There are several variables that affect how
long it takes
sugar crystals to grow from a water solution.
Crystals form from sugar dissolved in water
when the solution becomes
supersaturated. When a solution is saturated, it
has reached its
maximum equilibrium concentration. As water
evaporates from a sugar
solution, it will become more and more
concentrated until it is
saturated. Then if more water evaporates, the
solution can become
supersaturated. When a solution is supersaturated
there is a driving
force for crystals to form, which lowers the
concentration of the
solution back to the equilibrium value. However,
the crystals must
nucleate before crystallization can occur.
Crystals can nucleate from
random clusters of sugar molecules that sticks
together due to
statistical fluctuations, from impurities in the
solution like pieces
of dirt, or at interfaces of the solution with its
container or even
at the surface (interface with air). When people
grow sugar crystals
they sometimes use string or wooden sticks to help
Initially the rate of crystal growth depends on
the rate of
nucleation. Then, after the crystals are
nucleated, the rate of
crystal growth depends on additional nucleation as
well as the rate of
evaporation of the water and the surface area of
crystals. The faster the water evaporates, the
concentration increases. The larger the crystals
are, the larger the
surface where more molecules can be added. Both of
these factors will
increase the rate of crystallization.
As explained, crystallization occurs when a
supersaturated. The solubility of sugar (and other
molecules and ions)
depends on temperature, so another way to make a
solution is to cool the solution. As the solution
solubility of the sugar decreases, but the
concentration remains the
same. Eventually, the solution will reach a point
equilibrium solubility has dropped below the
concentration and the
solutions is supersaturated (assuming the solution
does not freeze
before this happens).
All samples of a sugar solution in perfectly pure
water with the same
concentration under the same conditions should
behave in the same way.
However, if there are impurities in the water,
they could affect the
crystallization of the sugar. Different ions and
pH can change
solubility which would change the crystallization
rate, so adding salt
or acid to the water (or using ocean water or
slightly acidic rain
water) may slightly change the solubility of
sugar. Here is what I
have been able to find in the published literature
about how adding
other things to water changes the solubility of
- If you add alcohol (ethanol or methanol) to
water, sucrose becomes
less soluble in the solvent mixture which should
cause it to
crystallize more readily.
- If you add propionic acid to water, sucrose also
soluble which should cause it to crystallize more
However, having extra molecules in the solution
could affect how
easily sucrose molecules can reach a crystal
surface to add to it,
which may slow down crystallization.
You could do an experiment to find if other things
in water affect the
rate of crystallization if you want to know!
For more information on crystal growth, please
Phase Equilibria of D-glucose and Sucrose in Mixed
Comparison of UNIQUAC-based Models. Peres, A. M.
and Macedo, E. A.
Carbohydrate Research 1997, 303 135-151.
Sucrose Solubility in Mixtures of Water,
Alcohol, Ester, and Acid. P.
Tsavas,S. Polydorou, E. C. Voutsas, and, K. G.
Magoulas, K. Naraghi
and, and P. J. Halling. Journal of Chemical &
Engineering Data 2002 47
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