MadSci Network: Chemistry

Re: Can you tell me a scientific principle for what makes rock candy form?

Date: Mon Mar 6 14:10:31 2000
Posted By: Artem Evdokimov, Postdoc
Area of science: Chemistry
ID: 951912300.Ch

Dear User,

The main principle behind making rock candy is called crystallization.

What is crystallization ? Matter can be found in several states, including 
gas liquid and solid forms. (You can include plasma, but physics is a bit 
unclear as to consider it a separate form of matter). We are now interested 
in solids - which can be amorphous or crystalline. Amorphous solids are 
often really very viscous liquids, for example, most glass will "flow" 
given time, which can be easily observed in windows, which get thicker at 
the bottom with passing of time (glass "flows" down). Other amorphous 
solids can be liquid/crystalline arrays and all sorts of intermediates 
between liquid anc crystalline phase. 

Now, what is a crystal ? Crystal is formed when molecules of the substance 
arrange themselves in infinite repetitive arrays. A good example of 
two-dimensional crystal can be created by putting small spheres (such as round 
candy or better, balls from ball-bearings) into a shallow tray. As you add more 
and more balls, you will reach the point when the bottom of the tray is more or 
less covered and the balls will start to aggregate into ordered patterns. This 
is very much like what happens during crystallization. The patterns which emerge 
when enough balls are put together will represent the lattices of the 
growing crystals. If you use objects of different shape, as long as they 
are all of the same size and shape, will form "crystalline" arrays too - 
you can easily arrange say gummy bears in an ordered pattern on a sheet of 
paper. Now imagine that the patterns are not two dimensional, but three 
dimensional and you will get a basic picture of what a crystal really is. 
From the point of view of everyday life, these patterns are infinite, or 
close to being infinite, simply because the size of the actual crystal is 
so much larger than the size of individual molecule.

There are a couple of primer books, some of which are quite old, dealing 
with crystals and crystallization. You would have to do a search in your 
local library, using the words "crystal" or "crystallization". Often the 
"science kits" sold in many toy and novelty stores will contain some 
references too, but of course, these kits are often costly and silly - most 
of the experiments which are offered in them can be performed at home with 
household ingredients and at a fraction of the cost.

Having dealt with crystals in general, I will try to elaborate on rock 
candy in particular:

Sucrose is not at all easy to crystallize, due to its "evil" properties, as 
can be seen from my older post. To grow crystals, namely, rock candy, one 
often has to use "seed crystals" i.e. tiny nuclei which will give rise to 
large, pretty crystals. Rock candy is grown around a stick or a piece of 
cord, which has been rolled around in crushed sugar crystals. Small bits 
stick to the stick or cord, and these, when in contact with supersaturated 
sugar solution or melt, result in large crystals which form rock candy. 
This is the reason why rock candy is not one large crystal - there are many 
hundreds of starting seeds, resulting in formation of many hundreds of 
individual crystals in each piece of the treat.

I hope that this provides some generalized information. Feel free to 
contact me if you require further details. I have included a couple of 
links - much more information can be gathered by searching with altavista 
or northern light using keywords "crystal growing" "rock candy" "crystal 
growth" etc.

Hope it helps,


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