How to Make Sugar Crystals in Tap Water

Rock candy is made by growing sugar crystals in water.

Growing sugar crystals in tap water is an time-honored science classroom experiment. Growing sugar crystals follows the same method used for growing rock candy, that is providing an environment (still water) for the sugar molecules to move, bind together and form new crystals. The experiment is among the most commonly completed due to its relatively simple procedure and materials needed, most of which are common household items.

Things You'll Need

• 1 cup water
• 2 cups sugar
• Tall glass jar
• String
• Pencil
• Plastic cling wrap

Instructions

1. Boil 1 cup of tap water in a medium sauce pan. Add 2 cups of sugar and stir until it is completely dissolved. Remove from heat and let cool for about 10 minutes or until it is no longer scalding (for safety reasons to avoid burn splashes).

2. Wash and dry a glass 1 to 2 pint jar and fill with the sugar water. Do not fill completely to the top of jar, or sugar water will spill over and make the experiment a sticky one. Leave at least 1/2 inch of head space at the top.

3. Cut a small length of string (about an inch longer than the jar). Tie one end of the string around the middle of a pencil with a knot.

4. Dip the string into the sugar water and remove. Roll the sticky string in solid dry sugar, and allow to dry (1 to 2 hours). Dipping the string in sugar acts as the "seed" crystal to encourage new crystals formed in the jar to cling to the string rather than the bottom of the jar (if you want them on the bottom of the jar, do not dip the string).

5. Stretch a piece of plastic cling wrap over the top of the jar. Poke a hole in the middle with a pencil. When the string is dry, lower it through the hole in the top of the jar (still attached to the pencil). Rest the pencil over the center of the jar allowing the string to hang suspended in the sugar water.

6. Wait 1 to 2 weeks for the crystals to grow. Do not touch the jar while the crystals are growing, keep it in a safe location out of the way but in a location that allows for you to easily observe the progress.

Tips & Warnings

• Add a few drops of food coloring to the jar to grow colored crystals.

References

• The Accidental Scientist: Science of Rock Candy
• Sea Why: How to Grow Your Own Sugar Crystals

Resources

• CrystalGrowing.com: Recipes for Growing Sugar Crystals