Density is a measure by which a pure substance's mass is divided by a pure substance's volume. When comparing two substances' densities, the denser of the two substances will have a greater mass-to-volume ratio than the other.
Common densities of substances found in the clinical laboratory are:
- Water at 4oC has a density of 1 g/mL
- Human serum has a density of 1.035 g/mL
- 70% isopropyl alcohol has a density of 0.88 g/mL
- Solid sodium chloride (NaCl) has a density of 2.16 g/mL or 2.16 g/cm3
Specific gravity is a unitless measure and is a ratio of the density of a specific substance divided by the density of water at 4oC. Since water at 4oC has a density of exactly 1 g/mL, a substance with a specific gravity of 1.8 indicates that the substance is 1.8 times denser than water.
Human urine typically is measured for specific gravity and has a normal range of 1.005 to 1.030.
