What is a hydrometer primarily used for?

A hydrometer is primarily used for measuring the specific gravity of liquids, which is the ratio of the density of a substance to the density of a reference substance, typically water. This instrument consists of a float that is calibrated to indicate the specific gravity directly on a scale. The way it functions is based on buoyancy: when the hydrometer is placed in a liquid, it sinks to a level that corresponds to the liquid's density.

This characteristic makes the hydrometer particularly useful in various applications, such as determining the concentration of solutions, monitoring fermentation in brewing, and measuring the salinity of water, among others. Each of these applications relies on precise knowledge of specific gravity, which can provide vital information about the physical properties of the liquid in question.

The other choices represent different measurements not associated with a hydrometer. For instance, measuring temperature requires a thermometer, while measuring pressure typically involves a manometer or pressure gauge. Viscosity, the measure of a fluid's resistance to flow, demands different types of instruments. Thus, the specificity of a hydrometer's function highlights its unique role in assessing specific gravity.