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Q & A: sugar water crystallizing

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Most recent answer: 10/22/2007
Q:
I did a sugar crystal experiment and observed that the crystals started forming from the top. Later it also started forming on the bottom of the jar. Why doesnít the crystals keep forming from the top and going down? How come the sugar water in the middle of the jar is still water?
- Melody
piscataway, NJ, U.S.
A:
Nice question. Hereís an educated guess.

The crystals form beause as the water evaporates the sugar concentration goes up. It goes past the saturation limit, the highest concentration that will stay in solution without forming crystals. Since the evaporation happens from the top, thatís where the concentration goes up the most first. Iím assuming the solution is sort of sticky, so it doesnít mix very quickly.

Now why do crytals then start to form on the bottom? I bet that some of the crystals from the top fall to the bottom, and start to grow there. Once a crystal breaks through the surface tension and gets off the top, itís too dense to float in the middle and will fall to the bottom.

Mike W.

(published on 10/22/2007)

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