Specific gravity: the density of a substance relative to water

Definition and related terms

Specific gravity is a special case of the more general concept of relative density, which allows any reference material. When water is used the term "specific gravity" is common. The underlying quantity compared is density, the mass per unit volume. In practice, water is often assumed to have a density of about 1000 kg/m³ at 4 °C for calculations, although the actual reference temperature can vary by convention (for example 20 °C is commonly used in some industries). Because temperature and pressure affect density, specific gravity values must be interpreted together with the temperatures at which both sample and reference densities apply.

How it is measured

There are several laboratory and field methods to determine specific gravity. Common approaches include:

• Hydrometers or lactometers: weighted glass instruments that float at a level indicating SG directly on a calibrated stem.
• Pycnometers: a closed container of known volume filled with the test liquid and weighed; density follows from mass and volume and thus SG can be computed.
• Digital density meters: instruments that measure oscillation frequency of a sample-filled tube to determine density with high precision.
• Archimedes' principle: measuring displaced fluid or buoyant force to infer density of solids and thereby compute SG.

Applications and examples

Specific gravity is widely used because it conveys whether a substance will sink or float in water and provides a convenient comparative value for many practical tasks. Typical applications include brewing and winemaking (monitoring fermentation by tracking SG changes), automotive and marine battery electrolyte checks, petroleum and chemical industry quality control, geology and mining (characterizing minerals and ores), and clinical testing (urine specific gravity gives information about hydration and kidney function).

Important considerations and notable facts

Because specific gravity is temperature dependent, published SG values should always be accompanied by the reference temperature or corrected accordingly. Air buoyancy produces a small error when weighing solids or liquids in air; high-accuracy work applies corrections or uses immersion methods. The term "apparent specific gravity" is sometimes used when such buoyancy effects are not removed.

It is helpful to remember qualitative examples: liquids and solutions with SG less than 1 will float on water (for instance many organic solvents), while materials with SG greater than 1 will sink (most metals and many concentrated brines). The magnitude of SG also aids material identification and quality control: small changes in SG can signal compositional or concentration changes in industrial processes.

For further technical detail on relative density conventions and temperature corrections consult specialized references and standard methods in analytical chemistry and materials testing.