Chemical Crystallography before X-ray Diffraction

by Roberto Poeti
The crystallography in chemistry

When I taught basic chemistry, at the Industrial Technical Institute in Arezzo town (Italy), one of the first lessons of the course was dedicated to illustrating a laboratory experience that students would perform at home. It was the formation of sodium chloride crystals from a salt solution. After about a week each student brought to school his saucer where they had grown small but regular cubic crystals of salt.

Formation of sodium chloride crystals                                                            Sodium chloride crystals under a microscope

After each student had examined his collection of crystals, they passed through the microscopy classroom to observe the enlarged crystals. The vision of the dress of the crystals was an experience that excited the boys.

The crystallization

This experience followed another, later in the course, this time in the school’s laboratory. The class was divided into groups of two students each. Each group had two tubes, a water bath beaker and two sulfides, ammonium chloride and potassium nitrate.  After preparing a solute solution in the hot tube, the test tube was removed from the water bath and slowly cooled to room temperature. When the solution reached the saturation point, the formation of beautiful crystals that had the shape of snowflakes began in the case of ammonium chloride and elongated spears of potassium nitrate.

Formation of lance-shaped potassium nitrate crystals

The crystallization of ammonium chloride

Formation of crystals in test tubes as snowflakes                 Growth of ammonium chloride crystals to the microscope

Experience provided an opportunity to talk about solubility and saturated solutions. A variant of the experience was to drop a drop of solutions into a slide and observe the formation and growth of crystals. The latter part of the experience was especially suitable after having had the experience of the salgemma, at the beginning of the course.

Because crystals

Observe crystals, their formation and their growth was one of the most contributing phenomena in the history of science to mature the concept of discontinuous structure and particle of matter. To propose to students the phenomenon of crystal formation, discussing their possible explanations, is a concrete way of responding to the hypothesis, which is itself very abstract, of the discontinuous structure of matter. And all this before you talk about atoms that do not represent a satisfying “response” of the discontinuous structure of matter for our student in the first year of s.s.s., but is simply one more notion to learn.

The contributions of crystallography

The study of crystals played a key role in the history of science and in particular chemistry, in several respects. We have already mentioned how the corpuscular hypothesis of matter was reflected in the properties of crystals, but their study also led to the discovery of the optical isomerism of crystals and molecules constituting crystals. The latter allowed tetrahedic carbon to be conceived in organic molecules. This fascinating story is told to us by two young chemists Kresimir Molcanov and Vladimir Stilinovic from the University of Zagreb. You find their essay an attached PDF file . They propose this by retracing the essential stages of the development of chemical crystallography before the use of X-ray difraction in a synthetic and captivating way.

The history essay of chemical crystallography



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