WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.137 Unit C C 8: Acids and alkalis. 00:00:09.800 --> 00:00:15.910 Acids dissolved in water (in aqueous solution) provide hydrogen ions, H, charge plus,. 00:00:16.060 --> 00:00:21.941 Hydrochloric acid produces H, charge plus, and chloride ions, C L, charge minus,. 00:00:22.091 --> 00:00:25.819 Chloride ions are different from chlorine atoms or chlorine gas. 00:00:28.500 --> 00:00:33.826 Alkalis provide hydroxide ions, O H, charge minus, in aqueous solution. 00:00:33.976 --> 00:00:39.515 Sodium hydroxide solution contains N A, charge plus, and O H, charge minus,. 00:00:42.167 --> 00:00:49.173 A base neutralises an acid to produce salt and water; metal oxides and metal hydroxides are common examples. 00:00:49.323 --> 00:00:52.161 An alkali is a base that dissolves in water. 00:00:54.833 --> 00:00:58.684 All alkalis are bases, but not all bases are alkalis. 00:00:58.834 --> 00:01:04.632 Copper oxide is an insoluble base; sodium hydroxide is a soluble base, an alkali. 00:01:07.300 --> 00:01:16.289 For ordinary GCSE aqueous solutions, acidic means P H below 7, neutral P H 7, and alkaline P H above 7. 00:01:16.439 --> 00:01:21.811 The familiar 0, 14 scale is useful but not an absolute limit for every solution. 00:01:24.467 --> 00:01:33.820 Wear eye protection and use the specified dilute reagents in school practicals; hazard and risk depend on both the substance and its concentration. 00:01:36.500 --> 00:01:44.327 Universal indicator gives an approximate P H by comparison with a colour chart: red and orange and yellow for acidic solutions, 00:01:44.477 --> 00:01:45.991 green near neutral, 00:01:46.141 --> 00:01:48.701 blue and purple for alkaline solutions. 00:01:51.367 --> 00:01:54.437 Litmus is red in acid and blue in alkali. 00:01:54.587 --> 00:01:59.855 It indicates acidity or alkalinity rather than providing an exact P H. 00:02:02.533 --> 00:02:10.712 Methyl orange is red in sufficiently acidic solution and yellow in neutral or alkaline solution, with orange in its transition range. 00:02:13.367 --> 00:02:19.541 Phenolphthalein is colourless in acid and neutral solution and pink in sufficiently alkaline solution. 00:02:19.691 --> 00:02:23.069 Use a suitable indicator for the expected endpoint. 00:02:23.219 --> 00:02:28.861 Colours in suitably acidic and alkaline solutions; indicator transition ranges differ. 00:02:33.533 --> 00:02:39.239 A calibrated P H probe can measure P H more precisely than matching an indicator colour. 00:02:39.389 --> 00:02:42.521 Rinse it between samples to reduce contamination. 00:02:45.200 --> 00:02:48.727 Lower P H means higher hydrogen-ion concentration. 00:02:48.877 --> 00:02:57.213 A decrease of one P H unit corresponds to ten times the H, charge plus, concentration; a decrease of two means a hundred times. 00:02:59.867 --> 00:03:05.967 A strong acid ionises almost completely in water: almost all its acid molecules form ions. 00:03:06.117 --> 00:03:08.741 A weak acid ionises only partly. 00:03:08.891 --> 00:03:11.706 Its molecules and ions are in equilibrium. 00:03:11.856 --> 00:03:15.342 This splitting into ions is also called dissociation. 00:03:18.000 --> 00:03:25.082 Concentrated means a large amount of dissolved acid per volume of solution; dilute means a smaller amount per volume. 00:03:25.232 --> 00:03:27.108 This is different from strength. 00:03:29.767 --> 00:03:34.270 A dilute strong acid and a concentrated weak acid are both possible. 00:03:34.420 --> 00:03:37.479 Do not use “strong” simply to mean “lots of acid”. 00:03:40.133 --> 00:03:47.652 At the same concentration, a strong acid generally supplies more hydrogen ions and has lower P H than a weak acid. 00:03:47.802 --> 00:03:51.590 Comparisons at different concentrations need more information. 00:03:54.267 --> 00:04:00.666 Diluting an acidic solution lowers H, charge plus, concentration and moves P H towards 7. 00:04:00.816 --> 00:04:03.919 It does not change a strong acid into a weak acid. 00:04:06.600 --> 00:04:14.755 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. 00:04:14.905 --> 00:04:19.483 Describe concentration rather than saying it is “a hundred times stronger”. 00:04:19.633 --> 00:04:24.396 Each unit decrease gives tenfold greater H, charge plus, concentration. 00:04:29.067 --> 00:04:34.463 A sufficiently reactive metal with a suitable dilute acid gives salt plus hydrogen. 00:04:34.613 --> 00:04:40.340 For magnesium: M G plus 2 H C L produces M G C L 2 plus H 2. 00:04:40.490 --> 00:04:43.625 Water is not an additional product in this equation. 00:04:46.300 --> 00:04:49.763 Metal oxide plus acid produces salt plus water. 00:04:49.913 --> 00:04:55.291 Example: C U O plus H 2 S O 4 produces C U S O 4 plus H 2 O. 00:04:57.967 --> 00:05:01.707 Metal hydroxide plus acid produces salt plus water. 00:05:01.857 --> 00:05:06.844 Example: N A O H plus H C L produces N A C L plus H 2 O. 00:05:09.500 --> 00:05:14.747 Metal carbonate plus acid produces salt plus water plus carbon dioxide. 00:05:14.897 --> 00:05:21.307 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. 00:05:23.967 --> 00:05:29.583 Hydrochloric acid makes chlorides, sulfuric acid sulfates, and nitric acid nitrates. 00:05:29.733 --> 00:05:34.011 The metal or ammonium ion supplies the first part of the salt name. 00:05:36.667 --> 00:05:39.739 Hydrogen makes a squeaky pop with a lighted splint. 00:05:39.889 --> 00:05:42.769 Carbon dioxide turns limewater cloudy. 00:05:42.919 --> 00:05:46.859 Use small gas samples and the teacher's safe practical procedure. 00:05:49.533 --> 00:05:59.323 The ionic equation for acid, alkali neutralisation is H, charge plus, aqueous plus O H, charge minus, aqueous produces H 2 O liquid. 00:05:59.473 --> 00:06:05.241 Ions such as N A, charge plus, and C L, charge minus, do not change during the reaction. 00:06:05.391 --> 00:06:08.983 These spectator ions are left out of the ionic equation. 00:06:09.133 --> 00:06:13.645 The water-forming ionic equation is independent of spectator ions. 00:06:18.300 --> 00:06:27.514 To make a soluble salt using an insoluble base, warm dilute acid gently and add the base in small portions until some remains unreacted. 00:06:27.664 --> 00:06:30.331 Excess base ensures the acid is used up. 00:06:33.000 --> 00:06:38.577 For hydrated copper sulfate crystals, react copper oxide with dilute sulfuric acid. 00:06:38.727 --> 00:06:42.001 Stir while adding the black solid until an excess remains. 00:06:44.667 --> 00:06:50.937 Filter the mixture to remove excess copper oxide as residue; copper sulfate solution is the filtrate. 00:06:51.087 --> 00:06:54.453 Do not use the solid residue as the desired salt. 00:06:57.133 --> 00:07:02.456 Concentrate the filtrate gently using a water bath, then leave it to cool and crystallise. 00:07:02.606 --> 00:07:07.875 Avoid heating to dryness, which can remove water of crystallisation or cause spitting. 00:07:10.533 --> 00:07:16.078 Filter the crystals, wash with a little cold distilled water and dry between filter papers. 00:07:16.228 --> 00:07:19.011 Hydrated copper sulfate crystals are blue. 00:07:19.161 --> 00:07:24.376 They contain water molecules in their crystal structure, called water of crystallisation. 00:07:24.526 --> 00:07:29.593 Keep the blue hydrated crystals; use a water bath and avoid heating to dryness. 00:07:34.267 --> 00:07:39.450 An insoluble excess base can be filtered off; a dissolved excess alkali cannot. 00:07:39.600 --> 00:07:45.061 A soluble acid, alkali salt preparation therefore needs measured reacting volumes. 00:07:47.733 --> 00:07:52.789 Use a pipette and filler to transfer a measured alkali volume to a conical flask. 00:07:52.939 --> 00:07:57.131 Add a few drops of a suitable indicator; never pipette by mouth. 00:07:59.800 --> 00:08:05.069 Fill a burette with acid, ensure its tip is full and read the initial level at eye height. 00:08:05.219 --> 00:08:09.805 Swirl the flask while adding acid, then add dropwise near the colour change. 00:08:09.955 --> 00:08:14.246 Read burette at eye level; swirl and add dropwise near the endpoint. 00:08:18.900 --> 00:08:24.031 The titre is the volume delivered from the burette: final reading minus initial reading. 00:08:24.181 --> 00:08:27.327 Repeat until the titres are close together (concordant). 00:08:27.477 --> 00:08:30.835 Follow the agreement required by your course or the question. 00:08:33.500 --> 00:08:38.801 The endpoint is the indicator colour change indicating suitable reacting proportions. 00:08:38.951 --> 00:08:43.803 It is not automatically P H 7 for every acid, base combination. 00:08:46.467 --> 00:08:51.747 To obtain pure salt, repeat with the measured reacting volumes but without indicator. 00:08:51.897 --> 00:08:55.855 Then gently concentrate, cool, crystallise and dry the salt. 00:08:58.533 --> 00:09:05.337 Do not add excess soluble reactant and attempt to filter it out: it remains dissolved and contaminates the salt. 00:09:05.487 --> 00:09:08.674 Titration avoids excess acid or alkali. 00:09:11.333 --> 00:09:19.478 All common sodium, potassium and ammonium salts, and all nitrates, are soluble in water under the GCSE solubility rules. 00:09:22.133 --> 00:09:26.399 Common chlorides are soluble except silver chloride and lead chloride. 00:09:26.549 --> 00:09:31.157 Common sulfates are soluble except lead, barium and calcium sulfates. 00:09:33.833 --> 00:09:39.967 Common carbonates and hydroxides are insoluble except those of sodium, potassium and ammonium. 00:09:40.117 --> 00:09:45.215 Calcium hydroxide is slightly soluble; use the specified rules for the question. 00:09:47.867 --> 00:09:52.730 A precipitate is an insoluble solid formed when suitable solutions react. 00:09:52.880 --> 00:09:56.883 Predict possible ion combinations and check their solubility. 00:09:59.533 --> 00:10:11.095 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. 00:10:11.245 --> 00:10:14.164 Sodium and nitrate ions remain dissolved. 00:10:16.833 --> 00:10:26.514 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. 00:10:26.664 --> 00:10:32.018 Here the desired salt is the filter residue, unlike the copper sulfate filtrate method. 00:10:36.700 --> 00:10:41.637 Measure a fixed volume of dilute hydrochloric acid and its initial P H. 00:10:41.787 --> 00:10:47.791 Add small measured portions of powdered calcium hydroxide or calcium oxide, stirring each time. 00:10:50.467 --> 00:10:56.343 Measure P H after each addition with a suitable probe or indicator and record the added amount. 00:10:56.493 --> 00:11:02.027 P H rises as the acid is neutralised; excess base can make the mixture alkaline. 00:11:04.700 --> 00:11:10.437 Control acid volume and concentration, powder choice and mixing method when comparing runs. 00:11:10.587 --> 00:11:13.177 Avoid splashing and wear eye protection. 00:11:15.833 --> 00:11:19.517 Plot P H against amount added and interpret the trend. 00:11:19.667 --> 00:11:27.395 A P H rise reflects falling H, charge plus, concentration, not the acid being converted into a weaker acid type. 00:11:30.067 --> 00:11:32.369 That completes Acids and alkalis. 00:11:32.519 --> 00:11:35.865 Revisit the notes and test yourself on the revision website.