Solubility Cause sometimes stuff just won’t dissolve.

SolubilityCause sometimes stuff just won’t dissolve

SolubilityDissolving is actually a very complicated process, if you want to look at the details.

Start with a solvent (we’ll use water) and a covalent solute (call it sugar)

SolubilityAfter dissolving, it looks something like this:

(the sugar molecules stay intact because the atoms are bonded together—each molecule just drifts away from its neighbors)

SolubilityWith an ionic compound (say, NaCl), it’s actually a little bit more complicated.

SolubilityWith an ionic compound (say, NaCl), it’s actually a little bit more complicated.

Each ion has come apart from all the others, because only charge was holding them together—the water was able to slip in between.

Three StepsLet's just look at the water molecules. Before dissolving, they look like this:

Three StepsAnd after dissolving, they're like this—spread out.

This is actually not favorable—water molecules want to be near each other.

Step 2The solute is even more dramatic. The ions get spread very far apart.

This is very unfavorable; those charges want to be together.

Step 3Mix them together: this is favorable.

OverallDissolving consists of two unfavorable steps (pulling things apart), and

one favorable one (putting them together).

So which wins?

Well....it depends.

Some compounds dissolve, and some don't. There's no simple way to predict which will and which won't, so you just try them and see.

You can come up with some guidelines, though.

Solubility RulesFor ionic compounds dissolving in water, there are two groups:

Soluble:Nitrates Alklali metals Sulfates Halides

Insoluble:Carbonates Hydroxides Oxides Phosphates Sulfides

(NASH CHOPS)




You just look at your compound and see if it has one of those groups:

MgSO4 = soluble MgS = insoluble CaCl

2 = soluble

CaCO3 = insoluble Na

3P = soluble




What about Na2CO

3?

It has an alkali (soluble), and a carbonate (insoluble)?

Soluble Wins!

ExceptionsAs with anything in life, there are some exceptions. The notable ones are:

Halides aren't soluble if they're paired with Pb2+ or Ag+

Sulfates aren't soluble if they're paired with Pb2+ or Ba2+

So AgCl, PbSO4, etc. will not dissolve.

(in fact, insoluble silver halides are responsible for the entire fields of photography and film)

Summary1. Dissolving is actually a fairly complex process

2. Covalent solutes remain intact, but ionic ones separate

3. Some compounds dissolve in some solvents, some do not.

4. NASH CHOPS lets you make predictions about which will and won't.

5. These are not perfect rules, just guidelines based on experiments.

Solubility Cause sometimes stuff just won’t dissolve.

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