MIXING AND DISSOLVING MATERIALS - Primary …

1 63 Science background for teachersMIXING AND DISSOLVING MATERIALSM ixing MATERIALS together forms a mixturewhere two or moresubstances are physically but not chemically combined and can beseparated again by physical methods such as sieving, filtering,evaporating etc. This means that the change can be reversed. A newmaterial is not formed as it is in a chemical reaction. Mixtures can bemade up of; solid in solid (muesli) gas in solid (pumice stone) solid in gas (smoke) gas in gas (air mostly made up of nitrogen and oxygen) liquid in gas (clouds, mist, aerosol) gas in liquid (oxygen in water) liquid in liquid (emulsion milk) insoluble solid in liquid (suspension muddy water) soluble solid in liquid (solution salt water). MIXING MATERIALS together may form a new substance but not amixture, as with Plaster of Paris and water. A chemicalreaction hastaken place in which case a completely newsubstance has beenformed and the original substances cannot easily be recovered.

2 Theword mixture is not always used correctly in its scientific sense. A cakemixture for example is so until water is added and it is baked. Achemical change has then taken place and it is no longer a mixture but a cake! A safe way of referring to such situations might be to callthem a mix . Activities involving baking mixtures have been left out ofthis section and are included in the section Heating and coolingmaterials .DISSOLVINGWhen the mixture is a solid in a liquidit will either produce asolutionor a solution is clear and will never settle out,a suspension is cloudy and will eventually settle out. If the solid(solute)dissolves in the liquid (solvent),a solution is at Primary level the solvent mostly used is water, olderchildren need to know that there are others. These will dissolve othersolids that may not dissolve in water eg nail varnish is dissolved bypropanone (acetone).

3 Various factors that affect DISSOLVING will befamiliar to children, such as stirring, temperature, time, amount ofsolute, amount of solvent and can be equated with their everyday lifesuch as stirring sugar in a cup of a solid dissolves in a liquid to produce a solution, but notof the original substances, because the change is a chemical one, forexample when Alka-Seltzer dissolves in , solid, liquid,gas, solution,suspension, dissolve,factors, saturated,soluble, insoluble,permanent,reversible, separate,sieve, change,solvent64 MIXING AND DISSOLVING MATERIALSFor younger children, it is sufficient to know that a change of somesort has taken place, older children can explore whether it ispermanent or a finite amount of solid will dissolve in a liquid and this is SOLUTIONS dependent on the solid and the temperature. When no more solid willdissolve the solution is saturated, but generally more will dissolve inhot than in cold liquid.

4 So if a saturated solution of sugar is madeusing hotwater, as it cools some of the sugar comes out of solutionand reforms making crystals. This principle is used to grow can also, of course, be made by leaving a salt or sugar solutionand allowing the water to evaporate leaving salt or sugar often confuse meltingand dissolvingand this may be apoint of discussion. Melting requires heat and DISSOLVING requires asolvent. In making a jelly both are happening when it is added to Using a thermometer and stopwatch. Working cooperatively. Choosing and using apparatus carefully. Observing and recording accurately. Measuring ideas and activitiesMIXING AND DISSOLVING MATERIALS (a)Look at and discuss with the children obvious mixtures such asDolly Mixtures, a jar of mincemeat, mixed vegetables, fruit salad,muesli. Get the children to closely observe and list theingredients in the mixtures.

5 Ask the question, Can they beseparated? (b)It may be appropriate at this stage also to do a sieving activity toseparate the various components of a mixture such as peas andrice and allowing the children to choose their own equipment orto make up their own mixture. Explain to them that to call it a mixture they must be able to separate it again. This wouldmake a good combined activity for younger children especially ifsome sieving is also done with soils (see Rocks and soils section).Investigatehow to separate sand and peas, or stones and food Design Technology activity where the children make simple mixtures such as icing sugar and coconut to make coconut ice ,design their own fruit salad or make chocolate crispy cakes usingmelted chocolate and Cornflakes or Rice Krispies , can also be pencil crayonsThis is a directed activity where thechildren need to follow a recipe for making pencil crayons.

6 They willneed: Large, marble-sized ball of modelling clay Small spoon of paste, (cellulose wallpaper paste made up withwater beforehand) Big spoon of powder paint or iron oxide (pottery supplier or localsecondary school). The paint makes playground pencils , the oxidemakes brown thick pencil leads that write on the ingredients in a polythene bag by squidging it from theoutside, then take it out and mix it in the hand, it resembles crumblyplasticene. Roll out the leads , this makes about three 8 cm lengthsand leave to dry overnight on paper towels. The leads can be coveredin fancy have morethan one thing inthem (some may beseparated bysieving) MIXING materialscan cause them tochangeYou will need:Put in a polythene bag Roll out sausage shapesand leave to dryand spooncellulose pastebig spoonpowder paint oriron oxideclay - largemarble size8 cmMix the mixtureoutside the rolling outthe leads , washyour AND DISSOLVING MATERIALS (a)Present the class as a whole with a wide range of MATERIALS tomix so that there can be a comparative, plenary discussion.

7 Eachgroup of children can have a smaller selection to work soluble and insoluble MATERIALS and get the children torecord what happens each time. Suggest they stir or shake themixtures and be patient! This would be a good lead into work examples are;Water withsand, sugar, salt, Alka-Seltzer, oil, coffee, lemon juice,flour;Vinegar withoil, lemon juice, bicarbonate of this stage the amount of each ingredient mixed is not criticalbut is used in the case of solids in liquids, 1 teaspoon of solid toat least 100 cm3of water to avoid saturation. Keep the variousmixtures for the discussion.(b)The results of the activity could then be grouped into thosematerials that dissolve and those that do not, having discussedsolubility first. In some of these mixings has anything new beenmade?You could use the concept cartoon on this with the children first and get them to suggest the factorsthat affect DISSOLVING from their own experiences such as making a cupof tea.

8 Get them to suggest a hypothesis, for example; I think thatstirring affects DISSOLVING and then plan a way to prove it. This wouldmake a good investigationor series of small investigations,eg InvestigateDoes stirring affect DISSOLVING ? This can have particularreference to fair testing due to the large number of variables that couldbe investigated. In this set of quantitative investigations, care shouldbe taken if using salt, as impurities in the salt often leave the watercloudy after it has dissolved. Consequently, it is not always clear atwhat point the salt and:SandChalkSugarSaltAlka-SeltzerOilFlo urCoffeePrediction:will it dissolve?Didit dissolve?Describe any other changesthat occurredThere are a varietyof factors whichaffect dissolvingThe coffeedissolved andwent Alka-Seltzerdissolved and materialsdissolve in waterothers just mixICTWord processing67 MIXING AND DISSOLVING MATERIALS (a)StirringLimit the number of soluble solids eg sugar whichworks very well, salt (test for cloudiness first) and water even ifthey are planning this themselves.

9 They can time how longdissolving takes with and without continuous stirring and stirringonce every minute. Suggest 1 teaspoon of solid, even thendissolving without stirring can take a long time.(b) TemperatureRepeat activity (a) using hot and cold one good stir at the beginning or continuous stirring,otherwise it can take a long time to dissolve the solid and thetemperature of the water will graph to showthe time taken for1 teaspoon of sugar todissolve in 100 cm3waterat 45 C with differentrates of stirringTime in seconds to dissolveTime between stirs in secondsThe shorter the timebetween stirs, themore stirs we longer it takes todissolve, the colder thewater gets!After 1 hour the sugarwe didn t stir at allstill hadn t dissolved!25 C45 C30 CWe used 100 cm3ofwater each time andkept stirring until itall package68 MIXING AND DISSOLVING MATERIALS (c)Size of the solid particlesFor convenience of time, use warmwater and stir.

10 Present the children with one solid in differentforms where the grains are different sizes eg sugar granulated,castor, icing, sugar crystals. Let the children look at the solidswith a magnifying glass first and predict which will dissolve thefastest. Brown sugar contains other substances, so is not thesame as white sugar and not appropriate to use.(d)Amount of solidAgain for time convenience, use warm waterand stir, eg once every minute. Present the children with only afew soluble solids and ask them if the amount they put in thewater affects the DISSOLVING 51015202530354045505560A graph to show the time takenfor 1 teaspoon ofsugar to dissolve in 100 cm3of water atdifferent temperaturesTime in secondsTemperature in CCan you predict howlong it would takeat 5 C?1234567 GranulatedCasterIcing8910A graph to showthe time takenfor differentsugars todissolveTime taken to dissolve in secondsDifferent sugarsWe used 1 teaspoonin 100 cm3of waterat 50 C each looked at the sizeof the sugar grains witha magnifying glass.