Question

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

A paper clip can float on water due to surface tension, which is the cohesive force between water molecules that creates a "skin" on the surface of the liquid, supporting the paper clip's weight. However, when soap is added to the water, it acts as a surfactant and lowers the surface tension by disrupting the cohesive forces between water molecules. With the reduced surface tension, the water's surface is unable to support the weight of the paper clip, causing it to sink.

Step-by-step solution

Define surface tension and explain how it helps a paper clip to float

Surface tension is a property of liquids that arises due to the cohesive forces between their molecules. At the surface of a liquid, these forces act to minimize the liquid's surface area, creating a thin "skin" that resists deformation. In the case of a paper clip floating on water, surface tension acts like an invisible film, supporting the weight of the paper clip and preventing it from sinking. This effect is also aided by the paper clip's shape, which distributes its weight over a larger area.

Discuss how soap affects surface tension

Soap is a surfactant, a substance that lowers the surface tension of a liquid by disrupting the cohesive forces between its molecules. When added to water, soap molecules have a hydrophobic (water-repelling) end and a hydrophilic (water-attracting) end. The hydrophobic ends of the soap molecules push apart the water molecules at the surface, while the hydrophilic ends bond with the water molecules, reducing the overall surface tension of the water.

Explain how the reduced surface tension causes the paper clip to sink when soap is added

When soap is added to the water, the surface tension of the water is reduced due to the presence of soap molecules. With the weaker surface tension, the water molecules are unable to create a strong enough "skin" to support the weight of the paper clip. As a result, the paper clip is no longer able to float on the water's surface and sinks.