The solubility of elemental calcium in water is unassessable because calcium reacts with liquid water to produce calcium hydroxide in a hydrolysis reaction. The reaction produces hydrogen gas and calcium hydroxide as byproducts.
A substance is considered soluble if it dissociates in another substance to form a homogenous solution. The substance being dissolved is the solute. The substance in which the solute dissolves is called the solvent. Together, the two are called a solution.
An important aspect of the dissolution process is that the chemical composition of the solvent and solute are usually not altered when the former dissolves in the latter. A dissolving ionic solid may dissociate into its ions when dissolved in water, but these ions should not react with the water forming new compounds for the process to be called dissolution. Although pure calcium cannot dissolve in water, calcium compounds can and often do dissolve.
Solubility is defined for specific phases of solid and solute. Aragonite and calcite are both allotropes of calcium carbonate having the same chemical composition, but their solubility in water differs because of the difference in the way their atoms are arranged. Dissolution should not be confused with solvolysis, in which a metal reacts with aqueous acid producing soluble products.