Sunday Science

Making Rock Candy

Simple science experiment you can do with your kids at home

Sunday is the favorite day of the week in our family for two reasons: One — the day starts with pancakes; Two — we get to do a science experiment.

Photo © Kris Wickremasinghe

My daughter is eight this year, and she’s entering that inquisitive age which is ideal for getting introduced to everyday science. Science doesn’t have to be something crazy looking old people do in white lab coats with flat bottomed flasks and test tubes filled with colored liquids. There is a lot of science we can observe with what we do in our everyday lives. If what we do in our everyday lives is not that interesting, then there is a lot of things we can do with everyday materials found around the home that may involve a few colored liquids and sometimes very tasty outcomes too. Making rock candy is one of those experiments. (Some of you may call it cooking — but us dads call it a science experiment!)

What you need

• 1 cup of water
• Small saucepan
• Lots of sugar (at least a bag of 500g)
• Clean empty jar (An empty Jam jar is ideal)
• A wooden skewer
• Food coloring
• Some flavoring — vanilla or rose essence

How do you make rock candy? A sneak peek at the science behind the process

First, you have to dissolve lots and lots of sugar in water. To help dissolve more sugar we heat up the water, as the ability of water to have more sugar (or any other soluble material) dissolved in it increases with temperature. At the boiling point, water is able to absorb the maximum possible amount of sugar. After making sure we have dissolved as much sugar as possible (this is called a supersaturated solution), we add a few sugar crystals (we can call them catalysts) into it and leave it for about a week. As the water cools it loses its ability to keep all that sugar dissolved and the excess sugar starts crystallizing around the existing sugar crystals. Over time evaporation of water also helps this process.

Word on safety

We are going to boil the water and dissolve lots of sugar in it until it becomes a thick sticky syrup. Very hot, thick sticky syrup is extremely dangerous and can cause really bad burns. So this is an experiment where most of the hard work will have to be done by a responsible adult. There’s still a few things the little scientists can help with including taste testing!

Detailed method

1. Before you start making the solution, get the jar prepared. It needs to be clean. Make a few holes in the lid and slide the wooden skewer through one of them. Make sure when the lid is closed, the skewer tip leaves a gap of about an inch between itself and the bottom of the jar — it should not touch.
2. Heat the water in the saucepan and bring it to boiling
3. Start adding sugar with a tablespoon, one spoon at a time. Stir in to make sure it is completely dissolved before adding the next spoon.
4. Keep adding sugar and stirring in. After a while, you will notice that it takes longer to dissolve as the consistency of the liquid thickens. The end result of the experiment depends heavily on making sure the solution is as concentrated as possible. So be patient, take your time to stir it in, and if you manage to get all of it dissolved, add a bit more and repeat. Last time we tried it, it took about 40 minutes for two cups of water.
5. When the syrup is getting thick, now is a good time to add the coloring and the flavoring. If you add the flavoring too early, it may all evaporate by the time you finish making the syrup. So try and delay it as long as possible.
6. When you feel like it is not going to take any more sugar, take the saucepan off the heat and carefully (Super carefully! You don’t want any of that stuff on yourself or your kids at that temperature) pour the syrup into the prepared jar.
7. Now is a good time for a little assistant to step in: The skewer needs to be dampened a bit by sprinkling a bit of water and needs to be coated with some sugar. It doesn’t need to be covered in sugar, just a few crystals hanging on to the skewer is enough.
8. Close the jar with the lid inserting the skewer carefully.
9. Now, wait! Leave it in a cool dry place for about a week. Don’t forget to check what’s going on inside the jar from time to time.

After about a day you can see the rock candy start forming on the skewer.

After 1 day — © Kris Wickremasinghe

After a week, if you are not extremely unlucky, you should see something like this:

After a week — © Kris Wickremasinghe

It looks absolutely amazing and tastes amazing too, especially if you are a kid!

If you were extremely unlucky, and the above didn’t happen, it is probably because you were a bit impatient and didn’t make the solution concentrated enough. Failing is part of science — so all you can do is try again!