Toowoomba, Queensland, Australia
Microscale is as much a state of mind as a different technology. While new kinds of equipment are becoming available, much can be done to downscale using simple and readily available equipment. The "state of mind" is one which looks at a standard macroscale experiment and sees how to repackage it, often with enhancements, in microscale.
The followingis experiment is open-ended, in that it allows students freedom to observe the effects of modifying different experimental variables, in a loosely structured way. Instructions below describe only one way in which the experiment can be set up; there are possible variations that might be explored. Teachers may choose to "tighten" the procedure to direct students towards specific observations and interpretations.
This is one of a series of experiments being developed by the author.
AIM To observe a range of processes and effects associated with electrolysis of an aqueous solution of copper sulfate.
Small glass petri dish with lid, concentrated solution of copper sulfate, platinum wire anode, fine copper wire cathode, solid copper(II) oxide, small spatula, adhesive tape, variable DC voltage source (2 to 6 volts), electric leads, ruler, matches, 10X magnifying glass.
Attach a long piece of fine copper wire to a short piece of platinum wire and pass the end of the wire from under the lid of the petri dish.
This procedure should be carried out in bright light.
a. Clean the tip of a piece of fine copper wire; inspect it with a magnifier.
b. Tape the electrode wires inside a petri dish so that their tips are separated by about five millimetres, and again where they pass over the sides of the dish.
c. Place about two drops of concentrated copper sulfate solution so that the tips of both electrodes are touching the solution. An extra drop placed to one side serves as a standard for evaluating the extent of colour change of the electrolyte during electrolysis.
d. Place a few specks of solid copper(II) oxide in the solution between the tips of the electrodes and spread them to form a band in the solution between the electrodes.
e. Place the lid on the petri dish, and connect the electrodes to a three volt source of direct current. (Platinum anode to the positive, copper cathode to the negative terminal.)
f. Watch the changes closely, using the magnifier to watch the changes that occur around the electrodes, and later, around the specks of copper oxide.
g. The amount of aerosol spray produced by the breaking of bubbles around the anode (and later the cathode) should be observed.
h. When the reactions have started, increase the voltage briefly to 5 or 6 volts, then reduce again to three volts; observe the effect of varying voltage.
i. Towards the end of the reaction process, bubbles may be forming at both electrodes. Light a match, remove the lid from the dish, and hold the burning match to the bubbles.
POSSIBLE VARIATIONS IN PROCEDURE:
Vary spacing of the electrodes by setting the tips of the wires at different distances apart. Vary the orientation of the electrodes by placing them parallel instead of tip to tip.
EVALUATION OF OBSERVATIONS
Consider the following observations. Where explanations are not possible, it may still be possible to make hypotheses.
1. Deposits start to form around the cathode as soon as electrolysis begins. The rate at which these develop varies as the electrolysis proceeds.
2. Bubbles formed at one electrode may stream through the electrolyte towards the other electrode.
3. The colour of the solution tends to fade, more quickly at the cathode side of the drop of solution.
4. When a growth of metal from the cathode reaches a grain of copper oxide, it grows very rapidly through or around the grain.
5. Grains of copper oxide tend to "disappear".
6. Fine drops of liquid form under the lid of the petri dish, mainly just above the anode.
7. Bubbles appear first at the anode, and much later at the cathode. Bubbles may be seen earlier at the cathode at higher voltages.
8. Sharp noises are heard when a lit match is held near the bubbles formed towards the end of the reaction process.
This list of possible observations is not complete.