Glass sugar diamonds, most commonly called rock candy, are a childhood favourite and possibly a favourite sweet treat for adults as well. What you may not know about this candy is that it is an example of the scientific process of growing crystals with sugar. The monoclinic crystals which form look like sugar diamonds. You can make rock candy at home using sugar, water and some simple household supplies. Making rock candy is a great way to teach kids how crystals grow, while promising them a sweet reward in the end.
Add 2 cups of water to a saucepan and bring it to a boil over medium-high heat. Add the sugar and stir with a wooden spoon until it has completely dissolved. You will know the sugar has dissolved when the water looks completely clear and it is at a rolling boil.
Lift the pan from the burner and carefully pour the sugar liquid into a glass jar, such as a canning jar. Be sure to do this over the sink or on a clear counter top so that if the solution spills, it doesn't drip onto your feet. Cover the jar with a piece of waxed paper.
Cut a piece of string that is about 2/3 as long as the jar is tall. Tie one end of the string around the screw or washer and tie the other end around the middle of the pencil. The screw or washer will help weigh the string down and keep it straight while you are making the rock candy.
Hold the end of the pencil and dip the string, with weight attached, into the sugar solution, allowing the weight at the bottom to touch the bottom of the jar. Lay the string on a flat surface and straighten it out evenly. Allow the string to dry for two days. Re-cover the jar with waxed paper.
Place the string back in the sugar solution after it has completely dried. Balance the pencil on the mouth of the jar so it will stay without you holding it. Leave the string in the solution for about seven days. Check back each day, and you will notice that crystals are forming on the string. After 7 days have passed, you will find that sugar diamonds or sugar crystals have formed.
Add food colouring to the sugar syrup in the jar before putting the string in the jar. This will give you coloured rock candy like you see in stores.
Don't touch the jar or the string while the crystals are forming. This will interrupt the process and ruin the formation of the crystals.