Science Puzzle
How Can a Paperclip Float?
Steel is about eight times as dense as water, so a paperclip dropped into a glass of water sinks straight to the bottom. But lower one gently onto the surface, lying flat, and it stays there, resting on top of the water. Add a single drop of washing-up liquid and the paperclip sinks at once.
What holds the paperclip up, and why does the washing-up liquid make it sink?
The Answer
Surface tension. The surface of water behaves like a thin, stretchy skin; the light paperclip rests on it. Washing-up liquid weakens that skin.
Water molecules pull on each other. A molecule in the middle of the water is tugged equally in every direction, so the pulls cancel out. A molecule at the surface has water below and beside it but only air above, so it's pulled sideways and down into the water. Together, the surface molecules form a layer that resists being stretched, a bit like a trampoline.
The paperclip presses down and makes a small dent in this skin; you can see the dip in Fig. 1. The stretched surface pushes back up. For something as light as a paperclip, that's enough to hold it. This isn't floating in the usual sense: an object floats by pushing aside its own weight of water, and steel is far too dense to do that. Push one end of the clip through the surface and it sinks.
Washing-up liquid is made of molecules that squeeze in between the water molecules at the surface and weaken their grip on each other. The skin loses its strength and down goes the paperclip. Pond skaters rely on surface tension to walk on water, with water-repellent hairs on their legs to keep from breaking through.
The principle: Surface tension. Water molecules attract each other, so the surface acts like an elastic skin that can support small, light objects, and detergents weaken it.