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Leaving patterns

By David, 21 January 2020 Activity

a cup of water with plastic over the top and a coin balancing on that.

Written by Elvina Lee

Here’s a cool activity that’s perfect for a hot summer’s day! Create a curious pattern with water.

You will need

  • Dark coloured cup
  • Water
  • Patterned metal object, e.g. cutlery or a coin
  • Piece of clear plastic*
  • Rubber band
  • Sticky tape (optional)
  • Handheld torch (optional)

* Hint: Consider re-using a clean piece of food packaging such as see-through biscuit wrap, instead of sending the packaging to the bin.

What to do

  1. Chill the patterned metal object in the fridge. (At this stage, you may wish to attach a loop of sticky tape to smaller objects – such as coins – to make them easier to pick up.)
  2. Half fill the cup with water.
  3. Someone is stretching a rubber band around a cup to attach a sheet of clear plastic over the top of it.Stretch the piece of plastic over the cup, fixing it in place with a rubber band.
  4. Let the covered cup sit at room temperature for about 30 minutes.
  5. Get the chilled metal object out of the fridge and place it in contact with the plastic.
  6. There is a faint image of a mirrored 20 cent coin.

    Lift up the metal object and admire the pattern left behind!


What’s happening?

When you leave the cup covered at room temperature, some of the water in the cup evaporates (changes from liquid to gas) forming water vapour. This water vapour collects in the space between the plastic sheet and liquid water, within the cup.

When you press the cold object onto the plastic, the water vapour cools down. It turns into tiny liquid water droplets (known as condensation) on the underside of the plastic surface. You will notice that the condensation pattern matches the cold object you pressed onto the plastic!

If the temperature of the room you are in is stable, the pattern should remain on the plastic for about 10 minutes. During this time, you can shine a light on the pattern from different angles. Does the colour of the pattern change over time? At what angle of light does your pattern look best? Let us know what you discover by leaving a comment below!

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