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.