• Question: How hard is it to make a crystal?

    Asked by cheeseman to Jean on 16 Nov 2012.
    • Photo: Jean Bourke

      Jean Bourke answered on 16 Nov 2012:

      The types of crystals I make are sugar crystals. Recrystallisation is where you have a crystalline solid that you dissolve and recrystallise to try and make it really pure. This happens because all the different molecules fit together to make the crystal, molecules that are the same will fit together to make clean pure crystals. If you make the crystals really quickly all the molecules will fly together really quickly and other bits of dirt can get trapped in between the layers of the stuff you want to purify.

      You could probably make sugar crystals with the regular sugar at home or the salt at home.

      Below I’m going to describe the scientific method for making crystals:
      1)Put a bit of your crystalline solid in a container (we use beakers, or test tubes)
      2)Add the solvent, use less than you need
      3)Heat it gently: warm solvents dissolve more solids (just like sugar in a cup of tea)
      4)Anything that doesn’t dissolve is dirt, filter this off (just like a coffee filter)
      5)Cool your solutions slowly
      6)Watch your crystals form
      7)After it’s a bit cool you can put your container on ice to cool even more, or if you’re not getting crystals you can force the process by putting it on ice.

      What happens is you make what we call a super saturated solution, this is one that has more solid dissolved in it than it would like. That way the extra solid will come out of solutions (we call this crashing out) and then as you further reduce how much solid can be in the solution, you can grown your crystal (you can do this by letting some of the solvent evaporate over time).

      The exact method I use is a bit different as my sugars do not dissolve in water, I have to use different solvents. I can dissolve my sugar in a solvent that it is happy to dissolve in, then I can slowly add another solvent (that will mix happily with first solvent but which my sugar does not dissolve in) little by little. As the conditions of the solution change, my sugar no longer wants to be in the solutions so it “crashes out” and forms crystals.

      Getting the mixture of solvents exactly right is the hard bit 🙂
      Sometimes it doesn’t work at all, when that happens you just dry your sugar by evaporating off the solvent and start again.

      Give it a try with some salt. You can just use some table salt and a bit of warm water from the shower. If you put in something for the first crystals to start forming on (we call this a seed crystal but you can use a grain or 2 of sand or a little bit of thread or string) it can encourage them to grow, and to grow in just one place making big crystals.