Hydrometer
Definition and meaning of Hydrometer in chemistry.
A hydrometer is an analytical instrument used to measure the density of a liquid. It floats freely in the test sample to provide a direct reading. The depth it sinks reveals the specific gravity compared to water.
In more detail
The device looks like a sealed glass tube with a weighted bottom. A thin stem with number markings extends upward from the weighted bulb. When you place it in a liquid, the tool sinks into the fluid.
It stops sinking when the displaced liquid equals its own total weight. This behavior follows the ancient physics rule known as Archimedes' principle. A denser liquid can support more weight in a smaller volume.
Therefore, the heavy glass instrument will float much higher in thick syrups. A less dense liquid like rubbing alcohol lets the tool sink deeper. Students often think the tool measures the actual weight of the liquid.
Instead, it measures the ratio of the liquid density compared to pure water. You take the reading right where the liquid surface crosses the thin stem. This simple design gives you the specific gravity without any complicated math.
Temperature changes the density of all liquids slightly. Because of this, you must check the liquid temperature before reading the scale. Most instruments are calibrated to work perfectly at exactly 20 degrees Celsius.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Underlying physics | Archimedes' principle of buoyancy |
| What it measures | Specific gravity or relative density |
| Calibration requirement | Must adjust for the sample temperature |
| Common units | Specific gravity, Brix, or Baume degrees |
A mechanic might use a hydrometer to test the acid inside a car battery. The instrument will float noticeably higher in a fully charged battery. This happens because the dense sulfuric acid pushes hard against the glass. As the battery loses power, the acid turns into less dense water. The glass tube then sinks deeper into the weak fluid. Another common version floats in salt brine to check aquarium water. A dense salt mixture will always lift the glass tube higher than pure tap water.
Frequently asked questions
How does a hydrometer give a density reading?
It sinks into the liquid until the upward buoyant force equals its own weight. You read the liquid level against a numbered stem to find the specific gravity.
Why does temperature matter when using a hydrometer?
Liquid density drops slightly as the liquid gets warmer and expands. You must use a temperature chart to correct your reading for accurate results.
Does a hydrometer measure the actual mass of the liquid?
No, it measures the relative density compared to pure water. It tells you how tightly packed the liquid is rather than its total weight.