Crystals are pretty. That’s the justification for this experiment – it’s Christmas and we like things to look nice. We’re going to make some crystals and look at how they form and then try and make some crystal decorations for your Christmas tree with them too!
What is a crystal?
You can dissolve some things in water – things like table salt and it’s like they disappear. If you boil that container though the water will leave and float away as steam but the salt you added won’t! It will stay in the pan and form back into a solid.
The salt will arrange itself really carefully and closely together in a really regular pattern – it’s like a big grid. Once you get a few showing the rest of the salt how to line up it all falls into place and all the salt gets into the pattern. This means that the structures grow and start off really small but get bigger and bigger and bigger as the water evaporates. What shape you get depends on the salt that you use.
2020 Edit: Want to find out more about crystals? Click here to find out about snow!
Take some boiling water and start adding your salt a bit at a time, stirring hard until all the salt dissolves. Eventually, you’ll reach a point where no more salt will dissolve and there will be some visible salt left in the bottom.
When you reach that point, add a tiny tiny tiny amount of water and stir again to dissolve that last bit of salt. We don’t want to be able to see any salt at all in the container! This solution is not saturated. It contains as much salt as it can possibly carry. Hot water can carry and dissolve a lot more salt than cold water though, so as the water cools down the water won’t be able to carry all the salt and it will want to form back into visible bits of solid salt. The salt will follow the pattern and arrange into a crystal! Let’s make some cyrstal shapes…
Sodium chloride (table salt) makes beautiful cubes and really quickly too!
Magnesium sulfate (Epsom salts) make needles – they are a bit trickier to work with, though, but do form lovely crystals.
Another thing that can help crystals to form is evaporation. If you have a hot solution of salt in water and leave it open to the air the steam will show you that water is leaving that container. That means there is less water there to help carry the salt – so more will crystallise. Our combination to get quick crystals, then, is to have a hot solution that is open to the air so it can evaporate. With the Epsom salts you might find that the crystals form a web over the water surface and trap all the water in as it tries to evaporate! You might see some fun results with bubbles trying to break through… like this!
How to make these crystals:
I’ve done this by placing some foil over the top of a mug of boiling water and pressed it down in the middle a little bit to make a bowl shaped bit of foil over the top. I’ve then poured my hot salt solution into the foil ‘bowl’. The hot water underneath keeps our salt solution really hot but it is open to the air so it evaporates fast. The salt that was dissolved will start to evaporate.
Making a crystal decoration
The crystals all have to line up in the right way to form a crystal – if we give them a hint about how to start arranging themselves then we can get the crystal to form in a certain place. I’ve used some pipecleaners to make some Christmas shapes like a candy cane. Then if I put those pipecleaners into a solution that has loads and loads of salt dissolved in it – that salt wants to come out and crystallise – then the pipecleaner’s fluffy surface gives hints to the salt about the pattern they should form to make a crystal. Hopefully, and with a lot of patience, you’ll be able to grow some lovely crystal decorations!
I’ve set this one up and will update this post with the progress as the crystals form into our crystal decorations…
Update from a few hours later…
I left it too long and didn’t think the bottle through.. Once the crystals had formed around the pipecleaner it was too chunky to fit through the opening of the bottle! The whole thing crystallised and I was left with a solid bottle full of crystal. Oops.
2020 Edit #2: There are some people out there much better at making crystal decorations than I am! I think it’s important to leave some of these experiments up where it doesn’t quite work out because you will have those experiences trying them out at home. Trying to do a different experiment every day while doing a job and organising Christmas – things will go wrong. So instead, here’s Tiffany Chan making beautiful crystal decorations as part of the Global Science Show!