An insoluble material when placed in a liquid of identical density will neither sink nor float but remain suspended, moving only with the currents.

The beaker on the left contains some nylon strips. These are suspended in a solution of Sodium Sulfate.
10.00 mL of this solution pipetted into a flask was found to weigh...

11.427 g

What is the density of the nylon?
Where will these strips be tomorrow? Remember, the beaker is not covered.
What happens if you add some distilled water and mix?


A simple flotation device used to measure density is the hydrometer.
It is a sealed tube with a narrow part at one end and some very dense material such as lead at the other. Such a tube if made correctly, floats "vertically" so that the narrow part sticks out of the liquid while the heavy end sinks.
The narrow part is calibrated for density. The hydrometer floats higher in liquids of higher density and lower in liquids of lower density.
Hydrometers are used in wine making. They are used to measure the conversion of sugar, which makes the wine more dense, to alcohol, which makes the wine less dense.
Hydrometers are also used to test car batteries for their charge. The liquid in a fully charged battery contains a lot of the more dense sulfuric acid. The liquid in a discharged battery is mainly water which is less dense.

RWK 1997