Submersion - MCAT Chemical and Physical Foundations of Biological Systems

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Question

Which of the following is false regarding surface tension?

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Answer

Consider a beaker containing water. The molecules at the surface of water interact with fewer molecules than the molecules at the center of the beaker. The surface molecules have no water molecules above them, whereas the molecules in the center of the solution interact with molecules in all directions. Since the surface molecules have fewer interactions, the interactions between these molecules are much stronger than the interactions of a center molecule; fewer interactions have opposing counterparts, resulting in stronger forces.

Recall that surface tension can be defined as the resistance to increasing surface area. This means that the surface area is minimized due to surface tension. Of all the geometrical shapes, spheres have the lowest surface area to volume ratio; therefore, surface tension forces water droplets to take spherical shapes. Surface tension occurs due to the intermolecular forces between liquid molecules. These intermolecular forces between molecules are cohesive, preventing the spreading of water molecules.

It is harder to move an object that is floating on water than an object submerged in water. This occurs because the interactions between the molecules at the surface are stronger than the interactions below the surface. Moving an object involves moving the water molecules around it. Since it is harder to move molecules at the surface (stronger interactions), it is harder to move an object at the surface. An object floating on water will have more resistance to motion.

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