3. Kinetic Energy and Matter

Kinetic Energy

Energy is the ability to cause changes in matter. Any matter that is moving has energy just because it’s moving. The energy of moving matter is called kinetic energy. Scientists think that the particles of all matter are in constant motion. In other words, the particles of matter have kinetic energy. The theory that all matter consists of constantly moving particles is called the kinetic theory of matter. 

Pay attention to the animations of the molecules in each phase in the following video. The movement of molecules depends on the kinetic energy. You can just ignore the man talking about whipped cream at the end, although it is funny. :)

Kinetic Energy and States of Matter

Differences in kinetic energy explain why matter exists in different states. Particles of matter are attracted to each other, so they tend to pull together. The particles can move apart only if they have enough kinetic energy to overcome this force of attraction. It’s like a tug of war between opposing sides, with the force of attraction between particles on one side and the kinetic energy of individual particles on the other side. The outcome of the “war” determines the state of matter.

  • If particles do not have enough kinetic energy to overcome the force of attraction between them, matter exists as a solid. The particles are packed closely together and held rigidly in place. All they can do is vibrate. This explains why solids have a fixed volume and a fixed shape.
  • If particles have enough kinetic energy to partly overcome the force of attraction between them, matter exists as a liquid. The particles can slide past one another but not pull apart completely. This explains why liquids can change shape but have a fixed volume.
  • If particles have enough kinetic energy to completely overcome the force of attraction between them, matter exists as a gas. The particles can pull apart and spread out. This explains why gases have neither a fixed volume nor a fixed shape.

Look at the Figure below. It sums up visually the relationship between kinetic energy and state of matter. 

CK-12 Foundation, Physical Science. http://creativecommons.org/licenses/by-nc-sa/3.0/