What is the purpose of a hydrometer in aqueous chemistry?

The hydrometer is designed specifically to measure the specific gravity of liquids. Specific gravity is the ratio of the density of a substance to the density of a reference substance, typically water for liquids. The hydrometer consists of a calibrated glass tube with a bulb at one end that floats upright in the liquid being tested.

As the hydrometer is placed in a liquid, it displaces a volume of the liquid equivalent to its weight. The level to which it sinks into the liquid correlates directly with the specific gravity based on the density of the liquid. This is particularly useful in various applications, such as determining the concentration of solutions or assessing the purity of liquids.

The other options relate to other measurement concepts not suited for a hydrometer's function. A hydrometer cannot measure pH as that requires a pH meter or indicator, nor can it measure gas pressure, which involves different instrumentation such as a manometer. Additionally, it does not measure temperature, which requires a thermometer. Thus, the primary purpose of a hydrometer remains centered on measuring the specific gravity of liquids.