“Mad Science” Mini-Lessons

3. Rock candy


Solutions, solubility. supersaturated solutions, purification by recrystallization


Heat source (e.g. hot plate, mbaula, stove); pot for heating syrup; holders to safely handle the hot pot; stirrer; potable water; sugar; clean string; clean, heatproof cups (one for each student group); stick or rigid bar to place across the top of each cup; paper, plastic wrap, plastic bags, or foil to cover the tops of the cups


Cut the string into segments about 20 cm long.  Make one segment for each student group.  It is best, but not required, for the string to be pre-treated to contain seed crystals.  To do this, make a small amount of sugar syrup as described below, completely soak the string segments in it, and allow them to dry on a plate (covered to exclude vermin) for about two days.  You may sprinkle the strings with sugar to induce crystallization.

For the syrup: make about ½ cup (70 mL) of syrup for each student group.  Place a volume of granulated sugar equal to the volume of syrup you want into the cooking pot.  Add half that volume of potable water to the pot.  Heat the pot until all the sugar dissolves, stirring constantly.  If sugar crystals cling to the sides of the pot, cover the pot while heating without stirring, checking and stirring occasionally.  Remove the pot from the heat when all the sugar is dissolved.  Allow the syrup to cool until it is cool enough to pour into the cups. Pour equal amounts into the cups, and allow the syrup in the cups to cool.  When the syrup has cooled to room temperature, place a stick across each cup, hang a string over each stick so that it hangs into the syrup, and cover each cup to keep out pests (cockroaches love syrup).

Check the cups daily.  Sugar crystals should grow on the string (and on the sides of the cup) after a few days.  The crystals should be lighter in color than the sugar used, demonstrating the purification technique of recrystallization.  The sugar is purified in the crystals, while the impurities are concentrated in the remaining syrup.

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Revised: 21 December 2016; Maintained by Richard Barrans.
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