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Name: Michael
Status: student
Grade: 6-8
Location: AR
Country: USA
Date: Winter 2009-2010


Question:
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.



Replies:
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 dissolves.

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 solution.

Nigel Skelton
Tennant Creek High School
AUSTRALIA


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.

Vince Calder


Michael,

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)
Canisius College



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