Here's why:

* Density is a measure of how much mass is contained in a given volume. It's calculated as:

* Density = Mass / Volume

* Substance refers to the type of matter. A substance has a specific chemical composition and therefore a specific mass per unit volume. For example, water is water, regardless of how much of it you have.

Let's illustrate with an example:

Imagine you have two containers of water:

* Container 1: Holds 1 liter of water.

* Container 2: Holds 2 liters of water.

Even though Container 2 has twice the volume of Container 1, the water in both containers will have the same density. The mass will be twice as much in Container 2, but so will the volume, resulting in the same density.

Important Note: This is true for pure substances. Mixtures can have varying densities depending on the proportions of their components.