Air Particle Density
When we revisit our earlier scenarios and try to visualize the air particles inside of the syringe(s) at each step, we begin to notice a second pattern that we may not have noticed before. When a syringe system is at rest (no one is pulling up or pushing down on a plunger), the syringe system will always return to the standard air particle density — where the standard air particle density is represented by 35 dots in 35 milliliters, or one dot per milliliter.
Air particle density is a measure of how closely packed together the air particles are. When the density is low, the air particles are spread far apart. When the density is high, the air particles are close together. You can calculate particle density by dividing the number of particles by the volume they occupy. This will give you the number of particles per unit volume.
Particle density is related to density when we discuss the density of liquids and solids, but slightly different.