Kitchen Science Experiments

Floating a Needle

Sun, 22nd Jul 2007

Listen Now    Download as mp3 Part 1,2 from the show Extreme Survival

What you Need

Glass of Water

A full glass or bowl of water

A Needle

A needle

Toilet Paper

A piece of toilet paper

What to do

 

The idea is to lower the needle onto the water as gently as possible without it breaking through the surface.

  1. Make sure the needle is dry and not soapy.
  2. Tear off a squareish piece of the toilet paper slightly larger than the needle in all directions.
  3. Put the needle on top of the paper.
  4. Gently lower the paper onto the water.
  5. Wait for the paper to sink - you may want to very gently push the edges down to help it.

Needle on the paper

What may happen

If you get it right the needle should float on the surface of the water.

Floating Needle

You should notice the the surface is pushed down by the needle.

If you push the needle through the surface it will sink. Also if you touch a little soap onto the surface it cause the needle to sink too.

Why does it happen?

The surface of a drop of water is always puling inwards pulling the  surface of the drop as small as possible, this is called surface tension. A sphere is the shape with the least surface for its volume so surface tension tries to pull water droplets into spheres which is why they are round.

If you very gently place a needle onto water so that it doesn't break the surface it;s weight will pull the surface downwards pulling it down, but because the surface is dented surface tension will pull it upwards stopping it sinking.

Floating a Needle

Soap makes water's surface tension far weaker, allowing you to stretch it out into thin films in bubbles, and allowing it to stick to grease when you are doing the washing up. However it means that there is not enough force to hold up the needle any more and it sinks.

With Soap

Where does the surface tension come from?

Water is made up of tiny lumps called molecules - each one has 2 hydrogen and 1 oxygen atoms,, hence H2O. These molecules are moving about all over the place but they are very strongly attracted to one another.

Water

Molecules below the surface will get pulled in all directions by other molecules which cancels out . The molecules on the surface however are pulled inwards because there are no molecules above them.

inwards all the time and pulled toward one another. This means that the surface acts like a slightly strange spring, as the surface grows or shrinks molecules are pulled into or out of the liquid out or from the surface

So if you look at the bottom of a dent in the surface there will be more water molecules above it than below it so it will get pulled upwards, shortening

 

A dent in water

 

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