Wikijunior:Big Book of Fun Science Experiments/How to make a needle float, and how to make it sink
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[edit] Does it float or sink?
If you put a steel needle in water, does it float or sink?
Well, usually it sinks, as it has a density than is greater that 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.
[edit] 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 do not. It keeps floating!
[edit] 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.
If you drop gently some water in the basin at a distance from the needle, the needle keeps floating.
But if you put even a single drop a liquid soap, like the one for washing-up, after 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.
