Physical Science: Session 2

A Closer Look: Microscopic Reasons for Macroscopic Properties of Solids, Liquids, and Gases

Recall the four principles or characteristics of the particle model:

1. All matter is made of individual particles.
2. There is empty space in between the particles.
3. All particles are in constant motion.
4. There are forces between the particles.

How do these microscopic ideas explain the macroscopic properties of solids, liquids, and gases we gave in the previous video session? Although all characteristics of the particle model help to explain these properties, we also pick out the most important characteristic related to each property.

Phase Property Most important characteristic Microscopic reason

Solid

Holds shape There are forces between the particles. The forces that attract individual particles to one another are strong enough to overcome the tendency of their motion to pull them apart. This results in a rigid structure.
Not compressible There is empty space between the particles. Each particle can be thought of as a rigid sphere. In a solid, the particles are touching each other and therefore cannot be pushed together more.

Liquid

Takes shape of container All particles are in constant motion, and there are forces between the particles. The energy of the motion of particles is strong enough to allow the particles to move past each other, despite the forces between particles.
Not compressible There is empty space between the particles. For a similar reason to solids, the particles can be thought of as touching each other, so no force can compress them further.

Gas

Fills container All particles are in constant motion. The energy of the motion of particles is so great as to overcome the attraction between particles when they collide. Individual particles then continue on until they hit the walls of the container.
Compressible There is empty space between the particles. The resulting space between the particles is much larger than the particles themselves (i.e. they are not touching) so it is possible to squeeze a container to make those spaces smaller, at the expense of increasing the pressure of the gas inside the container.
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