Reactions between acids and alkalis

UNDERSTANDING YOUR FOOD<br /> the ultimate educational resource<br /> TEACHER NOTES<br /> <br /> KS4<br /> science<br /> - some prior knowledge about acids<br /> and alkalis is desirable.<br /> Timing - 2 or 3 sessions of 40 - 50<br /> minutes to cover all the activities<br /> Two pupil activity sheets A1<br /> accompany this activity<br /> Requirements<br /> Task 1 (per group)<br /> • 30 g citric acid powder<br /> • 30 g sodium<br /> hydrogencarbonate powder<br /> • 2x50 cm<br /> 3water in small beakers<br /> • stirring rods (or access to<br /> magnetic stirrer)<br /> Task 2<br /> • equal concentrations (4.0 g in<br /> 100 cm<br /> 3) of citric acid solution &<br /> sodium hydrogencarbonate<br /> solution<br /> • Universal indicator & chart<br /> • measuring cylinders<br /> • small beakers<br /> • apparatus for testing for gases<br /> such as carbon dioxide and<br /> oxygen<br /> Task 1<br /> • Universal indicator & chart<br /> • samples of water, cola, fizzy<br /> lemonade, blackcurrant juice,<br /> orange squash, milk, freshly<br /> squeezed juice from<br /> oranges/lemons/other fruits<br /> Task 2<br /> • 250 cm<br /> 3conical flasks<br /> • 10 cm<br /> 3measuring<br /> cylinders/pipettes<br /> • distilled water<br /> • phenolphthalein indicator solution<br /> • burettes<br /> • funnels to assist filling of burette<br /> • various small beakers<br /> • 0.1M sodium hydroxide solution<br /> • Solution 1: citric acid solution of<br /> 7 g/dm<br /> 3<br /> samples of fizzy drinks; Tango,<br /> lemonade, 7Up and limeade all<br /> work well; coloured solutions can<br /> be used<br /> teachers’ notes A1.1<br /> A1. Some reactions between acids and alkalis<br /> Looking at sodium hydrogencarbonate and citric acid<br /> Pupils investigate solubility, acidity and neutralisation.<br /> Task 1<br /> Citric acid is far more soluble than sodium hydrogencarbonate. It is likely that all of<br /> the citric acid will dissolve in the water. About 6g of sodium hydrogencarbonate can<br /> be expected to dissolve.<br /> Task 2<br /> Citric acid has a pHof 7.<br /> When the two are mixed effervescence occurs. Pupils may suggest that this could<br /> be because a gas/carbon dioxide (because a carbonate has been used) is being<br /> produced. This can be tested for using limewater which goes from clear to cloudy.<br /> By mixing the volumes suggested on the pupil sheet, it should be possible to see<br /> that neutralisation occurs when acids are added to alkalis.<br /> The equation for this reaction is:<br /> citric acid + sodium hydrogencarbonate carbon dioxide + sodium citrate + water<br /> How much acid is present in drinks?<br /> Pupils investigate acidity and carry out a titration.<br /> Task 1<br /> All of the suggested foods are acidic to some extent except for water. The<br /> inclusion of the juice from fresh fruit is to show that many natural foods are highly<br /> acidic. Fresh milk may not give an acidic pH; but with time the production of lactic<br /> acid by bacteria causes milk to ‘sour’ resulting in a solution with pH <7.<br /> Task 2<br /> Pupils carry out a titration to determine accurately the concentration of an acid in a<br /> fizzy drink.<br /> SAFTEY NOTE: DO NOT ALLOW PIPETTING BY MOUTH<br /> Hints for the teacher:<br /> • Some of the soft drinks which the students will wish to test contain a range<br /> of acidulants, companies such as Britvic will often quote on labels total<br /> acidity, as an expression of citric acid.<br /> • Make sure the drinks supplied do contain mainly citric acid rather than other<br /> acids such as malic acid, which is found in apple drinks.<br /> • The correct use of burettes is quite a difficult skill and pupils will need<br /> guidance if this is the first time that they carry out a titration. If this is the<br /> case they should be allowed to practice using the equipment. How to read<br /> the burette correctly with respect to the meniscus and the scale must also<br /> be explained.<br /> • The end point is taken when the pink colour just remains in the solution.<br /> The standard citric acid solution should take between 7 and 8 cm3to be<br /> neutralised by the sodium hydroxide. Other results will depend on the drinks<br /> tested.<br /> <br /> SOME REACTIONS BETWEEN ACIDS AND ALKALIS pupil activity A1<br /> The reactions between acids and alkalis are very important reactions in everyday life<br /> as well as in both food technology and science lessons.<br /> acid + alkali a salt + water<br /> Sometimes other substances are produced depending on the nature of the acids<br /> and alkalis which take part in the reaction.<br /> Looking at sodium hydrogencarbonate and citric acid<br /> Sodium hydrogencarbonate (an alkali) and citric acid are often found in food<br /> products.<br /> Task 1<br /> You are given 30 g each of citric acid and sodium hydrogencarbonate. You are<br /> given 2 beakers each containing 50 cm<br /> 3<br /> of water.<br /> Find out how much of each substance you can dissolve in the water.This will<br /> compare thesolubilityof the two substances. Donotheat the water.<br /> Comment on your findings.<br /> Task 2<br /> You are given solutions of citric acid and sodium hydrogencarbonate. Use Universal<br /> indicator to find thepHof each solution.<br /> Mix 10 cm<br /> 3<br /> of the citric acid solution with 20 cm<br /> 3<br /> of the sodium hydrogencarbonate<br /> solution. What happens? Use Universal indicator to find the pH of the resulting solution.<br /> What gas do you think might be produced when the two are mixed? Devise how you<br /> could test for this gas. Write aword equationfor the reaction between these two<br /> substances.<br /> Present your results in a suitable way.<br /> Explain your findings.<br /> How much acid is present in drinks?<br /> The use of artificial sweeteners in soft drinks has helped to reduce the amount ofsugar<br /> we eat. This has also reduced the amount of tooth decay. Tooth decay is caused by<br /> the production ofacidsbybacteriainplaque.<br /> Tooth decay should not be confused withtooth erosion. The acids found in many of<br /> the foods and drinks we eat can cause the loss of tooth enamel. This is known as<br /> erosion. The frequent exposure of teeth to anyacidicfood or drink may cause this<br /> chemical erosion.<br /> Task 1<br /> Use Universal indicator to find out the pH of a range of drinks. Make sure you testtap<br /> water, cola, fizzy lemonade, blackcurrant juice, orange squash, milk and freshly<br /> squeezed orange or lemon juice.<br /> Present your results in a suitable way.<br /> <br /> pupil activity A1SOME REACTIONS BETWEEN ACIDS AND ALKALIS<br /> Task 2<br /> The acid that is used in most fizzy drinks is citric acid. It is quite easy to determine the<br /> acidity of drinks using a technique calledtitration.<br /> You are given a number of solutions. Find how much citric acid is in each.<br /> Method<br /> SAFETY - REMEMBER TO WEAR GOGGLES<br /> PHENOLPHTHALEIN SOLUTION IS FLAMMABLE<br /> AND HARMFUL<br /> SODIUM HYDROXIDE SOLUTION IS AN IRRITANT<br /> 1.Measure accurately 10 cm<br /> 3<br /> of solution 1 into a conical flask.<br /> 2.Addabout15 cm<br /> 3<br /> of distilled water to the flask.<br /> 3.Add 5 drops ofphenolphthaleinto the flask. Swirl<br /> the flask carefully to mix the contents of the flask.<br /> 4.Fill the burette to a convenient point withsodium<br /> hydroxidesolution. Sodium hydroxide solution is<br /> an alkali. (Your teacher will show you how to use<br /> a burette properly.)<br /> 5.Slowlyandcarefullyadd the sodium hydroxide<br /> solution to the flask. Swirl the flask carefully after<br /> each addition.<br /> 6.Keep adding the sodium hydroxide until the pink<br /> colourjust remainsafter swirling. Carefully read<br /> from the burette how much alkali you added.<br /> 7.Repeat the titration (steps 1 - 6) another 2 times.<br /> You will then be able to work out, from the<br /> three* readings, an average volume of sodium<br /> hydroxide added.<br /> 8.You can find out the total acidity of the solution<br /> using the following equation. The amount of acidity<br /> will be expressed as the amount of citric acid:<br /> amount of citric acid = 0.64 x volume of sodium<br /> hydroxide g per dm<br /> 3.<br /> 9.If there is time, repeat the titration using another<br /> drink in place of solution 1.<br /> 10.Write up your investigation.<br /> Present your results in a suitable way.<br /> Comment on your results.<br /> Phenolphthalein is an<br /> indicator. It is colourlessin<br /> acids. It will change to<br /> pink in alkalis.<br /> You will notice that apink<br /> colour appears whenthe<br /> sodium hydroxide is first<br /> added to the flask. This<br /> disappears when you<br /> swirl the flask as the alkali<br /> reacts with the acid.<br /> *Do not include the<br /> reading from the first<br /> titration if this was much<br /> larger than the second<br /> two.<br /> 11.If your teeth are in contact with acidic foods, not just fizzy drinks, tooth<br /> erosion may occur. Think of any advice you could give to individuals who<br /> like to drink acidic drinks so that the effects of the acid are minimised.