Carolyn N. Kinder
Crystals On A String
Objective The students will observe how some crystals are formed.
Materials Needed A hot plate or other heat source, a kettle, a spoon for stirring, about 500 g of sugar, alum, salt, three fruit jars, water, three pencils, three paper clips, string and scissors.
Procedure Tie a l5cm length of string to the middle of a pencil.
Then tie a paper clip onto the string’s other end. Place the pencil across the top of a jar, so that the paper clip hangs in the middle of the jar. If the clip hangs down too far, wind the string around the pencil until you find the correct position. The string and paper clip will act as seeds for the crystals. Pour 250 ml of water into the water, stirring it until it dissolves. Keep adding sugar until no more will dissolve in the water. Now pour the solution into one of the prepared jars, submerging the paper clip. Put the jar in a place where it will not be disturbed and observe it for the next several weeks, watching crystals form on the string. Because sugar crystallizes slowly, the formations will be small if the jar is moved at all. Repeat the procedure using salt and then alum. The alum will crystallize immediately; salt takes a little longer, usually overnight. Have your students observe the crystals with a magnifying glass and under a microscope. Can they see the same shape repeated again and again? How are they alike? How are they different? Why?
Discussion The crystals that the class is creating are formed chemically by putting simple substances into supersaturated solutions using heat and evaporation to change them from liquids to solids. All elements have their own unique crystalline shapes, and they will always form these shapes when solidifying. For example, salt always forms cubes when it crystallizes. This experiment is taken from Nona Whipple and Sherry Whitemore,
Science and Children, “crystal Creations: Crystals On A String”, January 1989, page 16.