Question

Why is lithium carbonate insoluble in water? Does this not break the rule that the salts of all the alkali metals are soluble?

Answer 1

Lithium carbonate is slightly soluble in water.

Explanation:

Actually, lithium carbonate is slightly soluble in water, its solubility at `25^@"C"` being listed at `"1.3 g/100mL"`.

However, lithium carbonate is not very soluble compared with other lithium salts or with other alkaline earth metal carbonates.

That happens because of lithium's small ionic size.

The strength of the electrostatic attraction that exists between the cations and anions that make up an ionic compound is directly proportional to the charge of the ions and inversely proportional to the distance between these ions.

This means that, when charges are kept constant, like you get for alkaline earth metal carbonates, the most important factor that affects solubility is the size of the alkaline earth metal ions.

Alkaline earth metals that have larger cations will be more soluble because of the decreasing strength of the electrostaic force of attraction.

SImply put, lithium carbonate is less soluble because the electrostatic attraction between the very small lithium cation and the carbonate anion overpowers the attraction between these ions and the water molecules.

Answer 2

The simplest answer would be that God wanted it that way. Nevertheless we may identify a few factors that contribute to its insolubility.

Explanation:

We know from basic solubility rules that all the salts of the alkali metals are soluble, and further that carbonates are generally insoluble (except for those of the alkali metals etc.; you know the drill, solubility rules follow a hierarchy!).

Because we are physical scientists, however, we should seek some solubility data.

From https://en.wikipedia.org/wiki/Solubility_table] we learn that the solubility of `Li_2CO_3` is `1.54` `g` per `100` `mL` (of water) at `0` `""^@C`, and `1.33` `g` per `100` `mL` at `20` `""^@C`.

In other words, solubility decreases with increasing temperature; quite an unusual phenomenon. For comparison, `Na_2CO_3` is `7` and `21.5` `g` with the same units. The decrease in solubility for `Li_2CO_3` with increasing temperature points to an entropy phenomenon. In other words, the lithium cation and the carbonate anion decrease overall entropy (i.e. by imposing order upon solvation) in solution. It is easy to see why `Li^+`, an alkali metal cation with the greatest charge to size ratio, should be entropically disfavoured. The greater polarizing ability of the lithium cation works to the same end,

If I've pitched this treatment at too high a level, I apologize. I don't know whether you are at university or doing A levels.