The Hydrometer
Simple Explanation
A hydrometer is a sealed, weighted float with a graduated stem, used to directly measure the relative density of a liquid. It floats higher in a denser liquid, and lower in a less dense one — the depth it sinks to is read directly off the stem's scale as the relative density.
Why Do We Need It?
A hydrometer gives an instant reading of relative density without needing to separately measure mass and volume — a single glance at where it floats gives the answer directly.
Worked Example
Compare hydrometer readings in two liquids
A hydrometer floats with its stem reading 1.03 in seawater, and 1.00 in fresh water. Explain which liquid is denser, and why the hydrometer floats differently in each.
Why Does This Work?
A floating object always displaces a weight of liquid exactly equal to its own weight — in a denser liquid, a smaller volume of liquid weighs the same as the hydrometer, so it needs to sink less deep to displace that weight, floating higher as a result.
Real-Life Example
Wine and beer making
Brewers and winemakers use a hydrometer to track fermentation.
As sugar in the liquid converts to alcohol (which is less dense than sugar solution), the relative density drops — tracking the hydrometer reading over the days tells the brewer exactly how fermentation is progressing.
Practice
A hydrometer floats lower (sinks deeper) in liquid A than in liquid B. What can you conclude?
MediumCommon mistake
Assuming a hydrometer floats LOWER in a denser liquid — it is the opposite: a hydrometer floats HIGHER (sinks less) in a denser liquid, since less volume needs to be displaced to support its weight.
Quick Review
- A hydrometer directly measures relative density by how deep it floats.
- It floats HIGHER in a denser liquid, lower in a less dense one.
- Used to test seawater salinity, battery acid, and fermentation progress.