It is often the chemist’s task to optimize the conditions in which he performs the reaction so that it proceeds as quickly as possible, because a faster reaction means less resources used to obtain the product, which affects the profitability of production itself.

However, there are situations in which we try to slow down the reactions. For example, if we mix a solution of copper(II) sulfate (bluestone) with a solution of sodium potassium tartrate (Rochelle salt), an insoluble blue-green precipitate will immediately form. The resulting product is a very fine crystalline powder. If we wanted to make crystals from that powder, we wouldn’t be able to do that, given that it is insoluble in water.

If, on the other hand, we wanted to grow beautiful, regular crystals of copper tartrate, we would have to apply one of the crystallization techniques that rely precisely on the slowing down of chemical reactions. This is how we can do it:

One of the convenient methods of crystallization is called crystallization in gels, where we usually use ordinary edible gelatine as a gel. The procedure is as follows: We dissolve potassium sodium tartrate and gelatine in water in a test tube while heating it, and let it harden. Then, we add an aqueous solution of copper(II) sulfate to the gel prepared in this way. And… we wait.

It is quite likely that we will not notice any changes on the first and second day, but somewhere around the fourth and fifth, very small crystals will appear, which will continue to grow in the next seven days. You can see in the photos how the crystals will look in about two weeks.

The role of gelatine is to slow down the movement of dissolved particles and thereby slow down the mixing of reactants. Given that only a small part of copper ions comes into contact with tartrate in the gel, very little product is initially formed that will serve as the centre of crystallization of our crystals. As time goes by, the amount of product continues to increase, but it is still low enough not to cause the powdery product to settle, but regular crystals grow from such a mixture, which you have to admit, look very beautiful.

Author of the text and photos: Alen Bjelopetrović, Dr.Sc.