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Name: Joseph
Status: student
Grade: 4-5
Location: NY
Country: USA
Date: April 7, 2011


Question:
I am doing a science project about sugar crystals. I found an article that says crystals will form faster when it is cooled slower. Why? I would think that it would be the other way around.



Replies:
I think it means that the crystals will grow bigger if cooled slower. If the solution is cooled faster, yes, the crystals most likely will form sooner.

Richard E. Barrans Jr., Ph.D., M.Ed.


This question does not have a single simple answer. For example, do you mean the growth of a single crystal, or do you mean the growth of crystals of multiple sizes and shapes? That is, do you just want to form crystals, or do you mean a single crystal? There are other complicating issues that make the problem difficult to answer. Crystal formation also depends upon whether the sugar crystal is "super cooled". It is possible to lower the temperature below the equilibrium temperature of the solution and the crystal. This is a non-equilibrium condition. In some cases, honey is an example, where the viscosity becomes large enough, it is possible to form a glassy condition where the molecules do not have enough time orient into a crystalline state. A classic case is itself -- glass. Glass "should" form a crystal of silicone dioxide, but the viscosity of the glass prevents it from forming crystalline quartz.

Vince Calder


Hi Joseph,

As you know, when you cool a sugar solution, the amount of sugar that can remain dissolved, decreases the colder it gets. Usually, you dissolve as much sugar in hot water, as the water will hold. Then you cool the solution. What happens is that since a cooler solution cannot hold as much sugar as it did when it was hot, the sugar starts to crystallize out.

But if you very slowly cool a pure solution, it becomes "supersaturated"; that is, there is more sugar still dissolved than the (cooler) solution can hold. Yet it still does not crystallize out at first! Then suddenly as you cool it further, crystallization starts. Because the solution is supersaturated, and has more sugar dissolved than the solution can hold, once crystallization starts, the crystals can grow at a very rapid rate. Sometimes this can be quite spectacular!

Once the excess sugar has crystallized out, and the solution is no longer supersaturated, things get back to normal. Further cooling results on the usual slow rate of crystal buildup that you would expect.

Regards, Bob Wilson


Hi Joseph,

The rate of temperature change of the water affects the number of sugar crystals and their size. If you cool the water more slowly, *a smaller number of larger* crystals will form, whereas if you cool the water faster, *a larger number of smaller* crystals will form.

(If you would like to know more details, keep reading, but this explanation might be challenging for 4th/5th grade level)

The process by which crystals forms is called "nucleation and growth". As the water cools, it cannot hold as much sugar, so the dissolved sugar must come out of solution. The first few molecules that come out of solution will form a little "mini-crystals". Once a mini-crystal forms, other sugar molecules will attach to it, and the crystal grows.

Once this process starts, each sugar molecule has a choice -- to form a new mini-crystal or to join an existing one. Sugar molecules would "rather" join an existing crystal -- the rules of physics make it "easier" for them to join an existing crystal (I can tell you more about this if you want -- feel free to reply). But they have to "find" that crystal first (it is actually a random physical process, not a conscious choice) -- if they can't find one, thought, they will form a new mini-crystal.

When you cool the water fast, sugar molecules are crashing out of solution, and they don't have time to find an existing crystal, so they just form their own new mini-crystal. This results in a bunch of small crystals. When you cool the water slowly, the sugar molecules have more time to find an existing crystal, so they tend to form larger crystals (but fewer in number).

Hope this helps, Burr Zimmerman



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