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Rocky Candy

Cool Science

overview

set up this rock candy experiment and watch your beautiful sugar crystals grow over the week. make sure to check and tend your crystals each day so the rock candy doesn’t stick to the sides of the jar. 

1 hour

maker checklist

  • 1 cup water
  • 3 cups sugar + 1 T extra
  • 2 large glass jars (about 32 oz)
  • candy coloring
  • sauce pan + stirring spoon
  • 2 wooden candy sticks (for 2 crystal candies)
  • 4 popsicle sticks
  • clothes pins or binder clips

instructions

1

create a simple syrup solution:
in a saucepan pour water and sugar at a 1:3 ratio – 1 cup water to 3 cups sugar – without stirring. let mixture come to a rolling boil before lightly stirring to make sure there aren’t any crystals stuck to the bottom of the pan.

your solution should be going from cloudy to clear. if there are any visible white pieces of sugar, you have not gone far enough!

once your solution is clear, let boil for 2 more minutes, then turn off the heat and set aside. let it cool!

the science behind it:

you can dissolve more sugar in hot water than in cold water. when you heat your water, stir the sugar into it, and then cool it back down, you have created a supersaturated solution. there is more sugar dissolved in the cooled water than would have been possible if you stirred sugar into plain old cold water. a supersaturated solution like this is unstable. over time (a week for your rock candy!), the sugar will come out of solution, forming a precipitate – that’s your crystallized candy!

2

prep your starter crystals:
dip a candy stick into the simple syrup (to make it sticky!) then dip it into dry, granulated sugar. let dry 1-2 hours.

make sure your stick is covered with syrup and sugar. this is how you create your starter crystals!

the science behind it:

the stick is the surface on which the crystals grow. as water evaporates from the stick, small crystals of sugar encrust the stick. these tiny seed crystals provide starting points for larger crystals. future growth is concentrated on these points.

3

set up your crystal candy stand:
pour your *cooled* (not hot!) solution into a large jar or cup. color it any color you like and add some flavoring too.

note: the more color you add, the more saturated the colors will be. you’ll need more than you think!

follow carefully – sandwich the un-sugared end of your skewer between two popsicle sticks, with the sugared end facing downward. clip each end of the popsicle sticks with clothes pins or binder clips (to hold in place) and rest on the top of your jar.

important! leave about 1 to 2 inches between the bottom tip of your skewer and the bottom of the jar, or else the candy will crystallize to the jar and get stuck!

5

observe the process:
the rock candy takes about a week to crystallize. check on it daily!

note: carefully break up the sugar crystals that form across the top of the liquid and around the sides of the jar.

the science behind it:
what makes the crystals grow?

two different methods cause them to grow! one is precipitation and the other is evaporation.

as time passes, the water evaporates, and the solution becomes even more unstable.

the sugar molecules come out of solution and collect on the candy rod. the rock candy crystals are growing molecule by molecule.