What Is Density?
Density is a fundamental physical property defined as mass per unit volume. It describes how tightly packed matter is within a substance. Denser materials have more mass in the same volume. Density determines whether objects float or sink (objects less dense than a fluid float in it), affects structural calculations, and is used to identify and characterize materials.
Density varies with temperature and pressure, especially for gases. For solids and liquids, temperature effects are usually small but measurable. Water has its maximum density at 4 degrees C (999.97 kg/m³), a fact that has profound ecological importance as it causes ice to float, insulating lakes from freezing solid.
Density Formula
Where ρ is density (kg/m³), m is mass (kg), and V is volume (m³). Rearranged: m = ρV and V = m/ρ.
Common Densities
| Material | Density (kg/m³) | Density (g/cm³) |
|---|---|---|
| Air (STP) | 1.225 | 0.001225 |
| Water | 1,000 | 1.000 |
| Aluminum | 2,700 | 2.70 |
| Iron | 7,874 | 7.87 |
| Gold | 19,300 | 19.30 |
| Osmium (densest) | 22,590 | 22.59 |
Frequently Asked Questions
How does temperature affect density?
Most substances expand when heated, decreasing their density. Water is anomalous: it becomes denser as it cools from 100 to 4 degrees C but then expands as it cools further to 0 C and freezes. This is why ice floats and pipes burst when water freezes. For gases, density is inversely proportional to absolute temperature at constant pressure (ideal gas law).
What is specific gravity?
Specific gravity (SG) is the ratio of a substance's density to that of water at 4 C. Since water's density is approximately 1000 kg/m³ (or 1 g/cm³), specific gravity numerically equals density in g/cm³. SG is dimensionless and convenient for quick comparisons: SG < 1 floats on water, SG > 1 sinks.
How is density measured?
Common methods include: direct measurement (weigh and measure volume), Archimedes' method (measure buoyant force), hydrometers (floating calibrated tube), and pycnometers (precise volume flasks). For gases, density is calculated from pressure, temperature, and molar mass using the ideal gas law.