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Q & A: Meniscus of mercury

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Most recent answer: 10/22/2007
Q:
When you have mercury liquid in a container, why is the meniscus upside down compared to all other liquids?
- Muhammed (age 17)
Sir George Monoux College, London
A:
If the cohesive forces between molecules of a liquid are stronger than the adhesive forces between the molecules of a liquid and those of the container then you will see a "convex meniscus." (In other words, that's if the liquid molecules stick to each other better than they stick to the container.) Surface tension, which arises due to the cohesive forces in the liquid, likes to minimize the surface area of the liquid, prefering a spherical shape. Adhesion between the liquid and the container, also known as "wetting", encourages as much liquid as possible to be in contact with the container.
When a convex miniscus starts to form, the initial effect is to decrease the surface between the liquid and the container and increase the surface between the container and the gas by the same amount. Only when the curvature gets large does the area of the liquid-gas surface go up much. So the tradeoff that determines whether the miscus is convex or concave is whether the gas-conatiner surface or the liquid-container surface is more 'costly'. The units of the cost are something called free energy, which is what falls to a minimum value in equilibrium. The more things lower their free energy when they're in contact, the more they stick together.

Mercury has very small adhesive forces with most container materials, and strong cohesive forces. Little drops of mercury will form into almost spheres when spilled on most surfaces (gravity will bend them out of shape).

Water on the other hand, is made up of electrically polar molecules. Glass is electrically polarizable, and attracts charged objects, so water sticks to glass.

Some containers may attract water molecules much less. Many kinds of plastics, called "hydrophobic", (e.g. teflon) have very small adhesive forces with water , and if you make a container out of these you will get a convex meniscus with water, too.

Tom (and mike)

(published on 10/22/2007)

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