cool gradually also. When the second and thirdjarshave both cooled toroom
temperature,put I small crystal of sodiumthiosulfateinto each of the 3jars.
Observe the result very closely.
Results:Compared with the other two solutions, the one made at room
temperaturetookrelatively little sodium thiosulfate, although it was a true
solution. More crystals dissolved when you heated the water, and the greatest
amountdissolved when you boiled the water.
The first solution, made at roomtemperature,was anunsaturatedsolution.
Thatmeans it held less solutethanit was reallycapableof holding.Therefore,
when you added the extra crystal, it dissolved. The secondsolution,withheat
applied, was asaturatedsolution.This means thatit held all the solute it is
normallycapable of holding, plus a little excessthatdidn'tdissolve.Therefore,
when you later added an extra crystal, itdidn'tdissolve, but fell to thebottom
with theotherexcess undissolved crystals. Thethirdsolution, made at boiling
temperature, was asupersaturatedsolution. The boiling forced it to absorb
much more solute than it wouldnormallyhold. Theextracrystalthatyoulater
addedseemed to be the center of an area where new crystals of sodium thio-
sulfate grew. Of course, they were not really new. Theyseparatedout from the
solutionas it cooled, and they would have done so even if youhadn'tadded
anothercrystal. The one you added merely served as aseedcrystal, or"starter"
for the others to clusteraround.The crystal growthcontinued until thesolution
was onlysaturatedratherthansupersaturated.
You can repeat this experiment with any crystalline chemical, but you will
have the best success with crystalsthatnormallycontainwater ofcrystallization
(see page 56), such ascoppersulfate, because they dissolve more readily.HOW TO SEPARATE SOLUTES FROM SOLVENTS
Gather thesematerials:Sodium chloride(NaCl);magnesium sulfate(MgS0 4 ) ;
crushed aspirin (CH3COOC6H4COOH);boric acid (H3B0 3 ) ;powdered calcium
hydroxide (Ca(OHh); sucrose (C12H 220 U); copper sulfate (CuS0 4 ) ; grain