Density is an intrinsic property of matter. We define density (d) as the mass or volume of a substance at a given temperature. We write d = m/v where d is density, m is mass, and v is volume. If we know two of the variables in this equation, we can solve for the third algebraically. The units for density are a mass unit divided by a volume unit. The units used to describe density often differ for the phases of matter: solids (g/cm3), liquids (g/mL), and gases (g/L).
After you read this section, try the practice problem examples 1 and 2.
Most of us have long understood that oil is lighter than water, or that iron is heavier than sugar. But in making such statements, we are implicitly comparing equal volumes of these substances: after all, we know that a cup of sugar will weigh more than a single ordinary steel nail.
Mass and volume, as we learned in the previous unit, are measures of the quantity of a substance, and as such are defined as extensive properties of matter.
You will recall that the ratio of two extensive properties is
always an intensive property – one that characterizes a particular kind of matter, independently of its size or mass. It is this ratio, (mass ÷ volume), that we are concerned with in this lesson.