Ask A Scientist Chemistry Archive

 
name         Queenie P.
status       student
age          13

Question -   What is the effect of water purity on the rate at which sugar dissolves?

Sorry, Queenie, I really do not know.  Lots of different things happen with speeds of chemical reactions like dissolution.
Personally, I doubt that hard water from the tap will dissolve sugar any slower than the best purified water.
But I am not sure.

Water is mostly water, so the dissolution rate of sugar should remain the same, unless:
  the impurities in the water are coating the surface of the sugar under the water.

If they do coat the sugar, they can make dissolution slower by keeping the sugar and the water apart,
or by slowing down the passage of sugar molecules into the water.
If you put some oil in the water and it is slightly soluble in the water, but also partly floating on the top of the water,
that might really almost stop dissolution.

It is even possible that something which coats the sugar will make it dissolve faster.
Soap, for instance, in small quantities might make sure that the water always wets all the sugar crystal surfaces right away.
But I am guessing about that.  It could slow things down instead.

The study of the speed of chemical reactions is called "kinetics"
In my opinion, there is no accounting for kinetics.
It always depends on tiny, invisible places and things, so all kinds of things happen.
So you better try the experiment for yourself.

What always matters in dissolution rate is the Stirring Speed.
So be sure you figure a way to keep your stirring the same each time,
so you can see the small differences due to the impurities.

Temperature matters too.  Warm water dissolves sugar faster than cold.
Have a thermometer, or keep two bottles of different waters together overnight before adding sugar,
to make them the same temperature.

If you have an impurity that makes water more viscous, like a 50% by volume of glycerin,
That will slow down the stirring right near each sugar surface, and the diffusion of sugar molecules through water.
So I think that would slow dissolution.  But 50% is so much, I am not sure you would call it an "impurity".
There is a water-soluble plastic called poly-vinyl-alcohol, which if you can find some,
will make water noticeably thicker using only 1-3%.
I found some laundry-soluble tacking material in a sewing-machine store once, probably made of that.

Sugar-water itself is rather viscous.
Make sure you are dissolving less than 10% of the sugar that your water could hold.

Jim Swenson
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Queenie,

Anything already dissolved in water will affect the rate at which anything else dissolves. In order 
to answer you question more exactly, one would need to know what's in the "impure" water and how 
much is there.

Why? It is because water interacts with things dissolved in it. That interaction "ties up" some of 
the water molecules -- in other words, the interaction binds or otherwise occupies the water 
molecules in the dissolution of anything it is able to dissolve. This leaves fewer unoccupied 
water molecules to assist in the dissolution of the sugar molecules.

Regards,
ProfHoff 867
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