Crystallization method

The theory and principle of crystallization we already know from class VI. Also the course of the experiment. Here are excerpts from the text from a physics textbook:

“Pour the salt and sugar solution from the glasses into separate saucers, put them on an asbestos plate and heat with a small flame for as long, until there is little water left at the bottom. Then put the saucers on a radiator or other warm place and after a few hours (or the next day) watch it, what's left at the bottom ".

When we repeat this school experience, "on the bottom” we will see crystals in each saucer. However, they will be very fine crystals, often smaller than those poured out of a salt or sugar packet, difficult to observe even with a magnifying glass. The reason is simple – crystallization by evaporation of the solution, because that's what this method of crystallization is called, is very fast. Too fast, so that one large crystal is formed. This method is used in the industrial crystallization of salt and sugar, when we want to receive these foods, and not growing nice crystals. In our experiment, I propose another method of crystallization from solution – slow cooling method.

The term "solution” self-explanatory. The sugar solution is sweet tea, and any soup with salt solution except sweet fruit soups. For crystallization, however, we need a supersaturated solution, i.e.. such a solution, in which even the smallest additional amount of substance cannot be dissolved. But the amount of the substance, which may dissolve in the solvent, depends on the temperature. The higher the temperature, the more e.g.. sugar will dissolve in tea. When such a solution cools, the solubility decreases, and an excess amount of the substance must separate from the solution. It is this physical phenomenon that is the basis of the crystallization process from solution.

Cane sugar and salt crystals (on the right).

The simplest method of crystallization by slow cooling is the closed bubble method, e.g.. jar with a twist lid”. Before proceeding to crystallization in another vessel, preferably metal, we heat the water up to boiling point. When the water starts to boil, pour into it, constantly stirring the water-soluble substance in such an amount, what can be dissolved in water. When we get a supersaturated solution, that is, part of the substance remains solid despite constant stirring, we stop heating, we wait, until the liquid stops boiling and has cooled slightly, and then pour it into the jar through a filter paper and close it with a tight lid. Before that, however, we need to stick a thread to the bottom of the lid, e.g.. silk (glue, insoluble in water and resistant to temperature, e.g.. Epidian 5 or Distal), Tie a small thread at the end of the thread, but a correctly formed crystal of the same substance, which we dissolved in water. Place the closed bubble over the radiator or on the sunny window sill and wait. When the cooling down is very slow, it's on a hanging crystal, which becomes the nucleus of crystallization, a very large and regular crystal will grow: Instead of a thread glued to the bottom of the lid, we can use a special rack bent from a wire and inserted into the bottom of the vessel.

In addition to sufficiently slow cooling, a condition for obtaining individual ones, fine crystals is the purity of the solution and the selection of the appropriate, easily crystallizing substances. We will get a clean solution, when we use distilled water for the experiment (it is easy to buy at gas stations and drugstores), all the utensils will be clean, thread or stand, and we will use filter paper when pouring the solution. It will retain impurities and, more importantly, non-dissolved particles in water, which could become additional nuclei of crystallization. We can use sugar as a crystallizing substance, regular home baking soda, sodium sulfate Na2SO4 or CuSO copper sulfate4. The hardest, and in domestic conditions it is almost impossible to obtain salt crystals.