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

Question

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

Luna Chen · Accepted Answer

Lithium carbonate is slightly soluble in water.

Actually, lithium carbonate is slightly soluble in water, its solubility at #25^@"C"# being listed at #"1.3 g/100mL"#. Comparing lithium carbonate to other alkaline earth metal carbonates or other lithium salts, it is less soluble. The small ionic size of lithium is the reason for that. The charge of the ions and their distance from one another are directly proportional to the strength of the electrostatic attraction that exists between the cations and anions that make up an ionic compound. This indicates that the size of the alkaline earth metal ions is the primary factor influencing solubility when charges are maintained constant, as in the case of alkaline earth metal carbonates. The stronger electrostatic force of attraction will cause alkaline earth metals with bigger cations to dissolve more readily. In other words, because the electrostatic attraction between the carbonate anion and the minuscule lithium cation is stronger than that between the ions and the water molecules, lithium carbonate is less soluble in water.

Peyton Adams · Answer

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

Basic solubility rules tell us that all alkali metal salts are soluble and that carbonates, in general, are insoluble (well, except for alkali metal salts, etc.; you get the idea, solubility rules are hierarchical!). But as physical scientists, we ought to look for some solubility information. From https://tutor.hix.ai 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, I apologize if I've pitched this treatment at an inappropriate level; I'm not sure if you're pursuing an A-level or attending a university.

Wyatt Atkins · Answer

undefined Lithium carbonate is insoluble in water due to its high lattice energy, which makes it difficult for water molecules to overcome the strong attraction between lithium and carbonate ions. This exception to the general rule that salts of alkali metals are soluble is attributed to the smaller size and higher charge density of the lithium ion compared to other alkali metal ions.