Hydrometer is a chemical teaching tool used to measure the density of liquids. It is made of glass, with a slender graduated tube as the main body and a lead particle counterweight at the lower end, which can float vertically in the liquid. The scale increases in numerical value from top to bottom, corresponding to the density values of different liquids. In teaching, place it into the liquid to be tested and obtain the density based on the scale reading when floating. During operation, ensure that the densitometer does not touch the container wall to ensure accurate readings. Suitable for middle school chemistry laboratory classes, helping students understand the concept of density and measurement principles.