MadSci Network: Chemistry
Query:

Re: Why are some substances more soluble than others ?

Area: Chemistry
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College
Date: Wed Aug 27 15:36:28 1997
Area of science: Chemistry
ID: 872640499.Ch
Message:

We need to elaborate a little bit on the unspoken half of your question, "in water," that is, "Why are some substances more soluble than others in water?"

We then need to point out that this question is not the same as asking, "Why can I dissolve motor oil in gasoline but not in water?" because the answer to that question is "Like dissolves like." That is, you typically find that non-polar solutes dissolve in non-polar solvents, while polar solutes dissolve in polar solvents.

Instead, you are asking about two polar substances (another example is sodium chloride, NaCl, vs. silver chloride, AgCl) dissolving in the same polar solvent.

Here the answer has to do with how the water interacts with ionic substances. When an ionic compound, like NaCl, dissolves in water, the water molecules surround each of the ions with a solvation sphere. In order to do this, the water must first break the ionic bond between the Na+ and Cl- ions. Water succeeds in doing this because solvated sodium and chloride ions are more stable than those same ions are in solid sodium chloride.

Silver chloride, on the other hand, is much more stable as the solid than as Ag+ and Cl- ions in aqueous solution. This is why you can precipitate silver chloride by adding AgNO3 to a solution of NaCl.

The answer to your question, then, is that Ba(OH)2 is much more soluble than other Group 2 hydroxides (all Group 1 hydroxides are very soluble!) because Ba2+ interacts more strongly with water than with hydroxide ions. Conversely, BaSO4 is more stable as an ionic solid than as a solution of separated ions.

Now, you are probably saying, "He hasn't answered my question yet! Why is this true for barium but not for, say, calcium?"

My best guess is that barium has the largest atomic diameter of the alkaline earth family, except for radium.

This means that barium probably fits very well into the rather large cavity in a sulfate ion; thus, the interaction of Ba2+ with SO42- will be stronger than for the smaller alkaline earths. In this case, it will be strong enough that barium cations prefer to interact with sulfate anions rather than with neutral water molecules.

The size of Ba2+ also has a bearing on why Ba(OH)2 is more soluble than, say, Ca(OH)2. Hydroxide, being negatively charged, will interact more strongly with a cation than will water (which is electrically neutral). But in solid barium hydroxide, there are only two hydroxides per barium ion, while at least six and probably seven or eight water molecules can fit around that same barium cation. Thus, cation-water interactions in aqueous barium hydroxide are stronger than cation-anion interactions in solid barium hydroxide.

On the other hand, Ca(OH)2 also has two hydroxides per Ca2+ in the solid, but the calcium cation probably can't interact with more than five or six water molecules in solution. This, plus the smaller size of calcium -- which concentrates the positive charge, making it stronger and more attractive to hydroxides -- would explain why Ca(OH)2 is only sparingly soluble in water.

I hope this answers your question without overwhelming you!

  Dan Berger
  Bluffton College
  http://cs.bluffton.edu/~berger


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