What Is The `density Of A Substance`?

Density in simple terms is the measure of how much something weighs compared to its size.  More precisely, it is the mass of an item divided by its volume.

What that means is that an item with high density will weigh more than an item of low density if the two items are the same size.  Or, in other words, it takes a larger item of low density to weigh as much as another item of high density.

Common units used are kg/m³ (metric) , and lb/in³ (U.S.)

Naturally occurring substances with very high densities are:
   Gold,  .697 lb/in³
   Platinum, .775 lb/in³
   Lead, .410 lb/in³

Naturally occurring substances with very low densities are:
   Air,  0.000045 lb/in³ (gas, at about room temp, 1 atm)
   Gasoline, 0.0025 lb/in³ (liquid)
   Aerogel, 0.00011 lb/in³ (solid)

The mass per unit volume of a substance.