# Particle Motion

At the very start of this unit, you learned about the particle theory of matter — the idea that all matter is made up of tiny particles in constant motion. So far we have gotten pretty good at visualizing those particles, especially when it comes to air inside a syringe. However, we have neglected to visualize those particles in constant motion.

Let’s take a look at a single air particle trapped inside a box:

Notice that the particle is in constant motion. It moves in a straight line and at a constant speed until it bounces off of a wall. When it does bounce off of a wall, it changes direction but keeps moving at the same speed. This is how I want you to visualize particles in constant motion. The speed of an air particle depends on the temperature of the air. The hotter the temperature, the faster the particles move. The cooler the temperature, the slower the particles move. To keep air particles from moving, you would have to cool the air to a temperature of zero kelvin (-273.15 °C or -459.67 °F). That is very, very, very cold. Basically, the coldest temperature possible. You will learn more about the relationship between particle motion and temperature next year.

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