Intermolecular Forces

GOAL(s):

• Classify phase changes as an increase or decrease in intermolecular forces.
• Determine the types of intermolecular forces that will exist in a substance based on molecular structure.
• Rank various molecules according to the strength of the intermolecular forces based on their molecular structure.
• Explain phase change data (boiling points, melting points, etc.) using intermolecular forces.

There is only one substance on Earth that exists in large quantities in all three physical states (solid, liquid, and gas): water. Water is essential to nearly all forms of life. While every person encounters each physical state of water every day, not many know the unique properties of this amazing molecule.

Several of water's chemical properties are unique. For example, water expands as it freezes, causing ice to be less dense than liquid water. You observe this when you place ice cubes in a glass of water and see them float. In nearly every other substance, the solid phase is more dense than the liquid phase. This unique property of water is of great consequence for aquatic life. As temperatures drop in the winter, ice forms on the surface of lakes, forming an insulating barrier and allowing the water below to remain in the liquid state. If ice were more dense than liquid water, lakes would freeze solid from the bottom up with deadly consequences for aquatic life.

Why is water so unique? The answer to this question lies in the molecular structure of water.