Ask A Scientist Chemistry Archive

name         Michael G.
status       educator
age          50s

Question -   This test question has caused some debate among my
fellow faculty members.  Please help us.

A factor that does not affect solubility is:
a.  polarity
b.  temperature
c.  freezing
d.  pressure
----------------
Michael,

As the question and answer choices are posed, I can understand the 
controversy because they make no reference as to the nature of the solute 
and solvent. Given the opportunity to specify solutes and solvents of my 
choice, I could make the case that any or all the answer choices would 
have an effect on solubility.

Regards,
ProfHoff 599
=====================================================
It is of course important to know the solubility of "what solute" in "what 
solvent". Ignoring that, I think that the targeted answer is (d) for two 
reasons:
     1. Elimination of the other three. Certainly solvent polarity has a 
 large effect on solubility and does temperature (in most cases). And 
 considering that most solutes that are soluble in a liquid solvent are 
 (usually) not soluble in the solid phase of the solvent.
     2. The more sophisticated reason is thermodynamic. Solubility has a 
 certain change in free energy,dG, associated with the general reaction 
 A(solid) or A(liquid) ------> A(solution) in some liquid solvent call it 
 'B'. The free energy change with a change in pressure (at constant 
 temperature) is: dG / dP = V. Now  almost all solids and liquids are 
 approximately "incompressible" at reasonable pressures changes say 1 to 
 10 atm (not 1000 atm) so the free energy change with pressure (and hence the
solubility change) is essentially independent of the change in pressure 
because the volumes do not change.
Vince Calder
====================================================
The factor that matters least when dissolving solids in liquids is 
pressure, because neither solids nor liquids are affected very much by 
pressure.  But if you are interested in dissolving gases in liquids, or 
anything in supercritical fluids, pressure matters very, very much.  So my 
vote is that the question is WRONG! Bad question! Bad question!

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
=====================================================
It is of course important to know the solubility of "what solute" in
"what solvent". Ignoring that, I think that the targeted answer is (d) for two
reasons:
    1. Elimination of the other three. Certainly solvent polarity has a
large effect on solubility and does temperature (in most cases). And
considering that most solutes that are soluble in a liquid solvent are
(usually) not soluble in the solid phase of the solvent.
    2. The more sophisticated reason is thermodynamic. Solubility has a
certain change in free energy,dG, associated with the general reaction
A(solid) or A(liquid) ------> A(solution) in some liquid solvent call it
'B'. The free energy change with a change in pressure (at constant
temperature) is: dG / dP = V. Now  almost all solids and liquids are
approximately "incompressible" at reasonable pressures changes say 1 to 10
atm (not 1000 atm) so the free energy change with pressure (and hence the
solubility change) is essentially independent of the change in pressure
because the volumes don't change.
Vince Calder
    There are some more considerations to what I provided above that
occurred to me after I responded.
    If the solute is a gas and the solvent is a liquid then pressure DOES 
affect solubility (Henry's Law). In addition, temperature and solvent 
polarity also play a role. Freezing also plays a role (depending upon 
whether the solute is soluble both in the liquid and solid phases of the 
solvent. However, the "usual" case, for example water and ice, most 
things -- dissolved minerals and air -- are not soluble in ice. That is 
why ice cubes look cloudy. The reason ice cubes you by in the market, 
motels, and bars are clear is that the freezing takes place in a flow of 
water over a cold surface. Then the minerals and air are continuously 
washed away and
pure ice forms clear ice cubes.
    Second, neither the inquiry nor I mentioned the solubility of various 
substances in super-critical fluids, i.e. fluids at temperatures and 
pressures above the critical point temperature and pressure. Here 
solvents like liquid CO2, and various other substances exhibit remarkable 
solvent properties very different than their behavior at room temperature 
and atmospheric pressure. This is a totally new phase of the solvent that 
is used commercially to de-caffeinate coffee, and various other 
interesting applications. Space here does not allow a discussion of 
super-critical solvents, but a "Google" search on "supercritical 
fractionation" or similar
search term will provide a wealth of web sites describing this phenomenon. 
In the super-critical domain, solids do not form, so choice (C) above in 
the inquiry is irrelavent.

Another "simple" question with a complicated answer!
Vince Calder (2)
=====================================================
Dear Michael and colleagues:
Any multiple-guess question and answer format necessarily has its 
limitations. In this case one has to distinguish between the
equilibrium constant for solubility and the amount of material solvated. 
In addition, what is meant by "solubility?" The usual
definition is that solubility is the concentration of a solvent when the 
solution is saturated, i.e., when the dissolving of a solvent and
its recrystallization are in equilibrium. This implies the existence of an 
equilibrium constant, and those are functions of temperature and not 
pressure (which is something we learn how to prove in physical chemistry 
and other thermodynamics courses).

Clearly polarity affects solubility because polar solvents are better at 
dissolving ionic and polar substances than nonpolar solvents.
It is also clear that temperature affects solubility because equilibrium 
constants are generally strong functions of temperature (given by the van 
t'Hoff equation). If the solvation reaction is endothermic the solubility 
will increase with increasing temperature (due to the increase in the 
equilibrium constant). If exothermic, the solubility will decrease with 
increasing temperature. NaCl is an example of the first case; NaOH is an 
example of the second case. This is basically LeChatelier's principle in 
action.

"Freezing" is a difficulty. I am not sure what is meant here. Clearly if 
one has a saturated solution and then freezes the solvent, some solute is 
going to  have to precipitate out if the frozen solvent is pure (usually 
the case, at least to a high degree of approximation. This is why 
recrystallization is such a useful purification technique.). However, I 
believe that the concentration would
not change and therefore neither would the solubility. On the  other hand, 
lowering the temperature to the freezing point would definitely affect the 
solubility (see previous paragraph).

As far as pressure is concerned, well, equilibrium CONSTANTS are only 
functions of temperature but equilibrium POSITIONS are functions of both. 
If the solute in question is a dissolved gas, increasing the pressure 
increases the amount of gas in the solution and therefore the 
concentration of gas  - and therefore the solubility. This is Henry's Law: 
C = k P where C is the concentration, P is the pressure and k is the 
Henry's law constant for the particular gas/solvent combination - 
effectively an equilibrium constant.
This law describes the bubbles which rise out of solution when you open a 
pressurized bottle of seltzer or other soda. If C is taken to be the 
solubility then one must say that the solubility depends on pressure.  On 
the other hand, if the solute is a crystalline solid then pressure will 
not make a bit of difference because liquids (and solids) are highly 
incompressible and the solute is nonvolatile,  so the concentration 
depends only on the equilibrium constant's value, which is independent of 
pressure in all cases.

So, if I had to choose one I would pick c. Clearly a and b would be wrong. 
I think this is a vague question, however, and the question should instead 
read "A factor that does not affect solubility OF GASES is..." and then 
the answer would definitely be c.

I hope this is helpful to you all.
best, prof. topper
=====================================================