102 DIY Science: Illustrated Guide to Home Chemistry Experiments
POCEDURER
1.ou have not already done so, put on your splash If y
goggles, gloves, and protective clothing.
- Place a weighing paper on the balance pan and tare the
balance to read 0.00 g. Add the crude copper sulfate
pentahydrate until the balance indicates about 250 g.
Record the mass of the crude copper sulfate to 0.01 g
on line A of Table 6-3. If the maximum capacity of your
balance is less than 250 g, weigh multiple samples until
you accumulate about 250 g total. Transfer the copper
sulfate to the second 250 mL beaker. - Set your balance to read 0.00 g and weigh a piece of filter
paper. Record the mass of the filter paper to 0.01 g in
Table 6-3, and use a pencil to write the mass on the filter
paper itself. - Fan-fold (flute) the massed filter paper and set it aside for
later use. - Set up your filter funnel with another piece of fan-folded
filter paper over one of the 250 mL beakers. - Use the 100 mL graduated cylinder to measure 150 mL of
hot tap water, and add it to the copper sulfate in the 250
mL beaker.
SBSTITUTIU oNS ANd modIfICATIoNS- If your hotplate has heating coil rather than a flat
heating surface, use a large tin can lid or a burner cover
between the coil and beaker rather than putting the
beaker in direct contact with the burner. - The quantity of copper sulfate used in this laboratory
assumes that you really do want to purify several
ounces of copper sulfate to use in the later labs in this
book. If you want only to demonstrate the principle
of recrystallization, you can reduce the quantity of
copper sulfate (and water) and the size of the beakers
proportionately. - You can substitute 50 g of crude washing soda for the
50 g of sodium carbonate heptahydrate. - You can substitute 23 g of anhydrous sodium
carbonate for the 50 g of sodium carbonate
heptahydrate. - Rather than using an ice bath, you can cool the
solution in a freezer. If you do that, make sure the
solution is in a sealed container to make sure it can’t
contaminate any of the food items in the freezer. Label
the solution to make absolutely certain it cannot be
confused with food items. (This should go without
saying; labeling containers is good lab technique, and
you should never store any product in an unlabeled
container.)
7.lace the beaker on the hotplate and bring the water to a P
gentle boil. Stir the solution until all of the crude copper
sulfate has dissolved. Any solid matter that refuses to
dissolve is an insoluble impurity and can be ignored.- Using the beaker tongs, pour the hot copper sulfate
solution through the filter paper as quickly as possible.
Some copper sulfate may recrystallize when it contacts
the cooler filter paper, funnel, or receiving beaker, but it
should go back into solution as you continue to pour the
hot solution through the funnel. - As the solution cools, copper sulfate will begin
crystallizing. Slow cooling produces larger crystals, and
fast cooling smaller crystals. Smaller crystals are easier
to handle and weigh, so we want to produce crystals as
small as possible. Our goal, then, is to cool the solution
as fast as possible from the boiling point to near the
freezing point. To do that, place the beaker of hot
solution in an ice bath (a larger container full of crushed
ice) or a freezer.
As the solution cools, stir it periodically with the ther mom-
eter. While you’re waiting for the solution to cool to 0°C,
remove the used filter paper from the filter funnel, rinse
the funnel clean, and put in the fan-folded filter paper that
you weighed earlier. Also rinse the original 250 mL beaker
clean and place it under the funnel.
When the solution has cooled to 0°C, use the stirring rod
to free any crystals that have grown on the sides of the
beaker. Swirl the contents of the beaker gently to keep the
crystals suspended in the solution, and pour the solution
through the filter funnel.
Use a few mL of acetone to rinse any remaining crystals
from the beaker and pour the acetone through the filterdR. pAUL joNES CommENTS:
I wouldn’t say that it can be “ignored.” As described in the
following step, any insoluble residue should be trapped
via hot filtration and discarded. Don’t add more water to
try to dissolve it. Also, hot filtration works best if you heat
the receiving vessel up prior to filtration. Use a little boiling
water to heat the filter paper and funnel. This reduces
crystallization on the paper.dAUL. Rp joNES CommENTS:
Yes, but it’s much cooler to grow big crystals by letting
the solution sit to cool slowly without stirring, and then
you get to watch them grow. You could even shoot a time-
lapse video.