Specific Gravity
Definition and meaning of Specific Gravity in chemistry.
Specific gravity is the ratio of a substance's density to the density of a reference material. Water is the usual reference for liquids and solids. Because it compares two densities, specific gravity has no units.
In more detail
A specific gravity above 1 means the substance is denser than water, so it will sink. A specific gravity below 1 means it is less dense than water, so it will float. Temperature affects this measurement a lot, because density changes as substances heat up or cool down.
For an accurate result, both the sample and the water reference need a stated temperature. Scientists often use 4 degrees Celsius for water, since water is at its densest at that temperature. Specific gravity is widely used across many fields.
Brewers use it to track sugar levels during fermentation. Mineralogists use it to help identify unknown rock and mineral samples. Doctors use it to check the concentration of a patient's urine.
An unusually concentrated or dilute reading can help flag kidney problems. Because the measurement is simple and needs no special units, it remains a quick, practical tool in many industries. A hydrometer is a simple floating instrument used to measure specific gravity directly in a liquid sample.
Winemakers and brewers use this tool to track how much sugar has turned into alcohol during fermentation. Geologists sometimes use specific gravity, along with color and hardness, to help tell two similar looking minerals apart.
Key facts
| Field | General Chemistry |
|---|---|
| Units | None, it is a dimensionless ratio |
| Reference material | Usually H2O at 4 degrees Celsius |
| Relation to density | Density of substance divided by density of reference |
| Used in | Brewing, mineralogy, and medicine |
The specific gravity of pure gold is 19.3. This means a piece of gold is 19.3 times as dense as an equal volume of liquid water (H2O). Both must be measured at the same temperature for the comparison to be accurate. This huge difference is part of why gold feels so heavy for its size.
Frequently asked questions
Is specific gravity the same as density?
No. Density has units, like grams per milliliter. Specific gravity is a unitless ratio that compares a substance's density to the density of water.
Why does specific gravity depend on temperature?
Density changes as a substance heats up or cools down, because most materials expand slightly when warmed. This means the specific gravity result can shift unless both the sample and reference are measured at stated temperatures.
What does a specific gravity of exactly 1 mean?
It means the substance has the same density as the reference material, usually water. It will neither float nor sink but stay suspended at the same level.