Basic Solubility Rules
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1BASIC SOLUBILITY RULES
Technically, all solids dissolve to some extent in water. Some compounds dissolve a large
amount in 100ml of water and some dissolve very little. For practical purposes, we say that any
compound with a solubility less than 0.1g/100ml of water is insoluble. !
The exact amount of solid that would dissolve can only be determined through solubility
equilibria calculations, but exact amounts are often unnecessary. Often we just need to know
wether a solid is likely to be present or not. Compounds with high solubility in water will
dissolve readily while insoluble compounds will stay solid and settle upon standing. Simple
solubility rules are useful for predicting wether solids will dissolve or whether a precipitate will
form when two aqueous solutions are combined.
SOLUBILITY RULES
As always, the rules are different for ionic and covalent compounds.
COVALENT
When assessing covalent compounds for solubility, the rule “Like Dissolves Like” tells us that
polar compounds will dissolve well in water and non-polar compounds will not.
IONIC
For ionic compounds, solubility is related to the energetic favorability(stability) of the ionic
bond, which is related to ionic charge, size, and stability. Some simple rules can be used to
determine the solubility of most compounds.
- ) are
-
1. Salts of ammonium (NH
4 +
), group 1A cations, nitrate (NO
), and acetate (C
H
O
3
2
3
2
!
always soluble.
-
-
-
-
2. Sulfates (SO
4 2
) and halides (Cl
, Br
, I
) are soluble with a few exceptions
.
(like AgCl & BaSO
)
4
-
-
-
3. Phosphates (PO
4 3
), carbonates (CO
3 2
), and metallic oxides (O
2
) are insoluble, with the
!
exception of group 1A cations and ammonium (NH
4 +
).
-
4. Metallic hydroxides (OH
) are insoluble except ammonium (NH
+
), group 1A cations and
4
!
group 2A from calcium down.
An insoluble compound will form a precipitate (solid) in water. (Ex. AgCl(s) = insoluble)
A soluble compound will exist as ions in water. (Ex. NaCl(aq) = soluble)
→ AgCl(s) + NaNO
AgNO
) + NaCl
(aq
(aq)
(aq)
3
3
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