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Name: Adam S.
Status: student
Age: 15
Location: N/A
Country: N/A
Date: 2000


Question:
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?


Replies:
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 increasing temperature.

Vince Calder



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