Experiments at the Children's Desk - Rock Candy!

Tuesday, July 22, 2014

Last week, we debuted our special Fizz, Boom, Read! Summer Reading display at the CHPL Youth Services Desk. It's been a very sweet week--we moved from gummy bears to rock candy! We started out with this recipe from About.com, and, as always, did it in our own special way.

First, we followed the recipe *almost* exactly how it was presented, using 3 cups of sugar to 1 cup of water. This proved difficult, especially given that we were using a hot plate rather than a traditional pot on a stove. It was difficult to get 3 cups of water to boil because the device is meant to heat water for food and does not go past a certain temperature. We then microwaved the mixture which we believe helped the experiment yield good results the first time around. We hung a string on a popsicle stick over a mason jar (how stylish!) and used a paperclip to weigh it down. Were we to be eating this candy, we probably would not have used a paperclip for safety reasons. The result was great after one day:

We decided to see what would happen if we put a new string in the old mix. We ended up with a jar full of caverns and groups of sugar crystals, but not another string of rock candy. We read that we might be able to re-use a mixture--but we would have to reheat it in order to rearrange the molecules once more. Below you will find an explanation for this issue as well as the basic scientific concepts behind growing sugar crystals.

The basic principle behind growing sugar crystals is to put so much sugar in the water (3 cups sugar to one cup water in our case) to the point where the water can no longer contain all of the sugar molecules. When this happens, the sugar will creep out of the
water, forming crystals.

Q & A

Q. What do you think we did to the water to make the sugar

A. We had to boil the water in order for the sugar to dissolve.

Q. Why do we have to boil the water?

A. When the molecules are heated, they move around more,
making room for more molecules. When the molecules freeze, they slow down and expand, making less room inside a solution for other materials. When making a sugar solution designed to make crystals, we have to completely saturate hot water with sugar until no more can dissolve. This makes it possible for the crystals to grow when the water cools.

Q. Why don’t the jars of sugar-water have lids?

A. This experiment also requires the water to slowly evaporate from the jar to make room for the sugar to form crystals.

Another way to ensure a more successful batch of rock candy is to wet and dry the string and rub sugar on it before putting it in your jar/container. The sugar acts as a "seed" and guides the molecules toward it. When we did not boil the water all of the way, we were significantly less successful. We also found that the project was much more manageable when we used 1/3 of the recipe, but be careful using such a small amount of water on the stove--you don't want to ruin your pan/pot!

This project was a LOT of fun, but following directions is really important. We imagine making these with flavors would be absolutely delicious!


Our most successful batch--this is the result after about 4 days. We were able to dissolve the sugar until the water was clear again because we made a smaller batch. Since the water was fully saturated, perfect crystals formed as we waited patiently!

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