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Chemistry Experiment: rock candy

Grow your own crystal rock candy!



This experiment is adapted from the one by Anne Marie Helmenstine, found at About.com.

Materials:
  • A cup of water
  • 3 cups of sugar
  • A clean glass jar, like a mayonnaise jar (but make sure you clean it properly)
  • Pencil, or any long stick
  • Cotton string (a piece that’s long but not longer than the jar’s height)
  • Pan or pot, and a stove
  • Lifesaver candy
  • Food colouring and something for taste (like fruit extract) (optional)

The experiment:

1. Pour the sugar and water into the pan, put it on your stove and boil it (CAUTION- let an adult do this!). Don’t cook it any longer or else you’ll get caramel, but make sure all the sugar chunks melt.

2. Stir well. You may add some extract from an orange or a lemon for flavour, and some food colouring for colour. Put it in the fridge to cool for like 15 minutes, until it’s just below room temp.


3. Prepare the string- make sure it’s a clean, non-toxic cotton string. tie one end of it to the lifesaver (to keep it from floating), and the other hand to the middle of the pencil or stick. You want the string to hang into the jar from the pencil (which will rest on top of the jar), but not touch the sides or bottom.

4. Pour the solution in to the jar, but leave a bit on the side and soak the string in it. Let it dry a bit, and you can also pour some sugar on it- it’ll help the crystals form easier.

5. Now, place the string into the full far, and set it all aside for about a week. Check on your crystals, but don’t touch them. If nothing begins to grow within 3 days, recook the solution and start over.

6. Once your rock candy looks nice and edible… eat it! Yum!

You can watch a video of how to make a rock candy right here. The video uses slightly different amounts and a pipecleaner instead of a string- make sure you use a real string though, otherwise it won’t work.

The chemistry behind the scenes:

This experiment has to do with solutions and dissolution. Water is a natural solvent, since many substances can dissolve in it. Sugar is a good example- sugar contains a lot of polar hydroxide groups that cause it to dissolve in water fairly easily.

However, all substances can only dissolve up to a certain point. At that point, the solution becomes saturated, and if you add any more sugar, it will remain solid because it has no room to dissolve.

When we heat water and sugar, we increase the saturation capacity, so more sugar can dissolve. This is because heat causes molecules to move apart and have more energy, so the sugar molecules can dissolve in hot water better than in cold water. Then, we cool the water down, and the solution becomes supersaturated- there’s too much sugar!

When we place the string in a solution and then in the jar full of the supersaturated solution, the sugar will be attracted to the existing solid molecules stuck to the string and form a nice crystalline structure, since sugar is a crystalline substance in nature. As days pass, water will evaporate but the sugar will remain in the solution, causing it to become even more saturated and thus more sugar will be added to our crystal, causing it to grow.

This is exactly how they make real rock candy you can buy in stores! Chemistry can’t only make volcanoes and pretty pictures, it takes care of tasty things too!

Chemistry is cool!

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