Every summer when I declutter my science supplies cupboard I come across a few hidden treasures. (The reality: “Oops, we never did get around to making shadow leaf prints / home made light bulbs / popsicle stick trebuchets,” accompanied by a pang of guilt. Is that just me?)
The Epsom salts were bought to make bath fizzies with C(11) last Christmas. The fizzies never happened, but on the bright side, we had an unopened pack of Epsom salts when CSIRO’s cool crystals email landed in my inbox.
J(10) asked if Epsom salt was like the stuff we put on our fries, which was a good opportunity to remind ourselves what we learned about salts when we concocted our own fizzy drinks a few months back: A salt is created when an acid and a base neutralise each other.
Epsom salt is another name for magnesium sulfate. We looked up magnesium and sulfur in our book, The Elements, and noticed how very different the salt is from its constituent elements.
What you need
Epsom salt (1/2 cup)
Hot water (1/2 cup)
Food colouring (optional)
What you do
Put the salt and water in the glass together with a few drops of food colouring. Stir for about five minutes, then put the glass in the fridge for at least three hours.
After just a few hours in the fridge, you get beautiful crystals like these.
We carefully drained the water to get a better look at our crystals.
How do crystals form? The scientific explanation
Epsom salt is an ionic compound. It’s made up of magnesium and sulfur ions joined together by ionic bonds. When we dissolve the salt in hot water, these bonds break and the two elements become separated .
Later, when we cool the salt solution in the fridge, the magnesium sulfate ions no longer have enough energy to move about freely. The ions begin to re-bond, first as single molecules and then – as the molecules themselves begin to join together – as crystals.
Science Kids at Home has some cool diagrams showing what’s happening at a molecular level.
More crystal science
Different types of molecule always the same shape of crystal, every time they form.
In the past we’ve also made crystals from table salt (sodium chloride), borax and sugar. The process for each of those is slightly more fiddly, but comparing the different shaped crystals is interesting. Borax crystals make pretty decorations, and sugar crystals are yummy!
And now the Epsom salt packet has been opened, we’re one step closer to making those bath bombs. 🙂