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Density

Density is a measure of mass (the amount of molecules) in a given object or volume. Density of fluid (volume) is measured in lbm/ft³ or kg/m³.

A fluid with a lot of molecules tightly packed together has a high density; one with fewer molecules would have a lower density. Water, for example, has a much higher density than air. A 10 gallon fish tank with water in it has much more mass than a 10 gallon tank with air in it. Since it has more mass, it will weigh more.

Density is also used to define whether a fluid is incompressible or compressible. If the density of the fluid is fixed (constant), the fluid is incompressible; neither the mass or the volume can change. Water is an incompressible fluid. The amount of volume and mass will stay the same, even under pressure.

Gases (like air), are compressible; they expand to fill a new larger volume. The mass doesn't change, but the volume increases, so the density of the gas decreases in the new volume.

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