Mercury is generally used in thermometers because

Mercury is chosen for use in thermometers primarily because of its high density. The high density of mercury allows for the creation of compact thermometers with precise and easily readable scales. Due to its dense nature, even a small quantity of mercury can produce a noticeable rise in the liquid column, facilitating accurate temperature measurement. Furthermore, mercury’s physical properties, such as its low freezing point of -38.83 °C and its wide liquid range, make it suitable for use in various temperature ranges. Its low coefficient of expansion also ensures that the volume change with temperature is relatively small, leading to stable and reliable temperature readings. Although mercury is toxic and poses health risks if mishandled or ingested, its physical properties make it an ideal choice for traditional liquid-in-glass thermometers. Therefore, its high density, combined with other favorable characteristics, makes mercury a commonly used fluid in thermometers.