[EvaH (OP)]

Neil asks:

Water expands when it freezes, yet mercury in a thermometer seems to do the exact opposite. The lower the temperature the lower the mercury level. Why is that?

Can you help?

[alancalverd]

Water is anomalous and expands because the intermolecular hydrogen bonds determine its bulk structure at low temperatures, introducing temporary polymerisation and order in the liquid phase below 4 deg C and an even more open crystal structure in the solid phase. Mercury remains monatomic and simply becomes more compact as you remove thermal (kinetic) energy from the atoms.

There are other anomalous materials. "Hot type" alloy expands as it freezes, increasing the sharpness of linotype print.

[evan_au]

In my simplistic understanding:
- as temperature rises, the amplitude of random atomic vibrations increase. This pushes apart the average distance between atoms in a solid or liquid.
- as temperature falls, the amplitude of random atomic vibrations decreases. Atomic attraction pulls the atoms closer together, reducing the average distance between atoms in a solid or liquid.
- so in "normal" materials, they shrink when cooled, and expand when heated.

The anomalous expansion of water below 4°C allows lakes to freeze over, with the ice floating on top, while creatures continue to survive below the (somewhat) insulating layer of ice.
- This makes a lot of areas more habitable than they would be if water was "normal" in its thermal properties

[axscientist]

The higher the temperature, the more the mercury (or alcohol) expands and rises in the tube. The lower the temperature, the lower the mercury goes.

[Zer0]

Fun Fact - Mercury has a melting point of -38.9oC, a boiling point of 356.7oC, and is the only metal to remain in liquid form at room temperature. Droplets of liquid mercury are shiny and silver-white with a high surface tension, appearing rounded when on flat surfaces.

P.S. - It is also a Very Very Toxic & Dangerous substance which can even devour Gold.
BeAware!
🖖

[evan_au]

Another fun fact: The melting point of mercury is almost the same in both Celsius (-38.8C) & Fahrenheit (-37.9F) scales.
- The actual crossover point between these two scales is -40C = -40F
- But at those temperatures, what is a degree between friends?