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The Solvation Process


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Figure 1

Consider an ionic solid, such as NaCl. Recall from the Periodic-Properties Experiment that an ionic solid is an extended three-dimensional lattice (a regular geometric arrangement) of cations and anions. NaCl (Figure 2) has the shape of a cubic crystal consisting of repeating units of Na+ and Cl- ions. One of the repeating units (the "unit cell") is shown in Figure 1.  This is the unit cell for a sodium chloride (NaCl) crystal lattice, in which Na+ and Cl- ions are arranged in a regular cubic pattern. This pattern can be repeated indefinitely to make a NaCl crystal.

In order for NaCl to be soluble, the Na+ and Cl- ions must break free from the crystal-lattice structure of the solid. When the ions are in solution, they are surrounded by water molecules, and the ions are said to be solvated, or dissolved in an aqueous solution, denoted (aq). Hence, the process of dissolving a NaCl crystal can be described by the following chemical equation (Equation 1): 

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Equation 1

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What are the microscopic events that occur when the crystal dissolves in water? The ions become solvated because of (1) the favorable electrostatic interactions with the solvent molecules and (2) entropy (randomness, as you will learn in unit 7). If the sum of these effects are stronger in the solution than in the crystal, the salt will dissolve. In this tutorial, we will describe only the effect of the electrostatic interactions on solvation.

H2O is a polar molecule; i.e., the molecule has negatively and positively charged regions. These charged regions are attracted to ions with the opposite charge. Hence, the positively charged regions of water molecules are attracted to Cl- ions, and the negatively charged regions of water molecules are attracted to Na+ ions. When several water molecules surround an ion in the crystal, the sum of the attractive forces between the water molecules and the ion may become strong enough to overcome the attractive forces between the cations and anions in the crystal. The water molecules form a shell of solvation around the ion, and the water-surrounded ion can break away from the crystal, as shown in Figure 2.

Figure 2This illustration shows a NaCl crystal being dissolved by H2O. Two ions have become solvated by water molecules and have broken away from the crystal. The water molecules form a "shell of solvation" around the ions. The ions that have broken away from the crystal and the water molecules forming the shells of solvation are shown in black and white in this illustration.

Note: Chloride ions are shown in green, sodium ions are shown in dark blue, oxygen atoms are shown in red, and hydrogen atoms are shown in light blue.

Eventually, all of the ions in the crystal may become solvated and break away from the crystal, forming a solution of solvated (aqueous) ions. The ions in the crystal become "free agents" in the solvent-- unattached to other ions and free to migrate about randomly in solution. In the solution, H2O molecules are constantly moving and exchanging with one another. Three steps in the formation of an ionic solution from a crystal are shown in Figure 3. The movie link in Figure 3 allows you to watch this process. 

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Figure 3
Figure 3:This scheme illustrates three steps in the formation of an ionic solution from a crystal. (1) The ionic crystal is immersed in a solvent (e.g., water). (2) Ions become solvated and break away from the crystal. (3) The crystal is completely broken apart as all of the ions become solvated, and a solution of solvated (aqueous) ions results.

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Overall, the charges in an ionic solution balance, so the solution is electrically neutral.

Nonionic (molecular) solids (e.g.http://www.youtube.com/watch?v=EBfGcTAJF4o, sugar) can also dissolve in water by a similar mechanism. When molecular solids dissolve, the individual molecules remain intact but become solvated by H2O molecules (and thus break away from the solid). Gas and liquid molecules can also be solvated by H2O molecules to form solutions.

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