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Kitchen Chemistry: Making Rock Candy
Message to Parents:
This science activity can be done by K-5th grade children with parental supervision.
Kitchen Chemistry: Making Rock Candy
Rock candy makes for a delicious science experiment you can do in your own kitchen. Rock candy can form on a wooden stick or a string and you can add colors and flavors to customize your candy in any way you can imagine!
2 cups (473 ml) of water
4 cups (946 g) of granulated white sugar
1 skewer stick or cotton fiber string
Food coloring and Flavoring (optional)
- Heat 2 cups (473 ml) water in a pot, bringing it to a boil. Get an adult to help you if you’re not allowed to use the stove–boiling water can be extremely dangerous if you spill it.
- Use purified water if possible. The sugar can attach to impurities in the tap water and create a crust that will prevent water from evaporating and prevent the crystals from growing on your string instead.
- If you don’t have access to a stove, you can use a microwave instead. Combine the sugar and water in a microwave-safe glass and heat it for two minutes on high. Stir the sugar water and microwave for another 2 minutes. Stir the mixture a third time and the sugar should dissolve almost entirely into the water.
- Make sure you handle the pot or microwave-safe glass with hot pads or oven mitts so you don’t burn yourself.
- Stir in the 4 cups (946 g) of sugar in 1/2 cup (118 g) increments. Stir with a spoon after each addition until the sugar is dissolved into the water. As the water becomes more saturated with sugar, it will take longer for it to dissolve in the water. It may take as long as two minutes for the sugar to dissolve.
- Stir the solution until the water is clear. If the solution is cloudy or you find the sugar stops dissolving, turn the heat up so the water reaches a vigorous boil. Hot water has a higher saturation point than cool water, so turning up the heat should allow you to mix in the rest of the sugar.
- Remove the solution from the heat and allow it to cool for 15-20 minutes. You don’t want any undissolved sugar to remain on the bottom of the pot. If undissolved sugar ends up in the jar or glass where you grow the rock candy, crystals will attach to the undissolved sugar and not to your string or stick.
- If you have undissolved sugar that didn’t mix with the boiling water, you may want to pour the solution through a strainer and only retain the liquid.
- The solution you have made is a super-saturated solution, meaning the water has absorbed more sugar than it would have absorbed at room temperature. As the solution cools, the water saturation point of the water will become lower, and it will no longer be able to hold the amount of sugar. The dissolved sugar will be unable to stay in liquid form, and will instead crystallize on the string or stick you provide.
- Add food coloring and/or flavoring if you don’t want plain rock candy. Try to coordinate the color to the flavor–blue with blueberry, red with strawberry, purple with grape–so the flavor is clear. Make sure you stir the solution thoroughly so the flavor and color is evenly distributed.
- You only need a few drops of flavoring, but try to make the solution dark with color for the best results.
- Try mixing in drink mix, like Kool-Aid, for color and flavor.
- Try adding a splash of fruit juice for lemon, lime, orange, or other fruity flavored candy.
- Try different extracts like peppermint, strawberry, vanilla, or even banana.
Pour the solution into a large glass or jar where you plan to grow your rock candy crystals. The glass or jar should be tall and cylindrical and made of glass–plastic may melt when you pour in the hot solution. Fill the glass almost to the top.
- Make sure the glass is clean and contains no dust particles. Even dust can give the sugar crystals something to cling to, and you only want them to settle and grow on your string or stick.
- Cover the glass with a piece of wax or parchment paper to prevent dust from settling on the top of the solution.
- This recipe, since it uses only one large jar of glass, yields one candy. If you want to make multiple small candies or do not have a large jar, you can divide the solution into smaller jars. The yield would then be as many as your jars.
Making Rock Candy on a String
- Tie one end of a string around the middle of a pencil and tie a weight (such as a paperclip) to the other end. The paperclip will act as a weight and keep your string hanging straight down so it doesn’t touch the sides. The string should be about 2/3 as long as the glass is deep–it should not be long enough that the weight will touch the bottom of the glass. This will give your crystals plenty of space to grow. Touching or hanging too close to the bottom or sides of the glass could make your crystals smaller or misshapen.
- Use a string made from natural fiber, like twine or cotton. Fishing twine or nylon string is too smooth and it will be difficult for the sugar crystals to find crevices to cling to and grow.
- You can also use a washer or screw to weight the string, or even another piece of rock candy, which may help your crystals grow faster.
- The pencil should be long enough that it can rest on the top of the glass without falling in. You can also use a butter knife, skewer, or popsicle stick instead. A butter knife or popsicle stick may be more stable, as they can lay flat on top of the glass and won’t roll.
- Dip the string in the glass of sugar/water solution, remove it, and lay on a piece of wax paper to dry. Lay the string out straight, since it will become stiff as it dries. As the water evaporates, you will see a few crystals form on the string. These are seed crystals and they will help larger crystals grow around these points.
- You must make sure the string is completely dry before you proceed to the next step and be very careful not to knock off any of the seed crystals when you place the string in the solution.
- You can skip this step or try to speed it along by wetting the string and rolling it in granulated sugar (just make sure the string is completely dry before you put it in the glass and the sugar isn’t falling off), but making seed crystals will make your rock candy grow faster and increase your chances of success in growing crystals.
- Lower the string into the glass of sugar/water solution, resting the pencil on the top of the glass. The string should hang straight down and never come in contact with the bottom or sides of the glass. Cover the solution with a paper towel. You don’t want to seal the glass with something that will keep the air out, like plastic wrap, as evaporation is an essential part of this process.
- As the water evaporates, the remaining solution becomes more saturated with sugar, and the water must force the sugar out. The sugar molecules will collect on the string, forming your rock candy crystals.
- Tape the pencil to the jar to keep it from rolling or moving while your crystals form.
- Place the glass somewhere safe, where it will remain undisturbed. For the biggest crystals, look for a cool, dark space, where the water will evaporate slowly, giving the crystals lots of time to grow.
- If you want to grow crystals quickly, but don’t care if they get big, put the glass in a sunny spot so that the water evaporates quickly.
- Vibrations can negatively affect your crystal growth. Keep your glass off the ground (and away from the vibrations of people walking on the floor) and away from sources of music or noise, like a stereo or television.
- Wait one week for crystals to form. Don’t touch or tap the glass, or you may disturb the crystal growth and even cause some to fall off the string. After a week you should see big, smooth crystals on the string.
- Carefully remove the string from the solution and lay it on wax paper to dry. Snip the paperclip off with scissors. If the rock candy is stuck to the glass, run some hot water on the bottom of the glass. This should loosen the sugar enough so that you can pull your candy out without damaging it.
Enjoy and have fun learning!