Crystal Candy Explained

The last experiment we did was "Crystal Candy," and you may find yourself asking
Caroll, how does this relate to science? What kind of lesson can I put with this?
Have no fear! I am here to explain.

What did the lab explain?
This lab is meant to explain the idea of crystal growth. In this experiment you can watch the crystals growing. (You don't want to sit and stare at the cup though... remember 'a watched pot never boils.')

This is as fun as watching paint dry...

After the cup has been left and the sugar has recrystallized you will see the faces, or sides, of the sugar crystals.

Why does this happen?
How do the sugar crystals form? Well, when you heat the sugar water you allow for more sugar to be added. If you had a cup of room temperature water and added two cups of sugar only some of it would dissolve and most of the sugar would sit in the bottom of the cup. However if you heat the water the sugar will dissolve. This is because heating the solution makes the molecules move faster and allows the sugar to dissolve into the water. This causes a supersaturated solution of sugar and water.

Crystals cannot form without something to grow on. In our experiment the skewer is our crystal holder. In nature it can be a rock or even another crystal. Crystal growth depends on solution (or chemical) type, time, temperature and the space available. A sugar solution is not going to produce quartz crystals. All minerals have their own chemical makeup and a crystal's growth and structure display this. Some minerals have specific shapes that they grow in. Sugar crystals grow very quickly but minerals like quartz, chert, and calcite take a long time to grow.

Calcite Left; Quartz Right

How fast crystals grow is also dependent on the temperature in which they are growing. Often, it is easier for a crystal to grow in hotter temperatures than cooler ones. What limits the size of crystals though? If you noticed in this experiment the sugar was growing on the skewer but it could not grow outside of the cup. This is the same in nature, a crystal cannot grow outside of the space available for it. A really good example of this is the geode. A geode is a small, durable rock that is often hollow on the inside. In this hollow area crystals can often be found. If the crystals growing in the space reach the center from all sides the crystal must stop growing.

Quartz Geode

This is what happens when you leave it sit too long.

~Caroll