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Name: Cynthia
Status: student
Grade: 6-8
Location: NY
Country: USA
Date: Winter 2012-2013


Question:
What makes sugar crystals form? Is it because of the structure? When comparing the speed and size of growing white sugar crystals and brown sugar crystals, if one grows faster than the other, why is that so?



Replies:
Hi Cynthia,

Thanks for the good question. Sugar crystals form because the solution has too much sugar dissolved in it. In order to "get rid" of the excess sugar, crystals grow and remove excess sugar from the solution. The type of crystal (square, rhombus, etc.) you obtain depends on how the molecules pack together in the solid phase. Growing crystals from brown sugar will be slower and will result in crystals of lower quality By "quality", I am referring to the size and clarity of the crystal. Brown sugar has impurities in it--that is why it is brown and not white. These impurities slow down the crystal growth process and give lower quality crystals. White sugar crystals do not have these impurities.

I hope this helps. Please let me know if you have additional questions. Thanks Jeff Grell


What “makes” a substance form a crystal is not the best way to frame the question, because it implies that there is “some factor” driving the molecules to arrange themselves in a crystal lattice. That is misleading; the process of crystal formation is much more complicated. For example, by slowly and carefully COOLING a solution or the liquid form of a molecule, it is possible to reduce the freezing temperature of a substance. This is known as “super cooling”. However, by carefully HEATING a crystal it is not possible to increase the temperature above the melting point of the substance. This implies that cooling and melting are fundamentally different processes. In a short answer format such as NEWTON it is not possible to dig very deeply into the mechanisms involved in the two processes. You can find out more details on the subject on the web site:

http://en.wikipedia.org/wiki/Crystallization

and

http://www.newton.dep.anl.gov/askasci/chem03/chem03139.htm

Vince Calder


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