Water appears to magically defy gravity and move upwards. It actually does defy gravity – but we understand the magic! The magic is called capillary action.
What causes water to walk?
Water walks because…
- water molecules are attracted to each other,
- water molecules are attracted to the molecules that make up paper, and
- water molecules are more attracted to the molecules that make up paper than they are too each other.
So, when the paper towel touches the water, the water is pulled towards it. Water molecules also attract each other, so the water pulls more water molecules with it and these water molecules pull more water molecules and so on, creating movement of water up the paper.
This movement of water up the paper towel is also helped by the air spaces in the paper towel – the gaps between the paper molecules. Water pulls itself up into these gaps where is is then able to reach the paper molecules that surround the gap where it is again pulled towards the paper molecules and into the next gap and so on.
The food coloring, which is in the water, moves because:
- they are also attracted to paper molecules, and
- they are also moving along with the movement of the water molecules.
When molecules of the same type are attracted to each other this is called cohesion.
When molecules of different types are attracted to each this is called adhesion.
Why are water molecules attracted to each other?
Water molecules are in a tug of war with themselves. Water is made up of oxygen and two hydrogen molecules. Oxygen is a big strong tough molecule and hydrogen is a much smaller, weaker molecule. Oxygen is winning the tug of war by pulling electrons (which are negatively charged) closer to itself and, in doing so, oxygen is becoming slightly negative.
Hydrogen, on the other hand, is losing the tug of war; the electrons are being pulled away from it, so it is becoming slightly positive. Water molecules are therefore slightly negative on the oxygen side of the molecule and slightly positive on the hydrogen side of the molecule – like a tiny magnet. They are not actually magnetic, it is an electric charge, but magnets are a useful analogy. When a negatively charged oxygen atom is attracted to a positively charged hydrogen atom on another water molecule, this is called hydrogen bonding.
Why are water molecules attracted to paper molecules?
Paper molecules, called cellulose, also have these oxygen-hydrogen tug of wars going on. These tug of wars create charged regions on the cellulose molecules that attract the water molecules, just like water molecules are attracted to each other.
Why are water molecules more attracted to paper molecules than each other?
Paper molecules, cellulose, are covered in oxygen-hydrogen tug of wars. They are everywhere on the cellulose molecule. This makes it very attractive to water. It is likely that a number of these oxygen-hydrogen groups attract a single water molecule.
Why does the paper get wet before the color arrives?
You may have noticed the paper towel gets wet ahead of the food coloring. If you haven’t check it out next time you do it. The movement of the water molecules carries the food coloring molecules along with it but the food color molecules move slower than the water because they are so much bigger than the water molecules and find it trickier to move around the paper molecules.
Rainbow Walking Water Experiment
This experiment is so simple to set up, and so rewarding to watch. Whilst it takes a little time (up to two hours) to complete, it kept my kids interested as they popped in to check on progress. You do start to see the effect straight away.
What do you need for the walking water experiment?
- Clear cups – glass or plastic. It is best if they are the same size. If you wish to do the rainbow walking water version like we did, you will need six.
- Food coloring
- Paper towel. We used only three pieces. We cut each piece in half, then folded each half into thirds lengthways. Do whatever fits best in your cup. We needed to chop an inch or so off the end of ours so it fitted comfortably in the cup.
- Extra paper towel for cleaning the spoon
How to do the walking water experiment?
- Arrange your cups in a circle. Add water to every other cup, i.e. to three of the cups. Try to get the same amount of water in each cup.
- Add food coloring to the cups with water. Primary colors give the rainbow effect, but experiment however you like. Stir the water to make sure the food coloring is evenly mixed (remember to wipe the spoon between cups so the colors don’t transfer).
- Fold the strips of paper towel and place one end in a paper cup and the other end in the adjacent cup, like this:
- You will start to see movement straight away. Watch as the water creeps up the towel and collects in the neighboring cup. See the earlier part of this post for an explanation of how this works. Notice the clear line between the colors in the previously empty cups:
- After a few hours
- To see the experiment at speed, see the video below:
You may have some further questions – my children did:
- Will the water keep walking and all the cups end up the same color?
- Does the water stop walking? If so, why?
- What happens if your towel is already wet
- What happens if you put some cups lower than other cups
Here is a side view taken late in the experiment:
It is a bit tricky to see, but do you notice that the levels are all the same? Why is that?
Why does the water stop walking?
At a certain point you will notice that the walking water stops. The water does not completely move from one container to the other. The water stops walking when the level of each container is the same. The forces that move the water one direction have become balanced with the forces moving the liquid the other direction, which is why the levels are the same.