Relative Density
Definition and meaning of Relative Density in chemistry.
Relative density is the ratio of the density of a substance to the density of a reference substance, typically water at 4°C. It is a dimensionless quantity used to characterize and compare the physical properties of different materials.
In more detail
Since water at 4°C has a density of 1.0 g/cm³, the relative density of a substance is numerically equal to its density expressed in g/cm³. This dimensionless nature makes relative density convenient for quick comparisons without needing units. A substance with relative density less than 1 will float on water, while one greater than 1 will sink, which is why relative density is useful for predicting buoyancy. Relative density is commonly used in geology, materials science, and laboratory practice to identify, classify, and compare the physical properties of minerals, metals, liquids, and other materials.
Key facts
| Field | Physical Chemistry |
|---|---|
| Formula | Relative Density = density(substance) / density(water at 4°C) |
| Reference Standard | Water at 4°C (1.0 g/cm³) |
| Character | Dimensionless; pure numerical ratio |
Mercury has a relative density of approximately 13.6, meaning it is 13.6 times as dense as water. This property explains why mercury sinks rapidly in water and why it is ideal for use in barometers and thermometers.
Frequently asked questions
Is relative density the same as specific gravity?
Yes, they are synonymous in modern chemistry. The term 'relative density' is now preferred by IUPAC, though 'specific gravity' remains widely used, particularly in older texts and industrial applications.
Why is water at 4°C the reference standard?
Water reaches its maximum density at 4°C (1.0 g/cm³), making this a consistent and reproducible reference point for comparing the densities of different substances across varying temperatures and conditions.