stayed the same when you boiled it.Benedict'ssolutionprovides a chemical
test for sugar. When you add it to asolutioncontainingsugar and boil the two
together,the liquidturnsbrick red. This onlyhappensif thesolutioncontains
sugar.- Fill oneofthe thistle tubes with plain water. Cover themouthwith the
cellulosewrappingpaperand hold it firm with arubberband. Invertthe thistle
tubeand place it in thejarof sugarsolution(jarNo.1).With the clamp,attach
the thistle tube to theuprightstandat such a heightthatthepaperis well below
the surface ofthe sugar solution. (Thediagramshows you how to fix your
apparatus.)Allow this tostandat least for severalhoursorpreferablyovernight.
FilljarNo.2with plain water.Put!cupofwarm water in theotherthistle
tubeand add 3teaspoonfulsofcoppersulfate to it.Thencover themouthof the
thistletubewithwrappingpaper,and invert the thistletubein thejarofplain
water, as you did before. Holdit in the right place with the clamp. Allow this
tostandovernightalso.
Results:Since plain water is colorless andcoppersulfate is blue, you had no
difficulty observingwhathappenedinjarNo.2.The bluesolutionin the thistle
tubewas less blue, whereas the formerly colorless water in thejarturnedblueish.
This means thatmoleculesofcoppersulfategot into thewaterin thejarand
vice versa.
Since the sugarsolutioninjarNo. 1 and the plain water in the thistletube
are bothcolorless, youcan'tsee any changes withyoureyes. But you can tell
if there were any by testing the plain water for sugar.Take2 teaspoonfulsof
the liquid from the thistle tube and pourit into a fresh test tube. Add 2 tea-
spoonfulsofBenedict'ssolution. Heatthe testtubeover the alcohol burner
until the liquid boils.Noticethatthecolorturnsbrick red,provingthatthe
"plainwater"nowcontainssugar. If you test the sugarsolutionin thejar,too,
you will findthatit doesnotturnquite such a deep brick red, showingthatit
lost some sugar. Sugar molecules from thejargot into the thistle tube and water
molecules from the thistletubegot into thejar.
You havejustproved againthat molecules pass back and forththrough
membranes.In this case, thewrappingpaperacted as amembrane.
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