Although many people commonly associate thermometers with a mercury filling, the dangerous properties of mercury have spurred manufacturers to use other liquids inside the glass gauges. Some thermometers may still contain mercury but many typically contain less toxic liquids.
The first thermometers manufactured contained water but the fact that water freezes at 32 degrees Fahrenheit led manufacturers to switch to mercury. Many older thermometers contained mercury. Because mercury has very toxic properties that can lead to sickness or death if the thermometer breaks, more modern thermometers use a form of colored alcohol.
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You can identify the liquid in a thermometer based on its color. Silver liquid indicates that the thermometer contains mercury, while red liquid is alcohol to which red coloring has been added. Although uncommon in modern thermometers, a clear color indicates water.
The liquid in a thermometer typically stays in a bulb at the bottom of the device. Because liquids expand as temperatures rise, the liquid climbs the thermometer tube as the temperature warms.