Wikijunior:Big Book of Fun Science Experiments/How to make a needle float, and how to make it sink

Normal Behavior of Needle when in water
If you put a steel needle in water, does it float or sink?

Well, usually it sinks, as it has a density that is greater than water's. You know that wood floats and steel sinks, except when the steel object is shaped as a boat, as it contains air, that is much lighter than water. Effectively, if you put a steel needle on the surface of water using your fingers, there is no way to keep it afloat.

Experiment

 * Step 1: You need a quiet water surface. You can fill a basin or a large pot.
 * Step 2: Put on a table a piece of toilet paper, and put the needle on it.
 * Step 3: Using both hands, raise the toilet paper, keeping it horizontal, with the needle on it.
 * Step 4: Drop the paper onto the water surface. Immediately, it soaks, but it stays on the surface.
 * Step 5: After a few seconds, the toilet paper begins to sink, but the needle does not. It keeps floating!

Observations
If you look at it at a short distance, if possible using a torch, you see that the water surface is bent under the needle, like a sheet under a heavy bar.

Don't touch it, and don't move the water, as that would cause the needle to sink in some conditions, they mostly bear it.

If you drop gently some water in the basin at a distance from the needle, the needle keeps floating, the floating can continue for hours or even days.

But if you put even a single drop of liquid soap, like the one for washing-up, after a few seconds the needle sinks, and you cannot make it float any more on that polluted water.

So, you have seen that fresh water behaves as it were wrapped in a thin fragile film that is able to keep very small objects afloat; but soapy water has no such film, everything is soaked up, and what is heavier than water is bound to sink.

View a short video that demonstrates this experiment

Physics
The steel needle floats on top of the water because of surface tension. Surface tension happens because the water molecules at the surface of the water are strongly attracted to each other more than they are to the air molecules above them. The water molecules make an invisible skin on the water’s surface that allows things like the needle to float on top of the water.

When soap is added to the water, the water molecules aren't drawn together as strongly. The soap molecules diffuse with the water molecules and weakens the liquid’s surface tension. The new attractions between the air, water, and soap molecules are weaker than the water-to-water attractions. As a result, the liquid’s surface is no longer strong enough to support the weight of the steel needle and the steel needle sinks as usual.