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Why is an alcohol thermometer suitable for measuring the temperature of very cold regions?

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Alcohol thermometers are used rather than mercury thermometers in very cold regions because alcohol has a lower freezing point than mercury. Pure ethanol freezes at -115 degrees C, while mercury freezes at -38 degrees C. 

Thermometers rely upon thermal expansion of the liquid in a bulb at the base of a thin glass tube. as the temperature increases the liquid in the bulb expands and pushes higher into the glass tube. As is cools, it contracts and moves lower in the glass tube. If it freezes it won't move in the tube, so a liquid must be used that has a freezing point that's lower than the temperature it's measuring. Since alcohol is colorless a dye is added. Alcohol thermometers can be used to measure the temperature inside a freezer. 

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