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 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

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.

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.

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.

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.

Key concepts

Cohesion

Cohesion refers to the attractive forces between similar molecules, like water molecules. These forces are pivotal in creating surface tension. Surface tension results because water molecules at the surface experience a stronger net attraction inward, as there are no neighboring molecules above them.

Water molecules hold tightly to one another due to these cohesive forces, forming a 'skin' on the surface. This effect allows objects denser than water, such as a paper clip, to float if placed gently. The paper clip displaces some water but does not break the surface due to cohesion keeping the molecules together. Cohesion makes a crucial contribution to the buoyancy experienced by light objects.

• Water molecules attract each other.
• This forms a strong 'network' or 'skin'.
• Such 'skin' holds up light objects placed on it carefully.

Hydrophobic and Hydrophilic Molecules

Hydrophobic and hydrophilic describe the affinity that certain molecules have towards water.

Soap molecules are excellent examples, demonstrating both features with their structure. Soap has a hydrophobic tail that repels water and a hydrophilic head that attracts it. When soap is dispersed in water, these molecules rearrange themselves so that their hydrophobic tails are distanced from the water.

This process disrupts the arrangement of water molecules at the interface, reducing surface tension. As a result, the network that held the paper clip afloat is weakened, leading it to sink.

• Hydrophobic: Repels water
• Hydrophilic: Attracts water
• Soap disrupts the water molecule arrangement and cohesion

Hydrogen Bonds

Hydrogen bonds are strong attractions between water molecules. Each water molecule can form up to four hydrogen bonds with its neighbors. This bonding is crucial for the cohesive forces among water molecules which contribute to surface tension.

Soap molecules break hydrogen bonds when they are added to water, thus reducing surface tension. The hydrophobic tails of the soap insert themselves into the water, thrusting apart the water molecules and breaking their hydrogen bonds.

• Hydrogen bonds form a strong network among water molecules.
• Soap interrupts these bonds.
• Breaking these bonds decreases surface tension.

Density and Buoyancy

Density measures how much mass is contained in a given volume. Objects float or sink in water based on their density relative to the water. Buoyancy is the upward force experienced by objects submerged or floating in a fluid.

The paper clip initially floats because the cohesive forces and the resulting surface tension provide enough buoyancy to support its weight. Despite being denser than water, its mass is spread across a small area of the water’s surface. The support provided by the surface tension compensates for its density. When soap is added, surface tension decreases and cannot provide the same buoyancy, allowing the denser paper clip to sink.

• Buoyancy: Upward force from a fluid
• Affected by density and surface tension
• Reduced surface tension decreases buoyancy, allowing sinking