How can air hold the water in an upturned glass? Why does water stay in a bottle with a hole in its base? Find out with these entertaining experiments.
From their earliest years, children enjoy playing with water, and so do many older students. In this set of experiments, we look at the forces that are significant when dealing with water, demonstrating some basic science principles – and some surprising results. All the experiments are safe to do at home as well as at school, and require just simple household objects as the equipment, plus plenty of water.
Many people have tried this experiment in some version, but can you work out what’s really going on?
Surprisingly, when the glass is inverted, the card and the water remain in place. Why is this?
Let’s consider the forces on the card. These are:
So in this experiment, the force of air pressure pushes upwards on the card at the open end of the glass, opposing the force of gravity and keeping the water in the glass.
How do we know that the upward force of air pressure is enough to oppose the downward force gravity, to hold the water in the glass? We can estimate these forces quite easily.
The weight of the card is much less than that of the water, so to simplify we can ignore the weight of the card itself. This means that the downward gravitational force on the card is the weight of the water column
= h x A x ρ x g
where h is the height of the water column, A is the cross-sectional area of the glass, ρ is the density of water (1 000 kg/m3) and g is the acceleration due to gravity (approximately 10 m/s2).
So if h is 10 cm (0.1 m) and A is approximately 25 cm2 (0.0025 m2), the downward force is approximately
0.1 x 0.0025 x 1000 x 10 = 2.5 N
For the upward force, this is the atmospheric pressure, P, multiplied by the area over which it acts, A.
Atmospheric pressure is approximately 100 000 Pa (pascals, or N/m2)
So the upward force on card = P x A
= 100 000 x 0.0025 = 250 N
So for a 10 cm water column, the upward force due to the atmosphere on the card far exceeds the downward force of gravity on the card due to the water.
We also need to recognise that the air above the water plays a significant role. If this remained at atmospheric pressure, the weight of the water would be sufficient to remove the card, however as soon as the water exerts a downward pressure on the card, it reduces the pressure of this trapped air, which is sufficient to enable the upward atmospheric pressure on the card to support the water. A 1/100 change in volume of this air is sufficient to balance the water, i.e., the pressure reduces by 1/100th which is equivalent to the pressure of the water.
You can also think about the questions below, and perhaps carry out further experiments to answer some of them:
In this experiment, we discover how cohesive forces within water act like an invisible ‘skin’ that can keep the liquid in an upturned cup – sometimes.
The reason why the water does not flow out through the very small holes in the nylon is because there are forces of cohesion between the molecules in the water. These forces make the surface of the water act like a ‘skin’ between the tiny holes in the nylon cloth. This effect is known as surface tension, and it is the same principle that keeps you dry under a woven nylon umbrella: there are tiny holes in the cloth, but the rain won’t get through due to the cohesive forces of surface tension between water molecules.
There are plenty more experiments you can do with surface tension and molecular cohesion. Perhaps look up ‘surface tension experiments’ on the internet and see what other activities you can find?
Here are two further simple experiments you can try.
Take a dish of clean water and a paper clip. Hold the paperclip in a strip of paper towel and lower it into the water. Then allow the paper to sink or carefully sink it with a toothpick. The paperclip will appear to float but is in fact being held by the surface tension of the water.
What else can you ‘float’ – for example, a ring pull from a drinks can? What happens to the paper clip if a drop of washing-up liquid is added to the water? How can you explain what you see?
This effect can also be used to make a ‘soap boat’.
Take a clean empty drinks can, plastic cup, or bottle and make three small holes close together, near the base. Fill the can with water, and when three jets come out, use your fingers to try and join the jets together. You will be able to do this, because of the cohesive forces between water molecules.
Another fun experiment to illustrate cohesion is pouring water down a string.
These activities use a simple bottle of water to reveal some surprising effects due to surface tension and gravity. It’s a good idea to do them outdoors because water spillage is likely (see figure 3).
Use the needle or nail to make one (or more) very small holes near the base of the plastic bottle by heating it over a flame (safely held) until it is hot enough to melt the plastic. In schools, this should be done by the teacher in advance of the experiment, for safely.
Once a container is sealed, water will only flow out of a small hole if that water can be replaced by air or more water. A bottle with one small hole can therefore hold water if the cap is sealed. Once the cap is unscrewed the water will flow out, due to the weight of the water. The hole needs to be small enough for the surface forces to hold the water.
Another interesting experiment to try with a full bottle of water with a hole near the base is: what happens when you throw the bottle up and catch it?
If you fill the bottle with water and hold it with the cap unscrewed for a few seconds (figure 4), the water will flow out of the hole.
Now throw the bottle up in the air (figure 5), and watch it carefully as it falls. Observe each part of its journey – on the way up, at the top of its flight, and on the way down.
When the water is in free fall (i.e. on the way down), water will cease to flow out of the bottle. This is because the water within the bottle becomes weightless relative to the bottle itself, as both the bottle and its contents are in free fall. Thus, in this situation the weight of the water does not force it out of the bottle.
This effect can also be demonstrated with a water-filled hollow tube (around 50 cm, with a diameter that can easily be covered by a finger). The bottom of the tube is covered with one hand while it is launched into the air, with this hand exerting the launching force and the other just supporting the tube. However, this can be a little tricky to demonstrate because it is essential that the hand covering the tube end is the last to let go when throwing the tube up and the first to contact the tube again on catching it, to avoid accelerating/decelerating the tube without the water column.
You can also try to answer these final questions: