Pour vinegar into the bowl until the eggs are completely covered. The eggs will start to bubble.
Leave eggs overnight.
While you're waiting, create a saturated salt solution in a jug by dissolving as much salt as you can into 500mL water. Keep adding salt until a few salt crystals are left on the bottom that will not go away, no matter how long you stir.
Day two:
The eggs should be soft and squidgy. Remove from the vinegar and gently brush any remaining shell off the egg until you can see the yolk through the membrane (clear covering).
Measure each egg.
Carefully put one egg into each of the small bowls.
Gently cover one of the eggs with water.
Cover the other egg with the salt solution from the jug.
Leave both eggs overnight. Measure the eggs. Have they changed size?
What's happening?
Egg shells contain calcium carbonate (CaCO3), and vinegar's active ingredient is acetic acid (CH3COOH). When these chemicals react together you end up with a salt called calcium ethanoate, some water, and bubbly carbon dioxide gas. The reaction for this equation looks like this:
CaCO3 + 2CH3COOH -> Ca(CH3COO)2 + CO2 + H2O
Once an egg's shell is dissolved by this reaction its membrane is revealed. This membrane is 'selectively permeable' which means it will let some things through but not others. An egg's membrane will allow small molecules like water to pass through, but not large ones like salt.
The egg left in water looked very different to the one in salt water due to a process called osmosis. Osmosis occurs when two solutions are separated by a selectively permeable membrane. Water moves by osmosis from a weak (dilute) solution to a strong (concentrated) solution, such as the solution inside the egg. So when you put an egg in water only, water flows in through the membrane making the egg expand.
The egg in the salt water shrunk. This is because the solution outside the egg is more concentrated, so the water flowed out from the dilute solution to the concentrated solution.
Applications
Osmosis is a vital process in cells. All living things use osmosis to move water into and out of their cells.
Cells are similar to eggs in that they are surrounded by a selectively permeable membrane. This is called a 'cell membrane'.
If you put a cell in a solution that is too salty, it can lose water to the environment, shrivel and die. If the solution is not salty enough, water rushes in and cells can expand until they burst. It is important that cells balance their water content in order to survive.
