Problem 1 - Chemistry An Atoms First Approach

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.

Step-by-Step Solution

Answer

A paper clip can float on water due to the high surface tension caused by hydrogen bonding between water molecules. This surface tension allows the water to support the weight of the paper clip. When soap is added, its hydrophilic and hydrophobic ends interact with the water molecules, breaking the hydrogen bonds and decreasing the surface tension. As a result, the forces holding the paper clip on the surface are no longer strong enough, causing it to sink.

Step 1: Understanding Surface Tension

Surface tension is a property of liquids that makes the surface of the liquid act like a membrane under tension. This property allows objects lighter and smaller than the liquid's surface to float on the top. Surface tension is caused by the cohesive forces between liquid molecules. In the case of water, its surface tension is high due to the hydrogen bonding between water molecules.

Step 2: Floating Paper Clip

A paper clip can float on the surface of water due to the high surface tension of water. Place the paper clip gently on the surface of the water so that the surface is not broken. The paper clip, being relatively small and light, will not break the surface tension of the water, allowing it to float. This is because the forces from the surface tension around the paper clip are enough to support its weight, keeping it from sinking.

Step 3: Adding Soap

Soap molecules have a polar end (hydrophilic) and a nonpolar end (hydrophobic). When you add soap to the water, the hydrophilic ends of the soap molecules are attracted to the water molecules, while the hydrophobic ends repel water. This interaction breaks the hydrogen bonding between the water molecules, decreasing the surface tension of the water.

Step 4: Sinking Paper Clip

As the surface tension of the water decreases after adding the soap, the forces holding the paper clip on the surface of the water are no longer strong enough to support the paper clip's weight. This causes it to sink. In summary, a paper clip can float on water due to the high surface tension of water, which can support the paper clip's weight. However, when soap is added to the water, it decreases the surface tension by breaking the hydrogen bonds between water molecules and thus the forces holding the paper clip are not strong enough anymore, causing it to sink.