The Accidental Scientist: Science of Cooking
recipe: Rock Candy

What’s Going On?

 

Why does the string need to be soaked and then dried?

The string will provide the surface on which the crystals will grow. As water evaporates from the string, small crystals of sugar will encrust the string. These tiny seed crystals provide starting points for larger crystals. Future growth will be concentrated around these points.

 

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.

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