SCSP

This work is based on trial and error experimentation by our team.  

After expending considerable effort trying to grow good sugar crystals (rock candy), we decided that the best material to use was alum [potassium alum, K₂Al₂(SO₄)₄(24H₂O)]. A small amount of alum is normally provided with the kit, sufficient for a demo, but perhaps insufficient for each student to grow his or her own crystals. We did find a ready source of alum, though not a cheap one. Alum is used in pickling, and we found a small bottle of it in the spices area of a local upscale food market.  A 1.9 oz bottle (McCormick) was $2.79, doubtless marked up significantly for retail sale. Nonetheless, we knew of no other places in the Princeton area which sell canning supplies and might stock alum in bulk. So the recipes below are scaled down in the hopes that every group of students can complete the crystal growing extension with the 1/4 cup alum which is provided in the kit. Alternatively, you could scale up the recipe and do one large demonstration experiment for the class.

Some comparisons between the alum supplied with the kit and the McCormick alum: the kit alum is more finely ground (powdered, vs. the granulated McCormick) and so dissolves faster. However, the kit alum makes a hazy solution (even when subsaturated), indicating the presence of some insoluble impurities. Often, impurities such as these will hinder efforts to grow large crystals because the impurities act as crystal nuclei (so one winds up growing many small crystals rather than a few large crystals).  In our lab, we were able to filter these out using a fine-pore filter paper (yielding a clear solution), but the coffee filters provided with the kit are too coarse and the impurity particles pass right through them. However, in the present case we found that the impurities (while visually unappealing) didn't seem to interfere significantly with the crystal growing, so we suggest you not bother trying to remove them. The McCormick alum did not seem to contain insoluble impurities.

To grow a very large crystal, one first needs a seed crystal. Nice faceted alum seed crystals measuring 3-4 mm across and about 1 mm thick were easily grown by the following procedure:

1) Add 3.0 grams of alum to 10 ml of tepid tap water. The solubility limit of alum in water at room temperature is about 1.2 g per 10 ml, so you will need to heat the solution slightly to dissolve the alum (but not very much, as the solubility is strongly temperature-dependent; 50^oC should do the trick easily).

2) Pour the hot solution into a crystallizing dish (can use the same ones used for growing crystals of salt and citric acid). Visible crystals will start to form in 10-20 minutes, and will grow to their final size after 1-2 hours. After this time, pour the excess solution out of the dish and blot the crystals dry with a paper towel. We typically got about two dozen crystals from this size batch of solution, which should be more than enough for an entire class. The crystals can, of course, be stored for use at any future time.

Rather than making a supersaturated alum solution (3 g alum per 10 ml water), which requires heating, it is possible to obtain seed crystals by preparing a saturated or slightly subsaturated solution at room temperature and simply allowing the water to evaporate overnight, as the students do for salt and citric acid. However, when the last bits of liquid evaporate, they deposit tiny crystals all over the large crystals, which will make the large crystals less attractive as seeds (that is, when such crystals are used as seeds, the tiny crystals will grow and make the final crystal less visually appealing). The concentration of 3 g alum per 10 ml water was found by trial and error; at higher concentrations, one forms many more but smaller crystals, while at lower concentrations, the crystallization process becomes much slower.

To grow large single crystals from these seeds, the best method is to suspend the crystal in a slightly supersaturated solution. This way, when alum starts to crystallize out of the supersaturated solution, it will preferentially (though not exclusively) deposit on the seed crystal, which the alum recognizes as a "friendly" (chemically identical) surface.

1) The simplest method we found to suspend the seed crystals was simply to hang them by a thread, though tying a thread around the crystal requires some dexterity and may be tricky for some students. We used a fairly coarse thread (button thread) which seemed a bit easier to tie.

2) Prepare a solution of 2.0 grams of alum in 10 ml of water. Again, the solution will have to be heated slightly for all the alum to dissolve, since this exceeds the saturation limit at room temperature. The simplest method would be to scale up this recipe and make one large batch of solution which is then divided up among the various student groups, but don't let the solution stand for too long (15 minutes is ok) before distributing it to the students because the excess alum (above the saturation limit) will start to crystallize out even in the absence of a seed crystal. Again, the concentration of 2 g alum per 10 ml water was found by trial and error. At higher concentrations, many additional crystals other than the seed crystal form in the solution, and form quickly. At lower concentrations, there is little excess alum to add to the seed crystal, so its size does not increase that much, and the crystallization process is much slower.

3) Once the solution has cooled off somewhat, suspend the seed crystal so that it hangs roughly in the center of the supersaturated solution. (If you suspend the seed crystal in the hot solution, which is not saturated, the seed crystal will dissolve!) The best way to suspend the seed crystal depends on the containers you have available. We used glass scintillation vials (a common container in our lab), which are cylindrical and hold about 20 ml. In this case, we simply wrapped a piece of wire across and around the top of the vial to act as a crosspiece from which to hang the crystal. If you're using the cups which the kit provides, then a simple popsicle stick laid across the top of the cup will make a fine crosspiece; however, the cups require much more than 10 ml of liquid, probably 50 ml at a minimum for this to work well, meaning that the amount of alum required goes up correspondingly.  It's best for students to prepare their setups (containers, crosspieces, and seed crystals hanging by a thread from the crosspiece to the proper height in the container) in advance, prior to step 2.  Actually, though this has nothing to do with chemistry, assembling a good setup is probably the trickiest part of crystal growing; this may mean that crystal growing is better done as a demonstration.

4) Within a couple hours, the crystal growing process should be essentially finished, and you should see that the seed crystal is substantially enlarged. To grow bigger crystals, the whole process could be repeated, using this crystal as a seed crystal. In addition to the suspended crystal, you will also see many other (smaller) crystals which have settled to the bottom of the container. These formed from additional nucleation events during crystallization.

As a side note, during the times we were letting crystals grow (seed crystals or the final crystal), we always covered the container loosely with aluminum foil to prevent dust from settling in the container. Since alum crystal growing seems fairly robust, this is probably not necessary.

A note on the scale here: the 1.9 oz of alum contained in the McCormick package equates to about 54 grams. Taking out 3 g for seed crystals leaves 50+ g for the students, or about 6 g for each of 8 groups. Thus, the growing recipe could be scaled up to 6 g alum in 30 ml water easily. The "1/4 cup" alum provided by the Invention Factory also weighs 50 g.