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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.
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