Density
Density is the mass of a substance per unit of volume.
density = mass ÷ volume, or d = m / V
Common units are g/mL for liquids and g/cm3 for solids. These are the same size, because 1 mL = 1 cm3.
Density is a property of the substance, not the sample. A gold ring and a gold bar have the same density, 19.3 g/cm3. That makes density useful for identifying materials.
An object floats in a liquid if it is less dense than the liquid. Water has a density of 1.00 g/mL.
Worked example: find a density
A metal block has a mass of 54.0 g and a volume of 20.0 cm3.
d = 54.0 g ÷ 20.0 cm3 = 2.70 g/cm3
That matches aluminum.
Worked example: find a mass
What is the mass of 15.0 mL of ethanol, density 0.789 g/mL? Rearrange to m = d × V.
m = 0.789 g/mL × 15.0 mL = 11.8 g
Common mistake
Thinking heavy objects sink and light ones float. A steel ship floats and a small pebble sinks. What matters is mass compared with volume, not mass alone.