The hydration sphere, a form of solvation shell, is a chemical structure that surrounds a solute in a solution in which the solvent is water. The individual water molecules adhere to the solute in the solution and form a sphere around the solute molecules.
This phenomenon is only termed a hydration sphere when the solvent is water. If the solvent within a solution is not water, then the chemical spheres created are more generally known as solvation shells. Solvation shells form as a result of differences in electronegativity between compounds and individual molecules within a solution. Typically, shells and spheres surround loose ions that have positive or negative charges.