Crystal Formation and Cooling Rate
Date: April 7, 2011
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.
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
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
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
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.
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
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,
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Update: June 2012