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How to Tell When Salt Dissolves
name Adam S.
status student
age 15
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?
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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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Hello Adam.
This is an interesting question. Most times, people are trying to
measure HOW MUCH salt will dissolve as a function of water temperature.
Since you are interested in measuring HOW FAST A GIVEN AMOUNT will dissolve,
we will need to be careful in setting up the experiment.
We know how much salt dissolves in water at different temperatures.
You can found this out by looking in the CRC Handbook of Chemistry and
Physics. I recommend that for each water temperature, you find out how much
salt will dissolve, and add exactly that much, or some fraction of this. Do
the same thing for EACH TEMPERATURE. That way, your data will not be
affected by differences in the solubility of salt at each temperature. Also,
it would be nice if you could add the salt as a single, big piece, instead
of a powder (like what comes in the salt shaker). I'll explain the reason
for this in a minute.
Then, you will have to consider how you will heat the water. I will
assume that you are going to put a heat source, like a hot plate, under some
sort of beaker. Next, you will have to think about whether you will stir the
water or not during your experiment. Depending on whether you do or do not
stir the water, you will be measuring TWO DIFFERENT quantities. If you do
not stir the water, then the salt which dissolves at any given time will
diffuse through the water by a process known as convection. As the water at
the bottom of the beaker heats up first (if the heat source is at the
bottom), it becomes a little lighter than the colder water at the top of the
beaker. This hotter, lighter water will therefore rise up to the top of
beaker, while the colder, heavier water at the top will sink to the bottom.
The salt that dissolves will therefore be carried around inside the water by
these convection currents, as they are called. This means that the
concentration of dissolved salt will vary from place to place inside the
beaker.
If you do stir the water, then the temperature of the water will be
more equal everywhere in the beaker, and convection will not be a problem.
Also, the salt which dissolves will be well mixed throughout the water,
making your measurements of the amount of dissolved salt more reliable.
However, if you stir the water, it would be better if you had a large piece
of salt instead of powdered salt, otherwise the large surface area of the
powdered salt in contact with the water will cause the powdered salt will
dissolve VERY quickly, and it will be difficult to make any measurements.
As for how to measure how much salt is in the water, you could use a
voltmeter (I bet your dad has one), set to measure resistance. Pure water is
a poor conductor of electricity. However, as salt dissolves in the water, it
will become a very good electrical conductor and the resistance will
decrease a lot. Measurements of resistance at various times could then be
plotted. I believe that you will measure the fastest rate at the beginning,
when the amount of dissolved salt in the water is low. This initial part of
the plot may be a straight line or close to it, and the slope of the line
would be one way of comparing the dissolution rate at different water
temperatures.
Good luck !
Jim Rubin
AWE
Bldg A90.2, Rm. 1-016
Aldermaston, Reading
Berkshire RG7 4PR
United Kingdom
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Adam,
You have given yourself an experiment that is very difficult to do. The
solubility of ordinary table salt (NaCl) is not very large and , worst of
all, it doesn't change much with increasing temperature. All the way from
freezing at 0 C to boiling at 100 C, it dissolves to the extent of only
about 30 grams of salt per 100 grams of water. Finally, without
sophisticated equipment, it is not at all easy to determine when all the
salt dissolves in your test solutions. Simply looking for undissolved
crystals would not be good enough.
Regards,
ProfHoff
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