Why mercury thermometer break if the temperature is too low?
Mercury freezes at -39.8° C (about -40° F.) My guess would be that the thermometer doesn’t break when the temperature gets too low, but when it starts to warm up again. Once it starts to warm again after the mercury has frozen, it will expand as a solid (below -40°) before it changes state back to a liquid. When it does this, it can not flow through the small aperture from the bulb to the guage. When that happens, the pressure increases in the bulb and the bulb breaks. Another possible cause (going the other way) is that once the mercury freezes, if it continues to get colder the solid mercury contracts even more. This creates a vacuum in the bulb and the pressure differential between the air pressure and the vacuum breaks the glass. .
