We've gotten several questions on sugar water revcently, so perhaps this answer can take care of most of them.
Sugar water is simply water in which some sugar has dissolved. The
best way to tell whether sugar dissolves faster in distilled water or
whatever type of tapwater is available in your town is to try the
experiment yourself. The most important thing is to make sure that both
types of water are at the same temperature. Of course you should also
make sure to use the same amount of sugar and of water for each type of
water, and to stir in the same way in the same container.
It really shouldn't be necessary to have a prediction before
starting the experiment. Still, if you want to know my guess, it will
hardly make any difference which type of water you use. Tap water often
has some calcium salts and other trace contaminants, but it is not all
that different from distilled water. If you are using very small
amounts of sugar, the initial pH of the water may matter, and that can
depend on the source.
Evidently this is a very popular assignment. I have no idea why,
since the results are unlikely to be dramatic, and the principles
involved are obscure.
Science education in this country often has shortcomings.
(republished on 07/19/06)