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Factors for Dissolving
Name: Zameer
Status: student
Grade: 6-8
Location: N/A
Country: N/A
Date: October 2006
Question:
I am 12 years old. How many different variables
their are to the rate of dissolving something?
Replies:
Zameer,
There are quite a few variables controlling the speed at which
something dissolves. We can split these up to two general classes:
Thermodynamic (things that have to do with stability and energy) and
Kinetic (things that have to do with the process or how the mixing
is done) conditions. In the thermodynamic conditions would be things
like: the type of intermolecular forces (how well the two molecules
interact), whether there is a large entropy gain (if the molecule
breaks apart and forms ions for example), the enthalpy lost or
gained by the system (whether the solution formation is exothermic
or endothermic) and so on. In the kinetic aspect, there are things
like: how finely the solute is subdivided (especially if it is a
solid), how hard does one stir the solution, and so on.
Greg (Roberto Gregorius)
Your question is a good one -- that means it is difficult to
give an easy answer. But let us start by making a difference
between "the rate of dissolving something" and "how soluble that
something is". Those two things are not the same. However, your
equating those two factors is a very common mistake. Very often we
are asked about "how fast" or the "rate" of something when the
person really means "how much" not "how fast". Of the two questions
"how much" is easier to answer than "how fast". For example,
"powdered sugar" dissolves much "faster" than "regular sugar"
because "powdered sugar" is ground to a much smaller particle size
than "regular sugar", even though both are the same substance and
end up having the same amount dissolved in water.
It is usually much easier to answer the question "how much" than
"how fast". The answer to "how much" means to measure the amount of
a substance that dissolves. But when you ask "how fast" a lot of
other variables enter in. For example: How much is the "something"
stirred? What is the size of the particles? Is the "something"
coated with another material? And a lot of other things. If you
mean "how much", all of these things do not matter. You just have
to wait long enough until all these variables do not matter.
To approach the question of "how much" there are some common
"rules". The more alike the "something" is and the "solvent" is,
then it likely that the "something" will be soluble. Chemists use
the rule of thumb "likes-dissolve-likes". An example is sugar,
which is very soluble in water, but is almost totally insoluble in
alcohol or gasoline.
Usually, but not always, the higher the temperature, the more
soluble a substance becomes. But you can find many exceptions. One
very universal exception is the solubility of gases, which always
(well, almost always) become less soluble as the temperature
increases. But there is a large difference in the solubility of
different gases. On the other hand, common table salt has almost
the same solubility in cold, room, warm and hot temperatures.
I think that your question is really about "how much" and not
really "how fast" because "how fast" depends upon many variables
that can be changed or cannot be controlled.
Vince Calder
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Update: June 2012
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