Strontium (Sr) is an alkaline earth metal located in Group 2 of the periodic table. Its interaction with water is complex, as the answer to its solubility depends on whether one refers to the pure, metallic element or its ionic compounds (salts). Elemental Strontium does not simply dissolve in water; instead, it undergoes a vigorous chemical reaction. Conversely, the solubility of Strontium compounds is highly variable, ranging from very soluble to practically insoluble. This variation is governed by chemical principles dictating how the Strontium ion interacts with other species.
The Reactivity of Elemental Strontium
When elemental Strontium is introduced to water, it does not passively dissolve like table salt. The metal is highly reactive, sharing properties with its neighbors Calcium and Barium, and immediately begins a chemical reaction. This process is exothermic, releasing heat energy into the surrounding water. Although the metal is dense enough to sink, the reaction quickly produces bubbles of gas.
The chemical interaction involves Strontium metal losing two electrons to form the \(Sr^{2+}\) ion, which reacts with water molecules. This process generates Strontium hydroxide (\(Sr(OH)_2\)) and hydrogen gas (\(H_2\)). The resulting Strontium hydroxide is only slightly soluble in water, and the reaction is rapid. The reaction is more energetic than the corresponding reaction with Calcium, but less vigorous than the one involving Barium.
Solubility Rules for Strontium Compounds
The solubility of Strontium salts is not uniform; it is largely determined by the identity of the anion (the negatively charged ion) paired with the \(Sr^{2+}\) cation. Many common Strontium compounds are highly soluble in water, meaning they fully dissociate into their constituent ions. Examples include Strontium chloride (\(SrCl_2\)), Strontium bromide (\(SrBr_2\)), and Strontium nitrate (\(Sr(NO_3)_2\)).
Other Strontium compounds are insoluble or only sparingly soluble. Strontium sulfate (\(SrSO_4\)), which occurs naturally as the mineral celestine, is highly insoluble. Similarly, Strontium carbonate (\(SrCO_3\)) and Strontium phosphate (\(Sr_3(PO_4)_2\)) are classified as insoluble. Strontium hydroxide (\(Sr(OH)_2\)) is slightly soluble, sitting in a middle ground compared to highly soluble salts like the nitrate.
Chemical Factors Governing Solubility
The dissolution of a Strontium compound in water is an energetic process governed by the balance between two opposing forces. The first is lattice energy, which is the energy required to break apart the tightly bound crystal structure of the solid salt. This energy holds the positive Strontium ions and the negative anion together.
The second factor is the hydration enthalpy, which is the energy released when the individual ions become surrounded by polar water molecules. When water molecules cluster around the charged ions, they form a stable, hydrated shell. A Strontium compound will be readily soluble only if the energy released by hydration is greater than the energy required to break the crystalline lattice apart. If the lattice energy is larger than the hydration enthalpy, the compound remains largely insoluble, explaining the low solubility of compounds like Strontium sulfate and carbonate.