Density
Definition and meaning of Density in chemistry.
Density is the mass of a substance per unit volume, expressed with the formula ρ = m/V, where ρ (rho) is density, m is mass, and V is volume.
In more detail
Density reflects how tightly matter is packed at the particle level: substances with heavier atoms or more closely spaced particles have higher density. It generally decreases as temperature rises (because volume expands while mass stays constant) and changes sharply at phase transitions, most notably water, which is denser as a liquid near 4°C than as ice. Density is an intensive property, meaning it does not depend on the amount of material present, which makes it useful for identifying substances and predicting whether one material will float or sink in another.
Key facts
| Field | Physical Chemistry |
|---|---|
| Formula | ρ = m/V |
| SI Unit | kg/m³ (often g/mL or g/cm³ for liquids and solids) |
| Property Type | Intensive (independent of sample size) |
Liquid water has a density of about 1.00 g/mL at 4°C, while ice has a density of about 0.92 g/mL, which is why ice floats on liquid water.
Frequently asked questions
Why does ice float on water?
Ice is less dense than liquid water (about 0.92 g/mL versus 1.00 g/mL) because water molecules form an open hexagonal hydrogen-bonded lattice when frozen, which takes up more space than the more closely packed arrangement in liquid water.
Does density change with temperature?
Yes, for most substances density decreases as temperature increases because thermal expansion increases volume while mass remains constant; water between 0°C and 4°C is a notable exception.