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 floats on water due to surface tension, which is a result of hydrogen bonds between water molecules. The surface tension allows water to support the weight of the paper clip without breaking these bonds. When soap is added, it reduces the surface tension by interfering with hydrogen bonds, weakening them. This leads to the water being unable to support the paper clip's weight and causing it to sink.

Step-by-step solution

Understand Surface Tension

Surface tension is a property of liquid surfaces that causes them to behave like an elastic sheet. It is caused by cohesive forces between the liquid molecules. In the case of water, the hydrogen bonds between water molecules create the force which causes surface tension.

Explain how a paperclip can float

When a paper clip is carefully placed on the surface of water in a beaker, the surface tension of the water can support the weight of the paper clip without breaking the hydrogen bonds between the water molecules. The paper clip does not actually float in the water, but rather stays on the surface due to the cohesive forces acting on the water molecules.

Identify the effect of soap on water's surface tension

When soap is added to the water, it causes a decrease in the surface tension of the water. Soap molecules are amphiphilic, meaning they have both hydrophobic and hydrophilic ends. The hydrophilic end of the soap molecule binds with the water molecules, while the hydrophobic end repels water. This weakens the hydrogen bonds between water molecules, effectively lowering the surface tension.

Explain why paperclip sinks when soap is added

When the paper clip is placed on the water surface that has soap mixed in, it sinks as the surface tension has been significantly reduced by the soap, rendering it unable to support the weight of the paper clip anymore. The weakened hydrogen bonds in the water do not provide enough cohesive forces to prevent the paper clip from sinking. In conclusion, a paper clip can float on the surface of water in a beaker due to the surface tension caused by hydrogen bonds between water molecules, and it sinks when soap is added since it lowers the surface tension of the water by weakening hydrogen bonds.