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Mercury is used in thermometers because it increases in length significantly due to rise in temperature, However, mercury has high density relative to water, which means more inter-molecular forces which makes it harder to expand than water. Then,why is mercury preferred to water in a thermometer?

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  • $\begingroup$ Your claims are incorrect, as R Millikan points out. $\endgroup$ – Carl Witthoft Jan 8 '14 at 14:30
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The density of mercury (13.534 g/cm^3) does not imply high intermolecular forces. It simply reflects that the mercury atom is much more massive than a water molecule. The atomic weight of mercury is 200.6, while the molecular weight of water is about 18, so mercury atoms take up $\frac {200.6} {18\cdot 13.534}=0.823$ as much volume as a water molecule. This doesn't say anything about the forces between the atoms.

Probably the worst thing about water for a thermometer (assuming you keep it from freezing) is that the expansion is not linear. Water hits a maximum density at 4C, so contracts as it drops in temperature from 0 to 4. Above 4C, it expands quite slowly for a while, then more quickly as it heats up. A plot of the volume of water as a function of temperature is below. It makes thermometers much easier if the curve is linear, which this is not. Over a small range, it is close to linear. It would work well for medical thermometers with a range of $35-42 C$, say

enter image description here

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I don't design thermometers and don't know exactly what the engineering tradeoffs are, but here are some advantages of mercury over water that pop to mind:

  1. Mercury doesn't turn to a solid and expand at 0°C.

  2. Mercury is not a clear liquid, which makes seeing it in a clear glass tube a lot easier.

  3. Mercury has much lower vapor pressure than water, so won't evaporate and condense at other parts of the tube.

  4. Mercury is a lot more linear in expansion as a function of temperature than water. Water isn't even monotonic a few degrees above freezing. Non-monotonicity is a obvious show stopper for a thermometer material.

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