Solubility of Ionic Compounds in Water
Some ionic compounds (salts) dissolve in water, which arises because of the attraction between positive and negative charges (see: solvation). For example, the salt's positive ions (e.g. Ag+) attract the partially negative oxygens in H2O. Likewise, the salt's negative ions (e.g. Cl−) attract the partially positive hydrogens in H2O. Note: oxygen is partially negative because it is more electronegative than hydrogen, and vice-versa (see: chemical polarity).
- AgCl(s) Ag+(aq) + Cl−(aq)
However, there is a limit to how much salt can be dissolved in a given volume of water. This amount is given by the solubility product, Ksp. This value depends on the type of salt (AgCl vs. NaCl, for example), temperature, and the common ion effect.
One can calculate the amount of AgCl that will dissolve in 1 liter of water, some algebra is required.
- Ksp = × (definition of solubility product)
- Ksp = 1.8 × 10−10 (from a table of solubility products)
=, in the absence of other silver or chloride salts,
- 2 = 1.8 × 10−10
- = 1.34 × 10−5
The result: 1 liter of water can dissolve 1.34 × 10−5 moles of AgCl(s) at room temperature. Compared with other types of salts, AgCl is poorly soluble in water. In contrast, table salt (NaCl) has a higher Ksp and is, therefore, more soluble.
| Soluble | Insoluble |
|---|---|
| Group I and NH4+ compounds | Carbonates (Except Group I, NH4+ and uranyl compounds) |
| Nitrates | Sulfites (Except Group I and NH4+ compounds) |
| Acetates (Ethanoates) (Except Ag+ compounds) | Phosphates (Except Group I and NH4+ compounds) |
| Chlorides (Chlorates and Perchlorates), bromides and iodides (Except Ag+, Pb2+, Cu+ and Hg22+) | Hydroxides and oxides (Except Group I, NH4+, Ba2+, Sr2+ and Tl+) |
| Sulfates (Except Ag+, Pb2+, Ba2+, Sr2+ and Ca2+) | Sulfides (Except Group I, Group II and NH4+ compounds) |
| Hydroxides (Only with (aq)
Ba2+, Li+, Na+, K+, Rb+, Cs+, Fr+ ) |
with all other positive ions (aq) |
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