There is another recrystallization technique that is used most frequently in laboratories and in manufacturing. The solubility of most solids as well as liquids (but not
gases!) increases with temperature. Therefore, a solid substance can often be recrystallized by dissolving it in a minimal or close to minimal amount of hot water (or some other solvent) and then cooling the solution. If the substance to be purified by this method is contaminated with an insoluble impurity, such an impurity can be removed by filtering the solution while hot.
Crystallization is a fascinating process. Watching crystals grow
can be mesmerizing. There are many excellent videos on YouTube showing how to safely grow beautiful crystals for fun, such as copper sulfate
crystals of ammonium dihydrogen phosphate. Our goal, however, is not only to have fun crystallizing compounds, but also learn how crystallization can be used for purification. The next section below describes three crystallization experiments that you may consider doing yourself. 1.4.6. Crystallization Experiments.
Should you decide not to do the experiments, please still read this subsection.
To carry out chemical experiments successfully and safely, we should adhere to certain rules.
(1) Know well the materials and chemicals to be used in the experiments.
(2) Having learned about properties and potential hazards of all of the chemicals to be used, decide what protective gear to wear.
(3) Use your brain. If we are Homo Sapiens, which is the Latin for "wise man", we are supposed to act wisely. In order to act wisely, we should think. Think over your plan for a chemical experiment, pay attention to detail, and make sure that the procedure is clear. Think as you carry out the experiment. Think how to dispose of the materials after the experiment.
A brief description of what is needed to perform the crystallization experiments is presented below. Chemicals.
The compounds you will be recrystallizing are NaCl (sodium chloride or table salt), KNO3
(potassium nitrate or saltpeter), and CuSO4
O (copper sulfate pentahydrate). As we all must know very well the chemicals that we work with, some information on these three compounds is provided below. NaCl.
Sodium chloride, also known as table salt or just salt, is nontoxic, cheap, and available from any food store. Almost all brands of food-grade NaCl contain a tiny quantity of an anti-caking agent
(sodium aluminosilicate) in the form of a fine, water-insoluble powder. That is why solutions of table salt in water are slightly opaque (cloudy). Two major types of table salt are sold in the stores, plain and iodized. Iodized salt contains a small amount of an iodine compound added to the NaCl as a supplement, in order to prevent iodine deficiency. The iodine additive to produce iodized salt is one of the four iodine compounds: potassium iodide (KI), potassium iodate (KIO3
), sodium iodide (NaI) or sodium iodate (NaIO3
). Most commonly, iodized salt contains about 0.005% of potassium iodide (KI). KNO3.
Potassium nitrate (KNO3
) is a long-known and widely used chemical compound that is sometimes called saltpeter
. Saltpeter used to be found in nature in considerable quantities, particularly in India. That is why KNO3
is often called Indian saltpeter
to distinguish it from its sodium analog, NaNO3
, which is found native in Chile and Peru and is called Chilean saltpeter
. In the old times, saltpeter was used on a large scale for curing meat and in the production of gunpowder, also known as black powder. The consumption was so large that the natural deposits of KNO3
have long been exhausted. These days almost all KNO3
on the market is synthetic
, meaning not naturally occurring but made from other chemicals. Potassium nitrate is manufactured on a large scale and used mostly in fertilizers. It has many other applications, such as in fireworks and for stump removal.
Potassium nitrate is a white crystalline solid and is reasonably safe to work with. It is often stated that KNO3
is non-toxic. Indeed, KNO3
is an ingredient of some toothpastes formulated for sensitive teeth. It is a good idea, however, to always keep in mind that "the dose makes the poison
". Eating too much of such innocent substances as sugar or table salt can result in poisoning and can be even lethal. Just do not eat KNO3
, avoid getting it in your eyes, and wash it off with water if you accidentally spill it on your skin. Keep potassium nitrate in a glass or plastic bottle. Do not
store it in tins or metal containers and avoid mixing KNO3
with other materials. You will understand why when we talk about reduction and oxidation processes (Volume 2). Saltpeter for our experiment can be purchased from many sources. An easy and cheap way to obtain saltpeter is to buy Spectracide Stump Remover, which is virtually pure KNO3
Copper sulfate pentahydrate is a very well-known and widely used compound. First off, what is this strange formula and name? Many compounds crystallize from aqueous solutions in the form of crystal hydrates
. A crystal hydrate is not
considered to be a mixture of water with the other substance because the molecules of water of crystallization are part of the larger molecular structure.
The number of water molecules constituting such crystals can vary. For example, gypsum is also a crystal hydrate that contains two molecules of water, CaSO4
O. These water molecules are called water of crystallization
and are shown at the end of the chemical formula of the main compound, followed by a middle dot. For many, but not all, crystal hydrates, the water of crystallization can be removed from the solid by heating.
In the old times, copper sulfate pentahydrate was known under a variety of names, including bluestone
or blue vitriol
, and vitriol of copper
. As you may gather from some of these names, CuSO4
O should be blue in color. And, it is beautifully blue indeed (Figure 1-12).