Hydrated Ion: Definition, Structure & Examples

Instructor: Stephanie Bryan

Stephanie has a master's degree in Physical Chemistry and teaches college level chemistry and physics.

What happens when salt is placed in water? In this lesson, we will explore the hydration of ions, or what happens when ionic solids dissolve in water and further talk about ions that even form complexes with water in the solid phase.

Dissolving in Water

Many ionic solids are soluble in water. What does this mean? Observationally, it appears that table salt, NaCl, disappears into water when it dissolves. Some other ionic solids which have a visible color, like copper (II) sulfate, can be seen drifting into the solution and spreading out. But what is actually happening at an atomic level? That is what we want to explore here.

Ionic Solids

Ionic solids are made up of a continuous framework of ions. These ions sit in a regular pattern that repeats itself. There must be both positive ions, called cations, and negative ions, called anions, in any ionic solid in order for the solid to have no net charge. Once these solids are placed into water, they begin to dissolve. Ions are picked up off this lattice framework and carried into the water solution.

Water as a Polar Solvent

Before we can talk about how and why water can dissolve and carry off these ions into solution, we need to look closely at the structure of water. Water is a covalent molecule with one oxygen in the middle and two hydrogen atoms at an angle of 104.5 degrees from this central oxygen. Because oxygen is much more electronegative than hydrogen, bonding electrons in the molecule spend more time near the oxygen atom than near the hydrogen atoms. This results in a dipole moment in the water molecule. A dipole moment in a molecule means that one part of the molecule is partially negative and another part of the molecule is partially positive. Because water has a dipole moment, it is a polar solvent.

The structure and polarity of the water molecule

Hydrated Ions

Once a soluble ionic solid, like NaCl, is placed into water, the polar solvent begins working. The positive side of water's dipole moment face the Cl- ions, creating a spherical solvation shell surrounding the anion and carrying it away from the solid and into solution. Similarly, the negative side of the dipole moment of the water molecules will turn toward the Na+ ions to create a solvated sodium ion. The solvated ion consists of an ion in the middle surrounded by aligned solvent molecules surrounding the ion in a spherical shell. When the solvent is water, we also call this a hydrated ion. Notice that the picture below is a two-dimensional image. In reality, the water molecules surround the ions in a three-dimensional sphere.

An example of hydrated NaCl ions

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