This Easter, have some intriguing science fun with eggs. You’ll never look at them the same way again!
Traditionally, Easter is the season of eggs – whether chocolate, painted or special in some other way. Along with the fun of egg hunts and creating (or buying) decorative eggs, we can use eggs to learn some science – as these five light-hearted activities show.
Most of the experiments described here are suitable for almost all ages and can usually be done in a few minutes with simple, easy-to-find materials – including quite a few eggs.
A normal hen’s egg consists of three main parts: shell, white (or albumen) and yolk. However, if you can get some really fresh eggs, you may see when you crack them open that the white itself has two distinct parts: a firm inner layer and a runnier outer layer. Between the white and the shell there is another structure: a thin but quite strong membrane. In the activities below, we’ll be looking at how this anatomy of eggs affects their properties.
The shell of an egg laid by a healthy, outdoor hen is surprisingly strong; battery-raised hens often lay eggs with thinner shells. Although a sharp tap can break the shell, the shell’s hard material and its rounded, regular shape mean it is remarkably good at withstanding a heavy weight or force (such as the mother hen). There are several experiments you can do to demonstrate this strength – including, if you dare, actually walking on eggs.
Students can try this simple experiment themselves to feel the surprising amount of force that eggs can withstand.
The shell is made of a thin, brittle material – so why doesn’t it break? The answer is the egg’s domed shape, which – like a dome or arch in architecture – distributes the applied force over the whole structure, decreasing the pressure on any one part and so reducing the chance of breakage. This shape also ensures that the force acts only to compress the shell, rather than to stretch it or push it sideways. Because the eggshell is a hard material, it is very strong when compressed and so doesn’t break.
In fact, the shape of an egg at the pointed end may be ideal for load-bearing. Engineers know that the arch shape that distributes weight best is the catenary. This mathematical curve has a shape similar to an egg at the pointed end, which is why eggshells will support even more weight when force is applied to the ends of the egg (as in the next activity), rather than around the middle.
It’s even more impressive to walk on eggs – although it’s riskier too!
Because the eggs are in their boxes, they are kept upright. The walker’s weight presses on the domed ends of the eggs instead of their sides, enabling them to withstand the maximum force. The weight is also shared between all the eggs you are standing on, thus minimising the pressure on each egg.
The same principle of weight distribution means that it is also possible to lie on a ‘bed’ of eggs). To do this, you’ll need about ten dozen eggs. Here, the weight is distributed over a large area, in a similar way to a bed of nails.
From the outside, raw and hard-boiled eggs look just the same, but there’s an easy way of telling them apart without breaking them open. This activity shows how to do this, either as a teacher demonstration or a class activity.
For the teacher or each group of students: one hard-boiled and one raw egg. The eggs in each pair should be closely matched in size and shape (weigh them to check the masses) and be at the same temperature.
Whereas the inside of the hard-boiled egg is solid, the raw egg is liquid inside. When the raw egg is set spinning and then stopped, the liquid inside it continues to move, which makes the egg start spinning again. But when a hard-boiled egg is spun and stopped, the solid interior cannot continue to move, so the egg will remain stationary.
More scientifically, we can understand this in terms of forces in a viscous (thick) liquid. When the raw egg is stopped by touching the shell momentarily, the liquid inside continues to move, producing forces across the liquid. If the egg is quickly released, these forces can then act on the shell to make the egg move again. With the hard-boiled egg, there is no viscous liquid to store the force, so stopping the spinning (which needs a little more force than with the raw egg) brings it to a complete standstill.
Ask your students to test which egg is more difficult to get spinning in the first place. Can they explain their answer, based on the principles described above? (They should find that the raw eggs are harder to start spinning for the same reasons that they are harder to stop.)
Did you know that as well as being remarkably strong, eggs can also bounce? First, however, we need to remove the shell.
Vinegar contains ethanoic (acetic) acid. This reacts with the eggshell, which is made of calcium carbonate:
2 CH3COOH + CaCO3 → H2O + CO2 + Ca(CH3COO)2
Ethanoic acid + calcium carbonate → carbon dioxide and calcium ethanoate
The resulting calcium ethanoate is soluble in water, so the shell begins to dissolve; if you leave it in vinegar for long enough, it will dissolve completely.
Why doesn’t the egg burst when it lands? The answer lies with the membrane, which is surprisingly strong and a little bit stretchy. This elasticity allows the egg to spread out as it hits a hard surface, which means it decelerates more slowly than a rigid egg in its shell would. Because the deceleration is reduced, so too is the force that is exerted on the egg (Newton’s second law of motion).
A hard-boiled egg that hass been very carefully shelled can be used to demonstrate atmospheric pressure.
Within seconds, you should see the egg appearing to be sucked right into the bottle (figures 3 and 4).
As the matches burn, they use up oxygen from the air within the bottle, forming soot. A solid (soot) takes up less space than a gas (oxygen), so the pressure within the sealed bottle decreases. As a result, the egg appears to be sucked into the bottle; in fact, it is the surrounding air pressure outside that forces the egg into the bottle.
A further consideration is that the burning matches heat the air around them, which you might expect to increase the pressure within the bottle. Clearly the reduction in pressure caused by the removal of oxygen outweighs the increase in pressure caused by the heat.
Repeat the experiment without the egg, but instead covering the mouth of the bottle with cling film as soon as the matches are dropped in. As the pressure in the bottle decreases, the cling film is drawn into the bottle, forming a concave surface.
Alternatively, this demonstration can be performed by (carefully!) pouring boiling water into the bottle before sealing it with the egg. As the steam condenses and the pressure inside the bottle is reduced, the egg is slowly drawn into the bottle. This can take several minutes.
Do you use eggs to do experiments in science lessons? Were these suggestions helpful? Why not leave a comment on the online version of this article, describing how you used these ideas and what other experiments you have tried? Did they work well? What could have been improved?