A pycnometer with a thermometer is a precise laboratory tool, essentially a specific gravity bottle with a built-in thermometer, used to determine the density or specific gravity of liquids, powders, or solids by ensuring measurements are taken at a specific, controlled temperature, crucial because density changes with temperature. These devices are often made from borosilicate glass for durability and accuracy, featuring a stopper with a capillary tube, and are vital in quality control, research, and accredited labs.