Exploring Surface Tension

So you want to explore surface tension?  Here are some great experiments you can easily carry out at home.  You will need to the following equipment:

  • A glass of water
  • A stack of pennies
  • A plate
  • Some milk (Full fat is best!)
  • Food dye, three or four different colors
  • Liquid detergent

Experiment One:

Fill a glass of water to the very top.  What do children think will happen if they drop a penny in?  Add several pennies to the glass and look closely at the rim of the glass, what can they see?  Why hasn’t the water spilled over the edge?  What will it take to make the water spill?

What is happening here?

The molecules in water like to stick together. This is why it forms drops.  You can see this in condensation on windows very clearly.  This is known as surface tension.  Some animals, like pond skaters, exploit surface tension to walk on water!  But surface tension can be weakened.


Experiment Two:

Pour a little milk onto your paper plate to fill it.  Put a few drops of food coloring onto the milk.  Do the food dyes sink into the milk and mix into each other?  Now squirt some detergent onto the plate.  What happens this time?

What is happening here?

The milk has a higher density than the food dye so the dye just floats on top.  Adding the detergent breaks down the fat molecules in the milk and weakens the surface tension, causing the dyes to mix together.

Taking it further:

Why not visit our liquid density experiment? Or find out more about the lives of pond skaters?

What was learned?:

Knowledge base:

Some molecules (polar molecules) like to stick together, creating surface tension in liquids.
Some substances (known as surfactants) can weaken surface tension.

Scientific enquiry:

Children use existing knowledge to form a hypothesis. (For example they might use knowledge of displacement to predict the water in the glass will overflow.)

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