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We were searching through a chemistry book, looking for molecules to build out of mini-marshmallows and toothpicks. Salt looked interesting, from a construction point of view.

A sodium chloride crystal model

Salt is common and widespread. And, like so many things, when you look a little more closely, it turns out to be very interesting…

To be more stable, the sodium wants to lose an electron (to become the sodium ion, Na+) and the chloride wants to gain an electron (to become the chloride ion, Cl-). The electrons are shared between the atoms and this results in each ion being surrounded by 6 ions of the opposite charge (see the model above).

As a consequence of the structure of the molecules of the ionic compound, salt crystals are typically a very regular cubic shape. Technically, this is a face-centered cubic array.

So, we tried crystallizing some by dissolving some table salt in boiling water and then evaporating the water off. This was very simple to do, and did produce some very nice regular crystals.

'Regular' salt crystals

This progressed into just pouring salt into tap water at room temperature, along with some food coloring. We even filtered some before leaving it for the water to evaporate (since it is fun to use a funnel and real filter paper if you have it!)

As well as the regular cubes we were expecting, we got the shapes below – apparently forming as a crust on the surface of the liquid. The room the solutions were in was very hot, so perhaps the rapid evaporation helped to produce this particular shape?

Salt crystals forming on the surface

Coloured sodium chloride crystals

They even grow salt crystals on the International Space Station.

What happens if you don’t have any salt? Humans needs both sodium and chloride ions, for fluid regulation. And the sodium is also important for the nervous system. For more detail on that aspect, see here.

Salt is also widely used in industrial processes; e.g. as a food preservative, in the pulp and paper industry, to set dyes in fabric, processing of some metals, de-icing of roads, and even in drilling.