Solubility
Cause sometimes stuff just won’t dissolve
Solubility
Dissolving 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)
Solubility
After 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)
Solubility
With an ionic compound (say, NaCl), it’s actually a little bit more complicated.
Solubility
With 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 Steps
Let's just look at the water molecules. Before dissolving, they look like this:
Three Steps
And after dissolving, they're like this—spread out.
This is actually not favorable—water molecules want to be near each other.
Step 2
The solute is even more dramatic. The ions get spread very far apart.
This is very unfavorable; those charges want to be together.
Step 3
Mix them together: this is favorable.
Overall
Dissolving 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 Rules
For ionic compounds dissolving in water, there are two groups:
Soluble:
Nitrates
Insoluble:
Carbonates
Alklali metals
Hydroxides
(NASH
Sulfates
Oxides
CHOPS)
Halides
Phosphates
Sulfides
Solubility Rules
For ionic compounds dissolving in water, there are two groups:
Soluble:
Nitrates
Insoluble:
Carbonates
Alklali metals
Hydroxides
Sulfates
Oxides
Halides
Phosphates
Sulfides
You just look at your compound and see if it has one of those groups:
MgSO4 = soluble
MgS = insoluble
CaCO3 = insoluble
Na3P = soluble
CaCl2 = soluble
Solubility Rules
For ionic compounds dissolving in water, there are two groups:
Soluble:
Nitrates
Insoluble:
Carbonates
Alklali metals
Hydroxides
Sulfates
Oxides
Halides
Phosphates
What about Na2CO3?
It has an alkali (soluble), and a carbonate (insoluble)?
Soluble Wins!
Sulfides
Exceptions
As 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)
Summary
1. 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.