Some dissolve in it as molecules while others, called electrolytes , dissociate and dissolve not as neutral molecules but as charged species called ions . Compounds which exist as solid ionic crystals dissolve in water as ions, and most of them are highly soluble in water. Salts which are less soluble in water than this at room temperature are called slightly soluble salts. The solubility of an ionic salt depends both upon its cations and its anions, but for simple salts in aqueous solution at room temperature the following general observations are useful. Metal nitrates are soluble almost without exception.
Metal sulfides are generally insoluble in water.
When solids dissolve, the solutes are no longer pure substances and their chemical activity can no longer be taken as unity. In dilute solutions, aqueous or otherwise, activities of solutes are often taken as equal to their molar concentrations. These equilibria are called solubility equilibria . The example below shows how the form in which they are written compares to other equilibrium constants. The form of equilibrium constant indicated as K sp is called the solubility product constant or, more commonly, the solubility product .