Lesson 2: The Competition

Key Idea

Molecules in a substance are in a constant tug-of-war. The kinetic energy of the molecules makes them move and spread apart. At the same time, a constant force of molecular attraction pulls the molecules back together.

This "competition" between movement and attraction determines if a substance will change phase. A phase change happens only when the molecules' movement becomes strong enough to overcome the attraction. If the attraction is still stronger, the substance remains in its current phase.

Key Idea

The molecules that make up any substance are always pulling toward one another. This natural pull is an inherent force called molecular attraction. This attraction is what holds the molecules of a substance together, especially in the solid and liquid states.

The strength of this attraction is a constant property for a specific substance. It does not get stronger or weaker when energy is added or removed. The pull between the molecules is always there, acting like a constant "glue".