## Density and Its Uses

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/cm^{3}), 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.