The first thing I have to say about this activity it that it’s sticky, messy and doesn’t always work, but when it does it’s pretty amazing. Our rock candy recipe has been perfected from many, many attempts to get it to work, so hopefully we’ve done most of the hard work for you.
The first time we tried nothing grew at all as the solution wasn’t concentrated enough, the second time we made it too concentrated and the whole solution crystallised. The third time it worked!!
Rock Candy Recipe
3 cups of caster sugar
1 cup of water
A lolly stick, wooden stick or circle of wire/string
Some sparkles and/or food colouring ( optional )
Rock Candy Instructions
Dissolve the sugar in the water, as soon as it’s dissolved, remove the heat and leave to cool a little bit. If you can get a bit more sugar to dissolve then thats a good thing. You want a saturated solution.
Pour the solution into a glass jar and suspend the lolly stick, we used some sellotape to hold it in place. Don’t let it touch the bottom or the sides or the jar.
Alternatively you could tie some string to a pencil and rest the pencil on top of the jar with the string hanging in the jar.
You should see crystals start to form after a few days.
We made a lolly and a circle to hang on the Christmas tree. I was a bit disapppointed with the circle, but we will be trying again with a star!
Why does sugar crystallise?
A crystal is a solid material with a naturally geometrically regular form. Some take millions of years to form, such as diamonds. The sugar crystals we made above take just a few days.
Most minerals dissolved in water will form crystals given enough time and space. The shape of the crystal formed depends on the mineral’s molecule shape.
In the case of our sugar crystals there are two process at work.
Evaporation – the water evaporates slowly meaning the solution becomes more saturated, so the sugar molecules come out of solution and collect on the string/wire or stick.
Precipitation – the solution we made was very concentrated which means there was too much solute to remain dissolved in the water, therefore it starts to precipitate.
What is a saturated solution?
A saturated solution is a solution that won’t dissolve any more solute at that temperature. Our water and sugar solution was saturated when no more sugar would dissolve.
What is a solute?
A solute is the solid being dissolved. When making rock candy, sugar is the solute and water the solvent.
What is a solvent?
This is the liquid the solute is being dissolved into.
What is a solution?
This is what we call the solute and solvent.