A common glass tube thermometer uses a thin column of red liquid rising and falling to indicate temperature. This instrument relies on thermal expansion, where a substance increases in volume as it gets warmer. The liquid is housed in a bulb at the base, and as it heats up, it is forced to climb the narrow capillary tube, allowing for a precise temperature reading. Many people assume this visible red substance is mercury, a belief rooted in the history of temperature measurement when mercury was the standard liquid used in these devices.
What Mercury Really Looks Like
The assumption that the red column is mercury is incorrect; elemental mercury is silvery-white in color. This metal is liquid at room temperature and was historically known as “quicksilver.” Mercury was an ideal thermometric fluid due to its physical properties. It possesses a high density and remains liquid across a wide temperature range, from about -38.8°C to 356°C.
Mercury does not “wet” or stick to the glass walls of the capillary tube, ensuring uniform movement as it expands. Its consistent thermal expansion allowed for highly accurate temperature measurements. If a thermometer contained mercury, the liquid column would appear shiny and metallic, not red.
Identifying the Red Liquid
The red liquid in most modern liquid-in-glass thermometers is actually dyed alcohol, typically ethanol, or another organic liquid like kerosene. These liquids are naturally clear and colorless, making the column almost impossible to see against the glass. A bright red dye is added solely to make the fluid line clearly visible to the user.
Alcohol-filled thermometers function on the same principle of thermal expansion as mercury thermometers. Alcohol has a lower freezing point than mercury, allowing it to measure very cold temperatures down to nearly -115°C. However, alcohol also has a lower boiling point, which limits its usefulness for measuring high temperatures. While not as consistently accurate as mercury, these colored-alcohol thermometers are accurate enough for most household use.
Safety and the Phase-Out of Mercury
The transition away from mercury in thermometers was driven by safety and environmental concerns. Elemental mercury is a neurotoxin that poses a health risk if inhaled. If a mercury thermometer breaks, the liquid metal can vaporize at room temperature, releasing an invisible, odorless, toxic vapor.
Mercury is an environmentally persistent substance that can contaminate water and soil. Regulatory bodies have progressively banned or phased out mercury thermometers in consumer and medical settings to protect public health. The widespread adoption of colored-alcohol and digital thermometers has largely eliminated the risk of accidental mercury exposure in the home.