# Density

In this lesson we will explore density, what it means, and how it is applied to the particle model.

## What is Density?

Density is how many particles are in a given space. To view this in everyday life, when you change classes the hallways become packed with students. It can be tight and uncomfortable for students moving from one class to another because it is so dense. Particles experience this phenomenon as well, they can be tightly packed into spaces, the more particles in a given space the higher the density.

The density of fluids is normally measured in g/mL, or how many grams a mL of the fluid weighs. Likewise, solids are normally measured in g/cm^{3}, or how many grams a centimeter cubed weighs. *Note this measurement is specific for Earth, because weight changes depending on the gravitational force.*

## Mass, Volume, and Density

Mass and weight are different. Mass is the amount of matter in a substance, while weight is the force of gravity exerted on that object. Think of mass as the size of and amount of particles in an object. This view is not perfect because not all particles are the same size and mass, but for now it is helpful to simplify the concept.

Volume is the amount of space something takes up. Often the notation of cubed is used on volume measurements (even when it is not it is implied, like mL), for example, cm3. The 3 stands for the three cardinal directions or planes in the universe, X, Y, and Z (see picture).

## Measuring Density

From above we already know that density is a measurement of mass and volume. This means that density is a ratio of mass : volume and we can easily calculate it if we have enough information about the object. Look below to see the formula to calculate density.

Of course temperature plays an important role in density because most substances change density as they heat up or cool down, likewise, we are assuming that the substance is pure with no other elements or compounds.