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Kinetic Theory of Matter

Introduces the theory that all matter consists of constantly moving particles.

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Kinetic Theory of Matter

These neat rows of cola bottles represent matter in three different states—solid, liquid, and gas. The bottles and caps are solids, the cola is a liquid, and carbon dioxide dissolved in the cola is a gas. CO2 gives cola its fizz. Solids, liquids, and gases such as these have different properties. Solids have a definite shape and volume. Liquids also have a definite volume but can change their shape with the shape of the container they are in. Gases have neither a definite shape or volume. What explains these differences in states of matter? The answer has to do with energy.

Moving Matter

Energy is the ability to cause changes in matter. For example, your body uses chemical energy when you lift your arm or take a step. In both cases, energy is used to move matter—you. 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.

There are three parts to the theory:

  1. All matter is composed of tiny particles (atoms, molecules, ions, etc...)
  2. Thes particles are in constant and random motion.
  3. These particles collide with each other and their walls of their container without loosing energy in the collisions (elastic collisions).

You can learn more about the theory at this URL: http://www.youtube.com/watch?v=_rsqBNhFG1Y

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 definite volume and 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 still have a definite mass.
  • 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 definite volume or shape. Becasue the gases can spread out this also means that they can be compressed into smaller volumes or can expand into larger volumes.

Look at the Figure below. It sums up visually the relationship between kinetic energy and state of matter. You can see an animated diagram at this URL:


  • The final state of matter is plasma. Plasma (from Greek πλάσμα, "anything formed"). It comprises the major component of the Sun. Heating a gas may ionizeits molecules or atoms (reducing or increasing the number of electrons in them), thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions. Other than in the sun, plasma is found in lightening, can be created in neon lights   with high-energy devices (like a plasma torch).


  • According to the kinetic theory, particles of matter are in constant motion. The energy of motion is called kinetic energy.
  • The kinetic energy of particles of matter determines the state of matter. Particles of solids have the least kinetic energy and particles of gases have the most.
  • There are four states of matter: solid, liquid, gas and plasma.


  1. Create a six to nine pannel cartoon that explains the kinetic model and the four states of matter.
    1. The cartoon strip must have information about the different energy levels of each state and the basic properties of the state (like liquids taking on the shape of the container). 
    2. The carton must be neat.
    3. Color is good but it is optional.

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