Ionic compounds dissociate when they dissolve in water.
Ionic compounds are those where the ions are held together by electrical attraction. When an ionic compound like NaCl (table salt) dissolves, the ions dissociate, and the interactions between them are replaced by interactions with water molecules. The maximum solute concentration is called the molar solubility and is different for different solutes. Separation and identification of ions using their solubility characteristics is called qualitative analysis.
Basic Definitions
Some ionic compounds are more soluble in water than others. Multiplying the molar solubility of each ion times the molar solubility of the other ions gives a number called the solubility product (Ksp). The lower the Ksp, the less soluble the compound. By taking a solution of ions in water and adding other ions that will react to form insoluble compounds, we can remove specific ions from the solution.
Different Elements
Silver, lead, and mercury all combine with chloride ions to form slightly soluble or insoluble compounds, so adding hydrochloric acid to a solution will precipitate (remove) these ions. Adding hydrogen sulfide will precipitate copper or cadmium ions from an acidic solution. Ammonium carbonate is one of the few soluble carbonates, but adding it to an alkaline solution containing barium, calcium, or magnesium will precipitate these ions. Once the ions have been removed from solution, they can be distinguished through further chemical tests--by adding nitric acid to a precipitate containing silver ions, for example.
Le Chatelier's Principle
Even for compounds like silver chloride that are only slightly soluble in water, a fraction of the ions will remain in solution. This fraction can be calculated using the Ksp for the compound. To remove more of the ion from solution, chemists make use of an application of Le Chatelier's principle called the common ion effect. Since the Ksp remains constant and is the product of the molar solubility of the ions from the compound, adding more of one ion will decrease the concentration of the others. Adding more excess chloride ions to a solution of silver ions, for example, would precipitate more of the silver from the solution.
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