Name: ______

Rock Candy

When you make rock candy, you can see the shape of sugar crystals on a giant scale. The key is giving them lots of time (about 7 days) to grow. As the water evaporates, sugar crystals form on the string or stick, and the shapes that they form reflect the shape of individual sugar crystals.

What is sugar?

The white stuff we know as sugar is sucrose, a molecule composed of 12 atoms of carbon, 22 atoms of hydrogen, and 11 atoms of oxygen (C12H22O11). Like all compounds made from these three elements, sugar is a carbohydrate. It’s found naturally in most plants, but especially in sugarcane and sugar beets—hence their names.

Sucrose is actually two simpler sugars stuck together: fructose and glucose. In recipes, a little bit of acid (for example, some lemon juice or cream of tartar) will cause sucrose to break down into these two components.

If you look closely at dry sugar, you’ll notice it comes in little cubelike shapes. These are sugar crystals, orderly arrangements of sucrose molecules.

What happens when you heat a sugar solution?

When you add sugar to water, the sugar crystals dissolve and the sugar goes into solution. But you can’t dissolve an infinite amount of sugar into a fixed volume of water. When as much sugar has been dissolved into a solution as possible, the solution is said to be saturated. The saturation point is different at different temperatures. The higher the temperature, the more sugar that can be held in solution.

When you cook up a batch of candy, you cook sugar, water, and various other ingredients to extremely high temperatures. At these high temperatures, the sugar remains in solution, even though much of the water has boiled away. But when the candy is through cooking and begins to cool, there is more sugar in solution than is normally possible. The solution is said to be supersaturated with sugar. Supersaturation is an unstable state. The sugar molecules will begin to crystallize back into a solid at the least provocation. Stirring or jostling of any kind can cause the sugar to begin crystallizing.

What Do I Need?
• 4 cups sugar - more if needed to make a supersaturated solution
• 2 cups water
• a small saucepan
•a wooden spoon
•a small, clean glass jar
•a measuring cup
•cotton string
•a weight to hang on the string (such as a screw or galvanized washer)
•waxed paper
•a pencil (to suspend the string in the jar)
What Do I Do?
1.Heat the water in the saucepan over medium-high heat until it comes to a boil.
2. Completely dissolve the sugar in the boiling water, stirring continuously with the wooden spoon until the solution grows clear and it reaches a rolling boil. Stir in more sugar, a teaspoonful at a time. Keep adding sugar until it starts to accumulate at the bottom of the container and won't dissolve even with more stirring. This means your sugar solution is saturated. If you don't use a saturated solution, then your crystals won't grow quickly. On the other hand, if you add too much sugar, new crystals will grow on the undissolved sugar and not on your string.
3.Remove the solution from the heat, and then carefully pour it into the jar over the sink. (Try heating the jar first by running hot water over it so it won’t crack from the sudden rise in temperature from your sugar solution.) Cover the jar with a small piece of waxed paper to prevent evaporation.
4.Tie the weight to one end of the string, and then tie the other end to the middle of the pencil. The string should be about two-thirds as long as the jar is deep. Dip the string into the sugar solution, remove it, lay it on a piece of waxed paper, straighten it out, and let it dry for a few days.
•Why does the string need to be soaked and then dried?
The string will provide the surface on which the crystals will grow. As water evaporates from the string, small crystals of sugar will encrust the string. These tiny seed crystals provide starting points for larger crystals. Future growth will be concentrated around these points.
Growing a Seed Crystal = One method of getting a seed crystal is to pour a small amount of your saturated solution onto a plate, let the drop evaporate, and scrape the crystals formed on the bottom to use as seeds. Another method is to pour saturated solution into a very smooth container (like a glass jar) and dangle a rough object (like a piece of string) into the liquid. Small crystals will start to grow on the string, which can be used as seed crystals.
5.Gently suspend the prepared string in the solution and let sit at room temperature, undisturbed, for several days. You can check each day to see how much your crystals have grown. It’s tempting, but don’t touch the jar until the experiment is finished—it usually takes about seven days. If you like, you can set a coffee filter or paper towel over the jar to prevent dust from falling into the jar.
•What makes the crystals grow?
Two different methods will contribute to the growth of the crystals on the string. You have created a supersaturated solution by first heating a saturated sugar solution (a solution in which no more sugar can dissolve at a particular temperature) and then allowing it to cool. A supersaturated solution is unstable—it contains more solute (in this case, sugar) than can stay in a liquid form—so the sugar will come out of solution, forming what's called a precipitate. This method is called precipitation.
The other is evaporation—as time passes, the water will evaporate slowly from the solution. As the water evaporates, the solution becomes more saturated and sugar molecules will continue to come out of the solution and collect on the seed crystals on the string. The rock candy crystals grow molecule by molecule. Your finished rock candy will be made up of about a quadrillion (1,000,000,000,000,000) molecules attached to the string.
6. At the end of the week, the crystals on your string should be clearly defined, with sharp right angles and smooth faces of various sizes. In the field of crystallography, these are called monoclinic crystals. Their shape is determined by the way the individual sugar molecules fit together, which is similar to the way the shape of a pile of oranges is determined by the shape of the individual oranges and the way they stack together. At this point, you can pull out the string and allow the crystal to dry. You can eat them or keep them. Have fun!
What Else Can I Try?
• Try adding food coloring or flavoring to your sugar syrup before making the rock candy.

Tips:

  1. Crystals will form on a cotton or wool string or yarn, but not on a nylon line. If you use a nylon line, tie a seed crystal to it to stimulate crystal growth.
  2. If you are making the crystals to eat, please don't use a fishing weight to hold your string down. The lead from the weight will end up in the water -- it's toxic. Paper clips are a better choice, but still not great.