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Chapter 10: Problem 1

It is possible to balance a paper clip on the surface of water in a beaker. If you add a bit of soap to the water, however, the paper clip sinks. Explain how the paper clip can float and why it sinks when soap is added.

Short Answer

Expert verified
A paper clip can float on the surface of water due to the surface tension created by the cohesive nature of water molecules, which creates a film strong enough to support the weight of the paper clip. However, adding soap to the water reduces its surface tension by disrupting the hydrogen bonds between water molecules with the soap's hydrophobic tails and hydrophilic heads. As a result, the film becomes too weak to support the paper clip, causing it to sink.

Step by step solution

01

Understand Surface Tension

Surface tension is the property of liquids that allows them to resist an external force, due to the cohesive nature of their molecules. In this case, the surface tension of the water supports the weight of the paper clip despite its higher density, allowing it to float.
02

Describe how a paper clip can float on water

When a paper clip is gently placed on the surface of the water, it can be supported by the surface tension. The water molecules at the air-water interface attract each other, creating a "film" on the surface. This film is strong enough to support the weight of a paper clip, provided it is placed on the water delicately, without breaking the surface tension.
03

Explain the role of soap

Adding soap to water decreases its surface tension. Soap molecules have a hydrophobic (water-repelling) tail and a hydrophilic (water-loving) head. When soap is added to water, it disrupts the surface tension by reducing the cohesion between water molecules. This is due to the hydrophobic tails of the soap molecules interacting with the water and breaking the hydrogen bonds between water molecules.
04

Describe why the paper clip sinks with soap

When soap is added to the water, the surface tension of the water is reduced. As a consequence, the film that was supporting the weight of the paper clip is no longer strong enough to hold it up. Hence, the paper clip is no longer able to float and sinks to the bottom of the beaker.

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