 00:03:37.413 Do not use “strong” simply to mean “lots of acid”. 00:03:40.067 --> 00:03:47.585 At the same concentration, a strong acid generally supplies more hydrogen ions and has lower P H than a weak acid. 00:03:47.735 --> 00:03:51.523 Comparisons at different concentrations need more information. 00:03:54.200 --> 00:04:00.599 Diluting an acidic solution lowers H, charge plus, concentration and moves P H towards 7. 00:04:00.749 --> 00:04:03.853 It does not change a strong acid into a weak acid. 00:04:06.533 --> 00:04:14.688 For example, P H 2 has ten times the H, charge plus, concentration of P H 3, and a hundred times that of P H 4. 00:04:14.838 --> 00:04:19.417 Describe concentration rather than saying it is “a hundred times stronger”. 00:04:19.567 --> 00:04:24.329 Each unit decrease gives tenfold greater H, charge plus, concentration. 00:04:29.000 --> 00:04:34.397 A sufficiently reactive metal with a suitable dilute acid gives salt plus hydrogen. 00:04:34.547 --> 00:04:40.273 For magnesium: M G plus 2 H C L produces M G C L 2 plus H 2. 00:04:40.423 --> 00:04:43.558 Water is not an additional product in this equation. 00:04:46.233 --> 00:04:49.697 Metal oxide plus acid produces salt plus water. 00:04:49.847 --> 00:04:55.225 Example: C U O plus H 2 S O 4 produces C U S O 4 plus H 2 O. 00:04:57.900 --> 00:05:01.641 Metal hydroxide plus acid produces salt plus water. 00:05:01.791 --> 00:05:06.777 Example: N A O H plus H C L produces N A C L plus H 2 O. 00:05:09.433 --> 00:05:14.681 Metal carbonate plus acid produces salt plus water plus carbon dioxide. 00:05:14.831 --> 00:05:21.240 Example: C A C O 3 plus 2 H C L produces C A C L 2 plus H 2 O plus C O 2. 00:05:23.900 --> 00:05:29.516 Hydrochloric acid makes chlorides, sulfuric acid sulfates, and nitric acid nitrates. 00:05:29.666 --> 00:05:33.945 The metal or ammonium ion supplies the first part of the salt name. 00:05:36.600 --> 00:05:39.672 Hydrogen makes a squeaky pop with a lighted splint. 00:05:39.822 --> 00:05:42.702 Carbon dioxide turns limewater cloudy. 00:05:42.852 --> 00:05:46.792 Use small gas samples and the teacher's safe practical procedure. 00:05:49.467 --> 00:05:59.257 The ionic equation for acid, alkali neutralisation is H, charge plus, aqueous plus O H, charge minus, aqueous produces H 2 O liquid. 00:05:59.407 --> 00:06:05.175 Ions such as N A, charge plus, and C L, charge minus, do not change during the reaction. 00:06:05.325 --> 00:06:08.916 These spectator ions are left out of the ionic equation. 00:06:09.066 --> 00:06:13.578 The water-forming ionic equation is independent of spectator ions. 00:06:18.233 --> 00:06:27.447 To make a soluble salt using an insoluble base, warm dilute acid gently and add the base in small portions until some remains unreacted. 00:06:27.597 --> 00:06:30.264 Excess base ensures the acid is used up. 00:06:32.933 --> 00:06:38.511 For hydrated copper sulfate crystals, react copper oxide with dilute sulfuric acid. 00:06:38.661 --> 00:06:41.935 Stir while adding the black solid until an excess remains. 00:06:44.600 --> 00:06:50.870 Filter the mixture to remove excess copper oxide as residue; copper sulfate solution is the filtrate. 00:06:51.020 --> 00:06:54.387 Do not use the solid residue as the desired salt. 00:06:57.067 --> 00:07:02.389 Concentrate the filtrate gently using a water bath, then leave it to cool and crystallise. 00:07:02.539 --> 00:07:07.808 Avoid heating to dryness, which can remove water of crystallisation or cause spitting. 00:07:10.467 --> 00:07:16.011 Filter the crystals, wash with a little cold distilled water and dry between filter papers. 00:07:16.161 --> 00:07:18.945 Hydrated copper sulfate crystals are blue. 00:07:19.095 --> 00:07:24.309 They contain water molecules in their crystal structure, called water of crystallisation. 00:07:24.459 --> 00:07:29.526 Keep the blue hydrated crystals; use a water bath and avoid heating to dryness. 00:07:34.200 --> 00:07:39.383 An insoluble excess base can be filtered off; a dissolved excess alkali cannot. 00:07:39.533 --> 00:07:44.994 A soluble acid, alkali salt preparation therefore needs measured reacting volumes. 00:07:47.667 --> 00:07:52.722 Use a pipette and filler to transfer a measured alkali volume to a conical flask. 00:07:52.872 --> 00:07:57.064 Add a few drops of a suitable indicator; never pipette by mouth. 00:07:59.733 --> 00:08:05.002 Fill a burette with acid, ensure its tip is full and read the initial level at eye height. 00:08:05.152 --> 00:08:09.738 Swirl the flask while adding acid, then add dropwise near the colour change. 00:08:09.888 --> 00:08:14.179 Read burette at eye level; swirl and add dropwise near the endpoint. 00:08:18.833 --> 00:08:23.964 The titre is the volume delivered from the burette: final reading minus initial reading. 00:08:24.114 --> 00:08:27.260 Repeat until the titres are close together (concordant). 00:08:27.410 --> 00:08:30.768 Follow the agreement required by your course or the question. 00:08:33.433 --> 00:08:38.735 The endpoint is the indicator colour change indicating suitable reacting proportions. 00:08:38.885 --> 00:08:43.737 It is not automatically P H 7 for every acid, base combination. 00:08:46.400 --> 00:08:51.680 To obtain pure salt, repeat with the measured reacting volumes but without indicator. 00:08:51.830 --> 00:08:55.789 Then gently concentrate, cool, crystallise and dry the salt. 00:08:58.467 --> 00:09:05.270 Do not add excess soluble reactant and attempt to filter it out: it remains dissolved and contaminates the salt. 00:09:05.420 --> 00:09:08.607 Titration avoids excess acid or alkali. 00:09:11.267 --> 00:09:19.411 All common sodium, potassium and ammonium salts, and all nitrates, are soluble in water under the GCSE solubility rules. 00:09:22.067 --> 00:09:26.333 Common chlorides are soluble except silver chloride and lead chloride. 00:09:26.483 --> 00:09:31.090 Common sulfates are soluble except lead, barium and calcium sulfates. 00:09:33.767 --> 00:09:39.900 Common carbonates and hydroxides are insoluble except those of sodium, potassium and ammonium. 00:09:40.050 --> 00:09:45.148 Calcium hydroxide is slightly soluble; use the specified rules for the question. 00:09:47.800 --> 00:09:52.663 A precipitate is an insoluble solid formed when suitable solutions react. 00:09:52.813 --> 00:09:56.816 Predict possible ion combinations and check their solubility. 00:09:59.467 --> 00:10:11.029 Silver nitrate plus sodium chloride solutions form silver chloride precipitate: A G, charge plus, aqueous plus C L, charge minus, aqueous produces A G C L solid. 00:10:11.179 --> 00:10:14.097 Sodium and nitrate ions remain dissolved. 00:10:16.767 --> 00:10:26.447 To obtain pure dry insoluble salt, mix suitable solutions, filter off the precipitate, wash it with distilled water to remove dissolved salts, and dry it. 00:10:26.597 --> 00:10:31.951 Here the desired salt is the filter residue, unlike the copper sulfate filtrate method. 00:10:36.633 --> 00:10:41.571 Measure a fixed volume of dilute hydrochloric acid and its initial P H. 00:10:41.721 --> 00:10:47.725 Add small measured portions of powdered calcium hydroxide or calcium oxide, stirring each time. 00:10:50.400 --> 00:10:56.276 Measure P H after each addition with a suitable probe or indicator and record the added amount. 00:10:56.426 --> 00:11:01.961 P H rises as the acid is neutralised; excess base can make the mixture alkaline. 00:11:04.633 --> 00:11:10.370 Control acid volume and concentration, powder choice and mixing method when comparing runs. 00:11:10.520 --> 00:11:13.110 Avoid splashing and wear eye protection. 00:11:15.767 --> 00:11:19.451 Plot P H against amount added and interpret the trend. 00:11:19.601 --> 00:11:27.329 A P H rise reflects falling H, charge plus, concentration, not the acid being converted into a weaker acid type. 00:11:30.000 --> 00:11:32.303 That completes Acids and alkalis. 00:11:32.453 --> 00:11:35.798 Revisit the notes and test yourself on the revision website.