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The search for water on Mars
The search for water on Mars

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Surface tension

Hydrogen bonds between molecules of water are quite strong so when water molecules come into contact with each other, they will be held together tightly. This tight packing creates a surface tension in the water that forces it to adopt the smallest shape possible. To understand this more, try this simple experiment.

Activity 2: Surface tension experiment

Timing: 20 minutes

You will need:

  • A clean (and detergent free) glass
  • A sewing needle
  • A fork
  • Water

If you don’t have these items, you can still learn without carrying out the experiment.

Fill your glass with water. Notice the water surface appears ‘thicker’ against the glass. This is because it is slightly pulled upwards due to surface tension. Using the fork, gently place the sewing needle onto the water surface. Notice the ‘dents’ in the water around the needle. Again, this is because of the surface tension of the water; the strength of the hydrogen bonds is such that water can form a surface even against air. Surface tension against air is also the reason why raindrops are spherical.

This image shows five photographs that illustrate the steps in Activity 2. From left to right, the first photograph shows a clean glass filled with water. The next two photographs show a fork with a needle balanced on it being lowered onto the water in the glass. The fourth photograph shows the needle floating on the water surface – the fork has been removed from the image. The final image shows an aerial view of the surface of the water, with the needle floating. ‘Dents’ in the water are visible around the needle.
Figure 9 From left to right: A clean (and detergent free!) glass is filled with tap water. A fork is used to gently place a sewing needle onto the water surface creating ‘dents’ in the water adjacent to the needle. Image credit: Susanne Schwenzer.