Illustrated Guide to Home Chemistry Experiments

Chapter 6 Laboratory: Separating Mixtures 101

LABORATORY 6 .3:

RECRySTALLIzATIoN: pURIfy CoppER SULfATE

In this lab, we’ll use a procedure called

recrystallization to purify crude copper

sulfate. Crude copper sulfate is a mixture of

copper sulfate with various impurities that

may include copper carbonate, copper oxides,

and other copper compounds. You can obtain

crude copper sulfate at a hardware or lawn

and garden store, where it is sold as a root

killer or for pond treatment.

RIREEqU d EqUIpmENT ANd SUppLIES

£ goggles, gloves, and protective clothing

£ balance and weighing papers

£ hotplate

£ beaker, 250 mL (2)

£ 1 00 mL graduated cylinder

£ stirring rod

£ filter funnel and support

£ filter paper

£ beaker tongs

£ thermometer

£ copper sulfate pentahydrate (250 g crude crystals)

£ acetone (a few mL)

£ sodium carbonate heptahydrate (50 g)

£ ice bath

Many chemicals besides copper sulfate are available
inexpensively in impure forms, such as technical and practical
grades. Although these chemicals may be insufficiently pure
for general lab use, many of them can be purified to the
equivalent of laboratory grade or even reagent grade by using
recrystallization.

Successful recrystallization depends upon two factors. First,
crystals are the purest form of a chemical, because there is no
room for impurities in the crystalline lattice. As crystals grow
in a solution of an impure chemical, the impurities remain in
solution. Second, some chemicals are much more soluble in
hot water than in cold. For example, the solubility of copper
sulfate pentahydrate, the chemical that we’ll purify in this lab, is
203.3g/100mL at 100°C, versus only 31.6g/100mL at 0°C.

In other words, a saturated solution of copper sulfate at 100°C
contains about six times more copper sulfate than the same
amount of solution at 0°C. If we saturate boiling water with
impure copper sulfate and then cool that solution to 0°C, about
5/6 of the copper sulfate crystallizes in pure form, leaving
the impurities in solution (along with about 1/6 of the original
copper sulfate).

Once the crystals have formed, they can be separated from the
supernatant liquid by filtration or by decanting off the liquid.
In either case, some of the contaminated liquid remains mixed
with the purified copper sulfate crystals. You remove this last
bit of contamination by rinsing the crystals with a small amount
of ice-cold water or another solvent that is miscible with water,
such as acetone, leaving only purified copper sulfate crystals.

We’ll use acetone for the final rinse, because copper sulfate
is almost insoluble in acetone. If we used ice-cold water, some
of our purified copper sulfate would dissolve in the rinse water,
lowering our final yield. Using acetone flushes away impurities

without dissolving the copper sulfate crystals. Volatile organic

solvents like acetone also evaporate faster and more completely

than water.

SRSATURATEUpE d SoLUTIoNS

A supersaturated solution is one that contains

more solute per volume of solution than the solution

would ordinarily contain at that temperature.

Supersaturation normally occurs when the temperature

of a saturated solution is gradually reduced. Rather than

precipitating out, the excess solute remains in solution

until some event occurs that causes the excess solute

to precipitate suddenly. That event may be as minor

as dropping a tiny crystal of the solute into the solution

or even simply tapping the container. This is called

nucleation. Crystals produced from a supersaturated

solution are not necessarily pure, because the

rapid crystallization may trap impurities within the

crystal structure.