1. Yes, most ionic compounds are soluble in water
Chemistry sounds arcane, but the answers to many of its questions can be found in everyday life. When we ponder whether ionic compounds are soluble in water, we can simply think about table salt.
Table salt is sodium chloride, an ionic compound comprising sodium cations and chloride anions. From experience, we know that it dissolves quickly in water, adding flavours to our porridge, soup, and sauce.
Like table salt, most other ionic compounds are water-soluble.
2. Because water is a powerful solvent
While the electrostatic forces of attraction holding an ionic compound together are strong, water is its Achilles’ heel.
Water molecules can attract most ions very well, luring the ions away from each other. This allows the ionic compound to dissolve, whereby water molecules and ions intermingle and mix thoroughly.
In water, the electrostatic forces of attraction between oppositely charged ions are overcome, allowing the ions to dissociate and dissolve.
3. But there are insoluble exceptions
The shells of crabs, clams, and eggs are made of calcium carbonate, an ionic compound. However, they are clearly (and thankfully) insoluble, lest the shells dissolve and the animals perish.
Likewise, our bones contain calcium phosphate, an insoluble ionic compound.
So why the contradiction?
For these insoluble ionic compounds, water molecules cannot attract their ions as well. Consequently, their ions tend to stick together, forming a big, stubborn clump that refuses to dissolve.
| insoluble ionic compound | found in |
|---|---|
| calcium carbonate, CaCO3 | animal shells, limestone, chalk |
| calcium sulfate, CaSO4 | plaster of Paris, cast, bandage |
| iron(III) oxide, Fe₂O₃ | iron ore, rust |
4. Pure Chemistry: Solubility rules
Must we memorise the insoluble exceptions?
Yes is the unfortunate answer, but there are general rules of solubility to guide us.
Which ionic compounds are soluble in water?
- Any ionic compound containing Group I or ammonium cation is soluble
- Any ionic compound containing nitrate anion is soluble
Which ionic compounds are insoluble in water?
With the exception of compounds containing Group I or ammonium cations:
- Any ionic compound containing carbonate, oxide, or hydroxide anion is insoluble
- Barium sulfate, calcium sulfate, and lead(II) sulfate are insoluble
5. Extension: Why is water a powerful solvent?
-
Usually, we place the shared pair of electrons at the centre of the overlapping electron shells. However, in reality, the shared pair of electrons are closer to oxygen than hydrogen. -
This diagram only highlights the shared pair of electrons of the O-H bond in water. The actual sharing of electrons is even, whereby they are closer to oxygen.
Water is a simple molecule with covalent bonding. Its oxygen atom forms a single covalent bond with each hydrogen atom, whereby they share a pair of electrons.
However, the sharing is uneven. The shared pair of electrons is closer to oxygen than hydrogen. This makes oxygen slightly negatively charged. Conversely, hydrogen – being slightly deprived of the shared electrons – is partially positively charged.
-
The hydrogen atoms of water molecules have a partial positive charge. They attract anions. -
The attraction between the anion and hydrogen atoms of water is so strong that it overcomes the original electrostatic attraction between the cations and anions. -
The oxygen atoms of water molecules have a partial negative charge. They attract cations. -
Likewise, the good interaction between the cations and water molecules overcome the original electrostatic attraction between oppositely charged ions in the ionic compound.
The partial charges of water molecules allow them to attract ions. This new attraction disrupts the original electrostatic forces of attraction between oppositely charged ions. Consequently, the ionic compound falls apart as the ions form a new union with water molecules.
