Table sugar crystals, magnified 100 times
Rock candy crystals, magnified 250 times
Rock Candy on a stick
Rock candy is a simple sugar candy that can double as a science experiment. The process can take up to a week, but it’s fun to watch the sugar crystals growing over time. Note that the exact quantity of sugar syrup you will use depends on the size of the jar you have. If you want to make several pieces of rock candy, use multiple jars and skewers, and double or triple the sugar syrup solution as necessary.
Ingredients:
2 cups water
4 cups granulated sugar
1/2-1 tsp flavoring extract or oil (optional)
food coloring (optional)
glass jar
skewer or thread (see below)
Preparation:
1. Prepare your materials: wash a glass jar thoroughly with hot water to clean it. Cut a length of thick cotton thread a few inches longer than the height of the jar, and tape it to a pencil. Place the pencil across the lip of the jar, and wind it until the thread is hanging about 1 inch from the bottom of the jar. Attach a paper clip to the bottom of the thread to weight it and ensure it hangs straight down. Alternately, you can use a wooden skewer as the base of your rock candy, and use clothespins balanced across the top of the jar to clip it into place.
2. Wet your thread or wooden skewer, and roll it in granulated sugar. This base layer will give the sugar crystals something to “grab” when they start forming. Set the thread or skewer aside to dry while you prepare your sugar syrup.
3. Place the water in a medium-sized pan and bring it to a boil. Begin adding the sugar, one cup at a time, stirring after each addition. You will notice that it takes longer for the sugar to dissolve after each addition. Continue to stir and boil the syrup until all of the sugar has been added and it is all dissolved. Remove the pan from the heat.
4. If you are using colors or flavorings, add them at this point. If you are using an extract, add 1 tsp of extract, but if you are using flavoring oils, only add ½ tsp, and make sure you don’t stand right in front of the pan—the scent can be very strong as it rises in the steam. Add 2-3 drops of food coloring and stir to ensure even, smooth color.
5. Allow the sugar syrup to cool for approximately 10 minutes, then pour it into the prepared jar. Lower the sugared string or skewer until it hangs about 1 inch from the bottom.
6. Carefully place your jar in a cool place, away from harsh lights, where it can sit undisturbed. Cover the top loosely with plastic wrap or paper towel.
7. You should start to see sugar crystals forming within 2-4 hours. If you have seen no change to your skewer or thread after 24 hours, try boiling the sugar syrup again and dissolve another cup of sugar into it, then pour it back into the jar and insert the string or skewer again.
8. Allow the rock candy to grow until it is the size you want. Don’t let it grow too large, otherwise it might start growing into the sides of your jar! Once it has reached the size you want, remove it and allow it to dry for a few minutes, then enjoy or wrap in plastic wrap to save it for later.
• 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
