Kinetic Theory of Matter

The kinetic theory of matter helps us to explain why matter exists in different states or phases (solid, liquid, and gas), and how matter can change from one phase to the next. Broadly, the kinetic theory of matter says that all matter is composed of particles that have a certain amount of energy which allows them to move at different speeds depending on the temperature (energy).

In addition, particles of matter of the same substance, such as the same element, are attracted to one another. The force of attraction tends to pull the particles closer together. The particles need a lot of kinetic energy to overcome the force of attraction and move apart. It's like a tug-of-war between opposing forces. The kinetic energy of individual particles is on one side, and the force of attraction between different particles is on the other side. The outcome of the "war" depends on the state of matter.

This is illustrated in the picture below.

image of solid, liquid, and gas
kinetic energy →
particle motions →
← forces of attraction between particles

In solids, particles don't have enough kinetic energy to overcome the force of attraction between them. The particles are packed closely together and cannot move around. All they can do is vibrate. This explains why solids have a fixed volume and shape.
In liquids, particles have enough kinetic energy to partly overcome the force of attraction between them. They can slide past one another but not pull completely apart. This explains why liquids can change shape but have a fixed volume.
In gases, particles have a lot of kinetic energy. They can completely overcome the force of attraction between them and move apart. This explains why gases have neither a fixed volume nor a fixed shape.

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