PAUL ROBERTS: Hello, my name is Paul Roberts, and I'm a chemistry teacher here at Andover
College. Today, we're going to be looking at the core practical of preparing hydrated
copper sulfate crystals. The reaction we're going to be looking at today is between solid
copper oxide and sulfuric acid.
The produced copper sulfate solution will then be partially evaporated, and after a
few days, pure copper sulfate crystals will be produced. In terms of safety in this reaction,
there are a few key points to consider. Firstly, dilute sulfuric acid of a concentration one
mole per decimeter cubed is used. Therefore, it is important that safety glasses and lab
coats are worn.
Any spillages on the bench or splashes on the skin should be washed off immediately
with plenty of water. This experiment also involves heating with a Bunsen burner. Extra
precautions must be adhered to, tying hair back, making sure there are no loose articles
of clothing, or any other flammable materials nearby. It is important to evaporate the copper
sulfate solution with a hot water bath.
Direct heating of the copper sulfate solution with a Bunsen burner could result in spitting
of hot copper sulfate solution, which will cause skin damage. To start off with, pour
20 centimeters cubed of one mole per decimeter cubed sulfuric acid into a measuring cylinder.
Then add this to a 100 centimeter cubed conical flask.
The reaction takes place faster at a high temperature, so place the conical flask in
a bath of hot water at 50 degrees C. In this example, I'm using a 400 centimeter cubed
beaker as a water bath. However, if you use a large water bath, it is advisable to clamp
the conical flask to prevent it falling over. Take care the water is no hotter than 50 degrees
C as this may scald.
After three to four minutes, the sulfuric acid should have reached temperature. Turn
off the Bunsen, carefully remove the conical flask from the water bath, dry off the base
of the conical flask, and add half a spatula of copper oxide. Swirl the reaction mixture,
and you will notice that the copper oxide will slowly disappear as it reacts with the
sulfuric acid, producing copper sulfate.
Keep adding half spatula fulls of copper oxide to the reaction mixture with swirling, until
unreacted copper oxide remains in the conical flask. This will indicate that the copper
oxide is now in excess and all the sulfuric acid has reacted. At this point, the blue
color of the copper sulfate solution should be noticed.
The excess copper oxide can now be removed via filtration. Fold a piece of filter paper
into quarters. Open one of the quarters out to make a paper funnel. Place the filter paper
in an appropriately sized funnel and add a small amount of distilled water to bed the
paper into the funnel.
Do not keep any of the water that may come out of the bottom of the funnel at this stage.
If you have a ring clamp, you can use this to hold the funnel in position over an evaporating
basin. Alternatively, the funnel can be lightly clamped with a conventional clamp.
Pour the reaction mixture into the filter paper, taking care not to overfill beyond
the line of the paper. Depending on the thickness of the filter paper and the amount of excess
copper oxide, it may take a few minutes for the copper sulfate solution to pass through
the filter and be collected in the evaporating basin. This is an ideal time to prepare the
Bunsen burner for the next part of the procedure.
Arrange the Bunsen burner away from flammable materials on a heat proof mat with a tripod,
gauze, and water bath. In this instance, I'm using a 400 centimeter cubed beaker, a third
full of water, as my hot water bath. Place the evaporating basin containing the filtered
copper sulfate solution on top of the beaker.
At this point, make a note of the height of the copper sulfate solution and the evaporating
basin. Like the Bunsen with the hole closed. Place the Bunsen under the hot water bath
and then heat on a blue flame. It will take a few minutes for the water in the beaker
to start boiling. When it does, the water and the copper sulfate solution will begin
to evaporate.
Once half of the water and the copper sulfate solution has evaporated, turn off the Bunsen
and allow the apparatus to cool. Whilst your solution is cooling, write your name and copper
sulfate crystals on the outside of the watch glass or a petri dish. When the evaporating
basin is cooled and its contents to the watch glass or petri dish, now, leave in a warm
part of the laboratory uncovered for at least 24 hours.
After two days, my copper sulfate solution has crystallized to produce some dark blue
crystals of pure copper sulfate. Although tempting, make sure you do not touch the crystals
with your hands. Refer to clips for further guidance. The crystals can be carefully removed
from the solution by using tweezers and then dry it on a paper towel.
Take care not to wash these crystals as copper sulfate is soluble, and your crystals will
soon vanish back into the solution. If too much heating has been applied, then your crystals
will be considerably smaller and less impressive. In this practical, I've demonstrated how to
prepare a pure sample of hydrated copper sulfate crystals. It will be a good idea for you to
reflect on the techniques I have used today, and your teacher may provide you with further
questions to help you understand this practical in greater depth.