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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.

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We were the happy recipients of a ‘slime kit’ this Christmas.

Combining a sodium alginate solution with calcium chloride solution we produced calcium alginate, which is a cross linked polymer.

What we saw:

The sodium alginate powder was dissolved in water to form a fairly runny slime solution. When this was added to calcium chloride it became rather more firm – a gel. The food coloring just makes the result more colorful.

Why it happened:

Sodium alginate has the empirical formula of NaC6H7O6.  This formula describes the units of which the molecule is made, but the molecule itself is a long chain of these units.

The reason the alginate changes consistency is that the sodium ions in the sodium alginate are replaced by calcium ions. The sodium ions have a charge of +1. The calcium ions have a charge of +2. The alginate molecules are long and have lots of sites that are able to bond with ions that have a positive charge. So the calcium ions take the place of the sodium ions but are also able to form cross links between the long alginate molecules. There is a nice diagram here.

The sodium alginate comes from seaweed. There is more about that process here.

Following on from our ‘lava lamp‘ we did a variation of the chemical reaction involved, that shows how the carbon dioxide (CO2) is heavier than air. The density is approx. 1.98 kg/m3, whereas air is only 1.2 kg/m3.

We placed a small cup in a deep dish and put a short burning candle in next to it. In the cup we put about 2 tsps each of

  • sodium bicarbonate, aka baking soda (NaHCO3)
  • citric acid (C6H8O7)

Then we added some water (H2O).

The chemical equation for this is

3 NaHCO3 + C6H8O7 –> 3 CO2 + 3 H2O + Na3C6H5O7

You see the mixture bubbling from the production of CO2, but you can’t see the CO2 itself. However, because it is dense, the CO2 spills over the top of the cup and collects in the dish below. When it is deep enough, it puts out the candle.
We also put several lighted matches down into the space at the bottom of the dish, and they went out too. This was quite an impressive effect for quite small amounts of chemical.

Here is how it looked:


Another noticeable effect is that the reaction is endothermic, it absorbs energy (heat), and therefore cools down. So the mixture left in the cup is much colder than what we started with. It was very noticeable to the touch, and you could see condensation on the cup as well. So, clearly we need to obtain a thermometer suitable for this kind of investigation.

The reaction made the cup quite cold and condensation formed.

Although mixing baking soda and vinegar produce CO2 as well, the mixture tends to foam. That means that the CO2 is released into the air more gradually, since the foam has to dissipate before the bubbles are released.