Amount of Solvent and Solubility
Date: Winter 2009-2010
I'm currently learning about the rate of solubility of
solids in liquids, and I do not know if the amount of solvent (in
this case, water) will affect the rate of solubility. My guess is
that, the more the solvent, the faster solute dissolves, because
there are more solvent particles to help break down the solute so
that it will dissolve faster. Please Help, I tried Googling it for 2
hours, all I could find was temperature, pressure and the size of
particles affecting the rate of solubility.
There is a very difficult concept involved in answering your question.
If I drop a sugar cube into a cup of water it will dissolve. If I drop a
similar sugar cube into a lake it will dissolve in roughly the same
time. If I drop the cube into a stream it will dissolve much more
quickly because the movement of the water brings so much more water into
contact with the sugar, and removes the dissolved material as soon as it
It is not how much solvent is available that makes the difference, but
how much of that solvent can come into contact with the solute. It is
for that reason that we stir our coffee.
Despite all this, it is very hard to say that AMOUNT of solute changes
the rate of solution . What is changing is the concentration of solute
immediately adjacent to the solute, and so the gradient of solution.
Stirring reduces the gradient, and by doing so increases the rate of
Tennant Creek High School
Pardon my "ARRGH", as taught in most educational classes.There are serious
flaws in the methodology. But measuring rates of solids has so many
uncontrollable variables that the results are essentially useless. Here is
a "short" list of the variables that need to be controlled: Crystal growth
due to the CONTROLLED evaporation of solvent (water). The presence/absence
of a "seed" crystal. If the solution becomes "super saturated" you may
reduce the temperature several degrees, only to find that WHAM the whole
sample solidifies, or forms a "sludge". Depending upon the method, the "rate
of cooling" may be an important variable.
I am not trying to put a "wet blanket" on your project but be aware that this
undertaking is fraught with problems. In general when I see the question
involving "rates" of any process, my "horns" go up. Rates of any process are
usually difficult to measure.
As you already know, physical (as opposed to chemical reaction)
rates are controlled by so many factors and it is very difficult to
pinpoint exactly how important any one factor is.
Let us assume for the moment that you could completely isolate any
one effect - in this case the amount of solvent. This means, you
have completely removed any stirring effects (turbulence from
pouring the liquid into the container), the temperature or input of
heat, the size and shape of the container, air flow, the purity of
the solvent, etc. etc. Remember that you would still have to deal
with the turbulence that would result from pouring in the solid -
remembering that stirring causes more physical contact and increases
solvation, etc. etc.
Still, if you could truly isolate all the other factors out and were
dealing only with the amount of solvent as the only factor, then you
have to separate the effect of the solid diffusing through the
solvent as it dissolves (this is unavoidable unless you could
somehow introduce the solid in all locations in the liquid at the
same time). So, now imagine that you placed a chunk of solid into a
liquid - the solid sinks to the bottom of the container, and sits
there (no stirring) and slowly dissolves. The amount of solvent,
becomes less of a factor as the amount of available solvent that
actually comes into contact with the solid and manages to dissolve
it, move away, and allow more "free" solvent to dissolve more solid.
... so you see that diffusion is a huge factor here. If on the other
hand you were to try to add an incredibly fine ground powder (so
much so that the surface effect of the solid is no longer a factor),
you can then imagine that this would be like a liquid dissolving
into another liquid - and here (without stirring) the amount of
solvent is still not so much a factor since the introduced liquid
(solute) is only in contact with the solvent that is immediately
around it. The solvent that is far away from the introduction point
has no effect on the solvation until the diffusion of the solute gets there.
In effect then, the amount of solvent is a small factor - when
completely isolated from other factors.
But, you can imagine that this changes if we were allowed to stir
the mixture. Stirring reduces the effect of the diffusion rate and
will allow available solvent to come into contact with the solute
more rapidly. ...but now, there is the question of how fast you stir
as an important factor.
See what I mean when I say that physical rates are very difficult to
isolate and very difficult to pinpoint? Factors interact, factors
have varying power as a function of other factors.
Greg (Roberto Gregorius)
Click here to return to the Chemistry Archives
Update: June 2012