Chemical Equilibria

Solids have phase indicators at the end of their names:

• “(s)” for solid (unknown crystallinity); or
• “(am)” or “(a)” for amorphous; or
• “(ppt)” for precipitated (amorphous, possibly hydrated); or
• “(vit)” for vitreous; or
• “(cr)” or “(c)” for crystalline.

For solids, like for aqueous complexes, the logK-value for the formation reaction is used. Note that this is the inverse of the solubility product, i.e.: logK_f = −logK_sp

Owing to the Gibbs phase rule, the number of solids that may be present simultaneously in a chemical system is limited.

Unlike aqueous species, which have concentrations that may become gradually small, solid phases either are stable, or not. The concentration usually changes in a narrow range of conditions between the maximum possible amount, and zero.

If a solid is present in a system, its composition is constant, and its activity may be set = 1 (log activity = 0). Its concentration in mol/(L of solution) may be calculated and plotted. If the solid is not present, its concentration is zero, and its activity is undefined. In that case it is useful to calculate the saturation index:

where K_f is the equilibrium constant for the formation of the solid, and IAP is the ion activity product for the solid (the actual product of activities for the participating aqueous species in the reaction, as defined in the equilibrium constant of formation). SI > 0 indicates oversaturation, while values < 0 correspond to undersaturation.

See also reactions for solids in databases.