How to Tell When Salt Dissolves
Name: Adam S.
I am doing an experiment on the effect of the temp. of
water on the rate at which salt dissolves. I was wondering if you had
any suggestions about how to tell exactly when the salt dissolves?
This is not such an easy experiment because the RATE of solution depends
on many factors in addition to temperature -- particle size of the solid
(because that determines the surface area in contact with the water),
stirring (because that affects the transport of ions away from the
solid/water interface). Start with 200-400 gm of water to provide a good
"heat sink" because, while the heat of solution of NaCl is fairly small and
independent of the concentration of salt, this is not true of other
candidate salts. The heat of solution can cause the temperature to
fluctuate, or make the temperature difficult to keep constant.
If you have access to an analytical balance, an easy accurate way to follow
the dissolution process is to "grab" small samples of the solution (~1-4 gm)
at various times, weigh the sample, evaporate the water carefully to avoid
spattering (temperature ~ 60-70C.), then weigh the dried sample. You should
see an increasing percent of salt as a function of time and temperature.
When saturation is complete, sequential samples will not show any increase
in the weight of salt. Be sure to use reagent salt(s) because commercial
salt from the grocery store contains some silica or other insoluble material
to prevent caking. You will know this because the solutions will be hazy.
Also be aware that the solubility of salts does not always increase with
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Update: June 2012