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Do paper clips sink or float?

Published in Physics of Buoyancy 4 mins read

Paper clips generally sink in water, but can be made to float due to the fascinating phenomenon of surface tension.

The Science Behind Paper Clips and Water

At first glance, a paper clip might seem destined to sink when placed in water. This intuition is largely correct, as the material it's made from plays a crucial role. However, a surprising scientific principle can defy this expectation, allowing a paper clip to rest on the water's surface.

Why They Normally Sink: Density

Paper clips are typically made from steel, a metal alloy. Metals are inherently more dense than water. Density is a measure of how much "stuff" (mass) is packed into a given space (volume). If an object is denser than the fluid it's placed in, it will sink.

  • Composition: Paper clips are made of metal (steel).
  • Density: Metal is denser than water.
  • Outcome: Therefore, a paper clip will normally sink when dropped into water.

This fundamental principle of buoyancy dictates that objects with a higher density than the surrounding fluid will displace less fluid than their own weight, causing them to submerge.

Material Density Relative to Water Outcome in Water (Normally)
Paper Clip More Dense Sinks
Water Baseline N/A
Wood Less Dense Floats

Why They Can Float: The Magic of Surface Tension

While paper clips are indeed denser than water and will normally sink, they can be made to float thanks to surface tension. Surface tension is like a thin, elastic "skin" that forms on the very top of a liquid, caused by the cohesive forces between liquid molecules. Water molecules at the surface are pulled inward more strongly than they are pulled sideways or upward, creating a net inward force that makes the surface behave like a stretched membrane.

It is this "skin" of the water that can actually hold the paper clip up! When a paper clip is placed very carefully and gently on the water's surface, it can rest on this elastic film without breaking through it.

How to Make a Paper Clip Float (Practical Insight):

To observe a paper clip floating due to surface tension, follow these steps:

  1. Choose a still container of water: A wide, shallow bowl or glass filled with still water works best.
  2. Use a dry paper clip: Ensure the paper clip you intend to float is completely dry and free of oils or dirt, which can disrupt surface tension.
  3. Create a 'launcher': Unfold another paper clip into a flat shape, or use a fork. This will act as a tool to gently lower the floating paper clip.
  4. Carefully place the paper clip: Lay the dry paper clip horizontally onto your 'launcher'.
  5. Gently lower: Slowly and steadily lower the 'launcher' with the paper clip onto the water's surface until the paper clip makes contact.
  6. Withdraw the launcher: Once the paper clip is resting on the surface, carefully slide the 'launcher' out from underneath.
  7. Observe: The paper clip should now be floating on the water's surface, creating slight depressions due to its weight without breaking the tension. If the surface tension is broken (e.g., by a ripple or adding soap), the paper clip will immediately sink.

This remarkable phenomenon demonstrates how a relatively small, dense object can be supported by the delicate forces at a liquid's surface, defying its usual tendency to sink. Learn more about surface tension experiments.

Practical Applications and Fun Experiments

Understanding surface tension extends beyond just floating paper clips. This principle is vital in many natural and artificial processes:

  • Insects walking on water: Many insects, like water striders, utilize surface tension to move across ponds and lakes.
  • Soap and detergents: Soaps work by reducing water's surface tension, allowing it to penetrate fabrics more effectively and clean.
  • Raindrops: The spherical shape of raindrops is due to surface tension minimizing the surface area.