Hydrometer
Definition and meaning of Hydrometer in chemistry.
Hydrometer is an instrument used to measure the density, or specific gravity, of a liquid by floating freely in it and reading the depth of immersion on a calibrated scale.
In more detail
The device is a sealed, weighted glass tube with a slender graduated stem. When placed in a liquid, it sinks until the weight of liquid it displaces equals its own weight, in accordance with Archimedes' principle. A denser liquid supports more weight per unit volume displaced, so the hydrometer floats higher; a less dense liquid lets it sink deeper. The reading is taken where the liquid surface crosses the stem's scale, giving specific gravity (relative to water at a reference temperature) directly, without any calculation.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Underlying principle | Archimedes' principle of buoyancy |
| Typical scale | Specific gravity, or degrees Brix, Baume, or API |
| Common applications | Battery acid, antifreeze, wine must, brewing wort, urine testing |
A hydrometer floats noticeably higher in saturated brine (density about 1.20 g/mL) than in pure water (density 1.00 g/mL), because the denser brine exerts a greater buoyant force for the same submerged volume.
Frequently asked questions
How does a hydrometer give a density reading?
It floats freely in the liquid and sinks until the buoyant force equals its weight; the immersion depth, read against a graduated stem, is calibrated to show specific gravity directly.
Why does temperature matter when using a hydrometer?
Liquid density changes with temperature, so most hydrometers are calibrated for a specific reference temperature (often 15 or 20 degrees Celsius) and readings taken at other temperatures need correction.