Solutions Intermolecular Forces Compounds  Elmhurst College
Review of Solutions: Solubility of Polar Solutes Temperature & Pressure  Chemistry Department
Solubility of Salts Non-polar Solutes    Virtual ChemBook

Solubility of Salts

IONIC SOLUTE - POLAR SOLVENT:

Dissolving a Salt Crystal:

When an ionic crystal such as NaCl is placed in water, a dissolving reaction will occur. Initially, the positive and negative ion are only attracted to each other. The water molecules are hydrogen bonded to each other. If the crystal is to dissolve, these bonds must be broken.

Negative chloride ions on the surface are attracted by neighboring positive sodium ions and by the partially positive hydrogen atom in the polar water molecule (See the graphic on the left).

Similarly, the positive sodium ions are attracted by both chloride ions and the partially negative oxygen atom in the polar water molecule.

A "tug-of-war" occurs for the positive and negative ions between the other ions in the crystal and the water molecules. Several water molecules are attracted to each of the ions.

Whether the crystal dissolves is determined by which attractive force is stronger. If the internal ionic forces in the crystal are the strongest, the crystal does not dissolve. This is the situation in reactions where precipitates form. If the attractions for the ions by the polar water molecules are the strongest, the crystal will dissolve. This is the situation in sodium chloride.

Salt Crystal Dissolved:

Once the ions are released from the crystals, the ions are completely surrounded by water molecules. Note that the proper atom in the water molecule must "point" toward the correct ion. The charge principle and the partial charges in the polar molecule determine the correct orientation.

Partially negative oxygen atoms in the water molecule interact with the positive sodium ion.

Partially positive hydrogen atoms in the water molecule interact with the negative chloride ion.

Link to Animation of Salt Dissolving - Northland Community and Technical College

Link to Animation of Salt Dissolving with audio - Essential Chemistry, 2/e by Raymond Chang