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KS1 Science at Home: Week 9

Key Stage 1 Experiment of the Week! –
A 'walking rainbow'!


How does it work?

The coloured water travels up the paper towel by a process called capillary action. Capillary action is the ability of a liquid to flow upward, against gravity, in narrow spaces.  This is the same thing that helps water climb from a plant’s roots to the leaves in the tree tops.

Paper towels, and all paper products, are made from fibres found in plants called cellulose.  In this demonstration, the water flowed upwards through the tiny gaps between the cellulose fibres.  The gaps in the towel acted like capillary tubes, pulling the water upwards.

The water is able to defy gravity as it travels upward due to the attractive forces between the water and the cellulose fibres.

The water molecules tend to cling to the cellulose fibres in the paper towel.  This is called adhesion.

The water molecules are also attracted to each other and stick close together, a process called cohesion.  So as the water slowly moves up the tiny gaps in the paper towel fibres, the cohesive forces help to draw more water upwards.

At some point, the adhesive forces between the water and cellulose and the cohesive forces between the water molecules will be overcome by the gravitational forces on the weight of the water in the paper towel.  

When that happens, the water will not travel up the paper towel anymore. That is why it helps to shorten the length that coloured water has to travel by making sure your paper towel isn’t too tall and making sure you fill your coloured liquid to the top of the glass.


Materials you need

  • 6 wide mouth glasses or jars
  • Paper towels {use the kind where you can select a size}
  • Food dye or liquid water colours (red, yellow, and blue)


1. Arrange 6 glasses in a row.

2. Fill the first one with a good squirt of red food colouring, the third with yellow, and the fifth glass with blue.  Leave the other glasses empty.

3. Add water to the glasses with colour until the coloured water almost reached the top.

4. Move the glasses into a circle and add the paper towels. Starting with the red, add one end of the paper towel and then put the other end in the empty glass next to it.

Continue around until the last paper towel is placed into the red glass.

5. After several minutes, the coloured water will almost travel the whole length of each paper towel.

Awesome science experiment for kids Make a walking water rainbow. 768x767

Questions to think about

1. Could you change the amount of water you start with?

2. Could you change the brand of the paper towel you use?

3. You could even use one of these sheets (or create your own) to record what colours are made when two colours mix.

Some research for next week:
We will answer this question with next week’s science experiment!