Remember the science-themed Christmas cards and decorations from a few weeks ago? After I posted that, I happened to get one of the items as a present: the crystal growing Christmas tree!

I set it up on a day where I was home to check in on it every once in a while, and I also set a time-lapse to record what was happening.

After assembling the kit, and pouring the solution that came with it in the petri dish, I waited. And waited. And waited. It took a very long time for anything to happen, but once I turned the heating on in the house, it sped up a little bit. This is a time-lapse of the first nine hours, reduced to half a minute.

The next morning, the tree looked like this!

What is this sorcery? Don’t worry, it’s just physics and chemistry in action.

What are the crystals?

The kit comes with a little plastic bag of what looks like water. It’s not water. It’s a very saturated solution of salts. I don’t know which salts are in the kit. The company that sells the trees says it’s a “magic solution”, but it’s most certainly not. (This reminds me of that time I tried to figure out the ingredients of Kracie Popin Cookin Sushi!)

I don’t have the equipment to analyse what’s in the crystal growing solution, but a few years ago the blog of a spectroscopy company got a hold of the same kit – and did an analysis of the crystals! They found potassium phosphate crystals.

How do the crystals end up in the tree?

The cardboard tree gradually absorbs the liquid from the dish below through capillary action. To get crystals to form, the water needs to evaporate. This happens much more easily from the edges of the cardboard than from the dish (or even from the middle of the tree). Where the water evaporates, the solid crystals stay behind on the tree. Because it was a very saturated solution to begin with, it was full of salt ions ready to form crystals as soon as the water disappeared, and they all end up on the tree.

This is why my tree grew faster when I turned the heating on. The water evaporated more quickly than it did in a cold room.

Why are the crystals green?

Potassium phosphate crystals are not naturally green. They’re just plain white, just like most other salt crystals (including regular table salt). What makes them green is simply dye! The tips of the tree branches contain green dye, which the crystal picks up on the way out.

Make your own crystal growing Christmas tree

You can get the tree on Amazon. (That’s an affiliate link, so if you buy it I get a small commission which helps support the site.) But if you want a challenge, you don’t actually need to buy a kit. You can also cut out your own cardboard tree and make your own solution. Steve Spangler has instructions, and the solution they use includes water, salt, bluing, and household ammonia.