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MCAT Chemistry Question — Meniscus

  • by John
  • Oct 28, 2014
  • MCAT Question of the Day

A student observes that mercury forms a convex meniscus in the graduated cylinder but that water forms a concave one. This behavior is best explained by the fact that:

 

A) the two liquids are being kept in graduated cylinders made of different materials.

B) the adhesive forces between water and the walls of the graduated cylinder are greater than the adhesive forces between the mercury and the walls of the graduated cylinder.

C) the cohesive forces between two mercury atoms are stronger than the cohesive forces between two water molecules.

D) the mercury has stronger cohesive than adhesive forces, whereas water has stronger adhesive than cohesive ones.

Explanation

The shape of the meniscus lets you know whether the substance has more attractive force for itself (convex meniscus) or for the walls of the cylinder (concave meniscus). Water, for example, experiences more adhesion between the water and the walls of the container, thus “pulling” the water up the sides of the container creating a concave meniscus. The adhesive force is greater than the cohesive force between the water molecules. The opposite is true for mercury, thus making (D) the right answer.

A: While this may be true, the student’s observation doesn’t lead to this conclusion.

B, C: The shape of the meniscus doesn’t let you compare the forces in water to the forces in mercury directly. It only lets you compare one force (adhesion) to another force (cohesion) for one substance.

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