Final answer:
The hydrometer is used to measure the specific gravity of liquids through the principle of buoyancy, indicating the liquid's density or concentration of dissolved substances.
Step-by-step explanation:
The purpose of the hydrometer is to measure the specific gravity or relative density of liquids. This scientific instrument operates on the principle of buoyancy. Consisting of a sealed, hollow glass tube that is wider at the bottom and contains a ballast like lead or mercury for stability, a hydrometer also features a narrow stemmed section with graduation marks for measurement.
When placed in a liquid, the hydrometer floats, and the level to which it sinks correlates to the liquid's density. Therefore, you can read the specific gravity of the liquid directly from the stem, where it aligns with the surface of the liquid. This tool has various applications such as checking the charge level in a lead-acid battery or measuring the concentration of substances dissolved in a solution.