Biology

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Video

Materials

• 1 raw egg
• Vinegar
• 1 drinking glass
• Plastic wrap (or something else to cover the glass with)

Short explanation

An eggshell consists of hard calcium carbonate. But the vinegar causes the eggshell to dissolve. What remains is a "naked egg" - an egg without a shell! However, it is still held together by two thin membranes and therefore feels like a rubber egg.

Long explanation

The shell of an egg consists almost entirely of calcium carbonate (CaCO₃ ). Common household vinegar, also called distilled white vinegar, consists of about 5 % acetic acid (CH₃COOH) and the rest is water. When the acetic acid comes in contact with calcium carbonate, a chemical reaction starts, where calcium carbonate is converted to calcium acetate, which dissolves in the water. The chemical reaction looks like this:

CaCO₃ (s) + 2 HC₂H₃O₂ (aq) → Ca(C₂H₃O₂)₂ (aq) + H₂O (l) + CO₂ (g)

The carbon dioxide that forms is clearly visible as bubbles pretty soon after the egg has been placed in vinegar. In fact, most of the shell is gone after only 12 hours.

Even though the shell is gone, the egg holds together. It is held together by two thin membranes just inside the shell - the outer and the inner shell membranes. The functions of these membranes are many and complicated, but can be summed up like this: they control the transport of chemical substances into and out of the egg as well as protection against bacteria. In this way, they are very similar to the cell membrane of an ordinary human cell. The eggshell also has the same functions, because there are actually thousands of pores in it that, among other things, let in oxygen and emit carbon dioxide. Together, the eggshell, the outer shell membrane and the inner shell membrane form a protective super trio.

The reasons why there are two shell membranes instead of one are also several, but among other things they complement each other in the bacterial protection and depots of substances can form alongside them. A protective air cushion is also formed between them. This air cushion is always extra large at the bottom of the egg. You've probably seen it there when you peel an egg. Then you may have also seen that the outer shell membrane is attached to the eggshell and the inner shell membrane to the egg white.

The egg swells in the glass. The reason is that it absorbs water. It starts doing that when the shell disappears. The shell otherwise prevents this - the pores in it are too small to allow liquid water with its surface tension to pass through. But with the shell gone, water can pass through the membranes. They let water through. The egg then becomes susceptible to osmosis. Osmosis is when water travels through a membrane that lets the water through but not the chemical substances that are dissolved in the water (yes, as mentioned above, the membranes do let through chemical substances, but only "well-chosen" substances). What determines whether water will migrate out or in through the membranes is how full the surrounding water is of other substances. If the naked egg is in pure water, water migrates into the egg and it swells. If the naked egg is in, for example, salt water or sugar water, water migrates out of the egg and it shrinks.

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What does the egg look like after 1 hour, 2 hours, 6 hours, 12 hours, 1 day, 2 days and so on?
• What happens if you place the naked egg in pure water, salt water, syrup etc.?
• How is the egg yolk affected compared to the egg white?

Variations

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