why mercury thermometer break if the temperature is too low?

Answer 1

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.
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Answer 2

njf13's answer doesn't make any sense. That answer could only be true if the pressure of the mercury in the bulb increases at very low temperatures (i.e., when more mercury is in the bulb); but in fact, the opposite is true. Look at it this way: If the mercury's pressure were great enough to burst the bulb, why wouldn't the mercury just squirt back up into the capillary tube instead of breaking the glass?

Here is a quote from the Wikipedia article, "Mercury-in-glass thermometer":

"Mercury, unlike water, does not expand upon solidification and will _not_ break the glass tube, making it difficult to notice when frozen." [emphasis added]

So, if there really are cases where the cold temperature causes the glass to break, I suspect it is caused by moving the thermometer too quickly from a warm environment to a cold one. Relatively quick changes in temperature often cause solids to crack; this is because part of the object is at the "new" temperature, causing it to expand or contract; while the other part of the object is still at the "old" temperature, and maintains its original size. If the object is brittle, the differential expansion/contraction will cause the object to crack.

Answer 3

If the thermometer gets too cold (-38 F), the mercury will freeze. If mercury expands when it freezes like water does, then the bulb could break. However, most liquids actually contract when they freeze. Look up the density of liquid and solid mercury at the freezing point to find out.

Answer 4

To know the answer, 1st u need to know ,why mercury is used to make thermometer..

ok ,in thermometer mercury works like this:

when temperature is high, it extends and so we can see the value on meter is rises.

thus when temperature is low, it shrinks (and we can see the value on meter shows lower )so vaccume is occured in thermometer ,and so it breaks..

(infect i have not heard about breaking of thermometer till yet,but if it happens i think ,it may break due to vaccume occored in it)

Answer 5

As the mercury solidifies in the bulb at around -39°C, its expansion as it begins to liquefy again will burst the bulb and it cannot expand into the capillary as a solid.

Answer 6

ive never had one break because of that. ( i live in missouri) but it never breaks when it gets below zero