Major trends in water

The aqueous solubility of ionic compounds is significant in analytical and synthetic chemistry, and in the creation of minerals through geochemical processes. The most important trends are as follows.

(i) Soluble salts are more frequently found when ions are of very dissimilar size rather than idetical size. So in comparing salts with dissimilar alkali metal cations, lithium compounds are the smallest soluble of the series with OH^- and F^-, but the most soluble with larger cations like  Cl^- or NO^-₃ . This principle is often helpful in preparative reactions and separations. If it is needed to precipitate a large complex anion, a large cation like tetrabutyl ammonium [(C₄H₉)₄N]⁺ can be useful.

(ii) Salts in which both ions have multiple charges are less expected to be soluble than ones with single charges. So carbonates (CO^2-₃)and sulfates (SO^2-₄)of the larger group 2 cations are insoluble. An significant issue is the negative solvation entropies of the ions.

(iii) With ions of dissimilar charges, particularly insoluble compounds result when the lower charged one is smaller (like this gives a very large lattice energy). So with M³⁺ ions, fluorides and hydroxides are usually very insoluble, where heavier halides and nitrates are very soluble.

(iv) From covalent contributions to the lattice energy, the lower solubility results. This occurs particularly with ions of less electropositive metals in combination with more polarizable cations. Late posttransition and transition elements frequently have insoluble sulfides; insoluble halides (but not usually fluoride) also occur, for instance with Ag⁺ and Pb²⁺.