April 9, 2003
A: FROM MENTOR KAT DRENNON IN KY
That is a very good question, Jewel. Let's start by thinking about water. You should now by now that when water freezes into ice, it expands. That is because the molecules in water form weak bonds called hydrogen bonds. These bonds occur between the hydrogen and either oxygen or nitrogen on adjacent molecules. Since a water molecule is made up of two hydrogens and one oxygen, all bonds formed between molecules are hydrogen bonds (not to be confused with the bonds between the atoms of a single molecule, which are covalent).
Similarly, rock candy, which is made mostly of sugar molecules, also forms hydrogen bonds. The molecular structure of sugar is more complex than water, obviously, but the principal is the same. Hydrogen bonds form between the hydrogen and oxygen atoms on the outside of adjacent molecules, thus making the beautiful formations that look like real rocks. Real rocks go through a similar process, but do not have as much oxygen in them. The bonds formed in real rocks are mostly covalent, which is why they are so much harder and less brittle than candy.
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A: FROM MENTOR JOAN LUSK IN RI
Many pure substances crystallize. Rock candy is basically sucrose (ordinary sugar, one of the purest compounds you can buy in the supermarket.) Another d household compound that easily forms large crystals is table salt, NaCL, sodium chloride. If you make a hot, concentrated solution of either and let it cool (and evaporate) slowly, undisturbed, you'll get big crystals. A string hanging in the solution can provide nucleation sites and a support for the crystals, giving you a sample to hang up.

Crystals form when copies of the same molecule find a way to pack together and fill up three-dimensional space in a repetitive pattern. Some molecules do this more readily than others, but most find a way to pack. Rocks often contain crystals of different mineral composition, sometimes appearing as large and beautiful shapes. These shapes take on many different forms. I looked for some web sites that would show you pictures of crystalline minerals and found http://www.mineralminers.com/index.htm and
http://dmoz.org/Science/Earth_Sciences/Geology/Rocks_and_Minerals/Rock_and_Mineral_Localities/

As specimens of crystals, rock candy and salt are fine so long as you don't get them wet - they'll dissolve in water.

Have you every noticed the crystals that form in honey that has been stored a long time? These are just sugar, and dissolve if you heat up the honey, and no harm has been done to the honey. But the main sugars in honey are glucose and fructose, not sucrose. According to http://www.mari-el.ru/homepage/nadir/what_healing_honey_must_be.html glucose is more easily crystallized from honey than fructose. It might make a good science project to look at the different crustal forms of different sugars under a microscope and to try to crystallize both glucose and fructose from honey, or to look at the relative amounts of glucose and fructose in honeys made from different nectars. Just a thought!