This is a fun and easy project, perfect for children (with adult supervision). These are the steps to make 1 "growth" of rock candy, however you can make more than one at a time.
3/4 cup of water
2 or more cups sugar
1/4 cups corn syrup
Food coloring (optional)
Form of stick (pencil,toothpick,pen,etc.)
A paper clip
A jar/glass cup
Start a pot of water boiling on the stove.
Tie a piece of string around the middle of the pencil/stick/butter knife. The string should be long enough so that when the pencil is placed over the top of the glass, the string will come up just short of the bottom.
Wet the string with a little water and roll it in the sugar. Put the paper clip on the end of the string to act as a weight and help the string stay straight. Lay the stick over the top of the glass jar so that the string hangs down inside the jar, but not touching the bottom of the jar.
When the water has started boiling, remove it from the heat and let the water settle.
Stir in the sugar and corn syrup, 1 tablespoon at a time. Continue adding sugar until it starts collecting at the bottom of the pot and won't dissolve even when you stir. This will take a while and a good bit of sugar but not too much.
If you want to add flavoring or color, stir it in now.
Pour the sugar/syrup solution into the glass until it is about 1" from the top.
Place the pencil over the jar and allow the string to dangle into the solution. Don't let the string settle on the bottom or sides of the glass.
Place the glass somewhere that it can sit for a while undisturbed (but do not refrigerate it). After a day or so, you should be seeing crystal growth on the string.
Leave the string in the solution until the crystals are big enough for you or have completely stopped growing.
Remove the string from the glass and let dry.
After you let dry please wait until crystal is formed.
To make colored crystals, you can add food dye when you add the sugar.
To add some flavor, you can add a few drops of lemon or lime juice (or other flavors) or tie a small piece of hard candy at the bottom of the string, like a piece of a peppermint or cinnamon candy.
Natural fiber string like cotton or twine will work best. Nylon fiber and other polymer string won't stimulate crystal growth very well. Or you can use wooden popsicle sticks (don't get the colored ones because the dye may not be edible!)
Glass jars with a hole cut in the lid work great and keep dust from settling into the solution. If you are using a normal glass, you can cover the top with tin foil.
If you want the crystals to grow larger, let a little air into the jar by raising the lid (you can rest it on a couple of pencils balanced on the jar rim).
If the string keeps floating, you can weigh the end down with something. Small pieces of store bought rock candy work best and will make your own crystals start to grow faster.
How does this work? The water-sugar mixture is saturated at boiling temperature (this means that it cannot dissolve any more sugar) When the mixture lowers in temperature it becomes supersaturated- meaning that it contains more sugar than it should at that temperature. When more sugar is added at room temperature the mixture "realizes" that it has more sugar than it should and the excess sugar crystallizes on the string.
A common water:sugar ratio is 1:2. For example, 1 cup of water to 2 cups of sugar.
This is a great science experiment.
Make sure an adult is supervising since you will be boiling water.
Do not mess with the jar or put your fingers in it. This disrupts the forming process of the crystalline structure. It may not prevent it from working, but it will hinder the progress of the crystal growth.
Too much sugar is bad for your teeth, and other parts of your body. Only have one piece of rock candy a day, and drink some water after you finish yours.
Rock candy is similar in appearance to crack cocaine. Make it very clear to police officers and others that it's candy.
If you are a child or teen and decide to make this, it's a good idea not to make around a stereotypical parent as they will very likely attempt to reduce the sugar content, for example from a water-sugar ratio of 1:2 to to 2:1 or less. This can result in some very dull rock candy that tastes "watered down".