Relative Density
Specific gravity (also known as relative density) is the ratio of the density of a substance to the density of a given reference material. For liquids and solids, the reference substance is almost always water at $4^\circ\text{C}$ (where water is at its maximum density of $1000 , \text{kg/m}^3$). For gases, the reference is usually dry air.
Because it is a ratio of two densities, specific gravity is a dimensionless quantity (it has no units). A specific gravity less than 1 means the substance will float in the reference fluid (water); a specific gravity greater than 1 means it will sink.
Practical Applications
Specific gravity is a widely used metric across many industries because it allows for quick, standardized density checks without needing to calculate exact volumes and masses:
- Brewing and Winemaking: Brewers use a hydrometer to measure the specific gravity of the "wort" before and after fermentation to determine the alcohol content.
- Automotive: Mechanics check the specific gravity of battery acid to determine the battery's state of charge, and check antifreeze coolant to ensure it won't freeze in winter.
- Gemology: Jewelers measure the specific gravity of gemstones to accurately identify them and distinguish real gems from fakes without damaging them.
The Formula
Example Calculation
You have a sample of liquid mercury, which has a known density of $13,593 , \text{kg/m}^3$. The reference density of water is $1000 , \text{kg/m}^3$.
- Divide Substance Density by Reference Density: $13,593 / 1000 = 13.593$.
The specific gravity of mercury is $13.593$. It is over 13 times denser than water, meaning iron and lead anvils will easily float in a pool of liquid mercury.