How do some insects walk on water?
Explore how some insects are able to walk on water.
Setting: Indoors
Time: < 30 minutes
Concepts: surface tension; insects; forces; surface area
Skills: Predicting, observing
Ages:
 9-11
 12-14
Subject(s):
 Physics
 Life Sciences
Materials:
• Drinking glass
• Sewing needle
• Spoon
• Scissors
• Piece of 1-ply facial tissue
• Liquid dishwashing soap
• Water
Safety First!
Be careful working with a sewing needle – it is sharp!
Photo/diagram/video – The photo above shows the materials and the photo below
shows the needle floating. Courtesy of Kim Taylor (© Let’s Talk Science)
What to do:
1. Cut a piece of facial tissue that is slightly larger than the
sewing needle.
2. Lay the piece of tissue carefully on the surface of a glass
of water.
3. Once the facial tissue is soaked and lying flat on the
surface, quickly and carefully lay the needle on top of
the facial tissue
4. Now, using either the handle of a spoon (or very
carefully with your finger), poke down at each corner of
the facial tissue so that it sinks to the bottom. What
happened to the needle? What’s keeping it from sinking?
5. Now put a tiny drop of dishwashing soap into the water.
What happened to the needle and why?
What’s happening?
The surface of any body of water behaves as though it is covered with an invisible skin, which
allows small, light-weight insects to walk across it. This skin effect is caused by surface tension.
Like molecules are attracted to each other. They like to “stick together” and this attractive force
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How do some insects walk on water?
is known as cohesion. Forces between unlike molecules is called adhesion. Molecules within a
liquid are equally attracted to all other neighbouring molecules so that the resultant force on any
molecule is zero. Molecules at the surface of the liquid have the same molecules on one side
(water) but different molecules on the other side (air).
The cohesive forces between water molecules is far greater than the adhesive forces between
water and air molecules, resulting in a net force on the surface towards the center of the liquid.
As a result of this force, the liquid assumes a shape that has the smallest surface area – a
sphere. This is the reason why water droplets and surfaces are curved.
Detergents lower surface tension and allow the water to coat the needle. The wet needle is now
heavier than water and sinks to the bottom of the container.
Why does it matter?
Many household cleaners contain detergents. Sometimes, these detergents can end up in
nearby bodies of water. With enough water pollution of this kind, the surface tension of these
bodies of water may reduce significantly. A variety of insects and animals, like water striders,
depend on the surface tension of water to survive. This is one of the many reasons why water
pollution should be monitored and reduced.
Investigate further!
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Try the experiment again using a larger sewing needle. Does it still float?
Try other small non-floating objects instead of the sewing needle.
Try this with liquids other than water, such as cooking oil or vinegar. Does the same
thing happen?
Check out this video: Richard Hammond's Invisible Worlds - Water Strider (Video – 3:59
min)
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