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PCAT Chemical Processes – Mercury Meniscus

Mercury is a liquid with greater cohesive than adhesive forces, when placed in a glass tube (such as in a barometer). The meniscus formed with such a liquid would appear:

A) Convex

B) Flat

C) Concave

D) Irregular

Click for Explanation

The classic concave meniscus that we see with water in a graduated cylinder is because the water has greater adhesive forces between the water and the glass than cohesive forces (the water sticking to itself). Since the water “wants” to stick to the walls of the container, it crawls up the sides of the container a bit to get more surface area sticking to the cylinder. Those higher edges create that classic concave meniscus.

The situation described in the question is the reverse of that – stronger cohesive forces. So the meniscus we see would be the opposite: convex instead of concave. Thus, the answer is (A) Convex.

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