What is the name of the sphere of water molecules surrounding each dissolved ion in an aqueous solution?

The sphere of water molecules surrounding each dissolved ion in an aqueous solution is called the hydration shell. This concept is essential in understanding how ions interact with water in solution. When an ionic compound dissolves in water, the polar water molecules orient themselves around the charged ions, with the oxygen side of water, which has a partial negative charge, attracted to cations (positively charged ions) and the hydrogen side, which has a partial positive charge, attracted to anions (negatively charged ions). This organized arrangement of water molecules forms a hydration shell that stabilizes the ion in solution and facilitates its movement.

While the term "solvation layer" also describes a similar concept and could refer to the interaction between solute and solvent molecules, it is often used more broadly than just in the context of water. Cohesion layer and shell structure do not accurately describe the specific interaction between water molecules and ions, making "hydration shell" the most precise and commonly used term in the context of dissolved ions in aqueous solutions.