A hydrometer is a scientific instrument used to measure the specific gravity or density of liquids. It typically consists of a sealed glass tube with a weighted bottom that allows it to float upright in the liquid being tested. The scale on the hydrometer indicates the density, which can help determine the concentration of solutions, such as sugar in water or alcohol in beverages. Hydrometers are commonly used in various fields, including brewing, winemaking, and aquarium maintenance. By providing accurate density measurements, they assist in quality control and ensure that products meet desired specifications.