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Egg osmosis

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The image to the left shows the effect of different types of liquid on red blood cells. This is what you just demonstrated using the egg.  Can you work out how isotonic sports drinks are meant to work? How are they different from water or a fizzy pop drink? The image to the left shows the effect of different types of liquid on red blood cells. This is what you just demonstrated using the egg. Can you work out how isotonic sports drinks are meant to work? How are they different from water or a fizzy pop drink?

Osmosis is the process where water passes into our tissues through a semi-permeable membrane. You can show osmosis at work using eggs.

Ingredients

  • Several raw eggs
  • Vinegar
  • Corn syrup
  • Water
  • Food coloring
  • Scales to weigh egg (if possible)

Recipe

Weigh the uncooked eggs and carefully place them into a large glass of vinegar, cover and leave for 2 days. It might start to look a bit like beer but please don't drink the liquid, its not nice!

Remove the eggs and rinse gently under the tap. All of the shell should be gone by now (you can wipe off little bits still left over). Weigh the egg again. You need to be really careful with the egg now as it is still raw and if you burst the outer skin, it will make a big mess.

Put some of the eggs in corn syrup and some in water. Leave overnight, rinse and weigh again. (If you have no scales you can look at the egg and record any differences that way!)

Try adding some food coloring to the water and see what happens.

Other things to try:

Soak hard boiled eggs in vinegar then drop them from a low height - they should bounce! Don't do this with the uncooked eggs!

Try soaking in lucozade energy drink after the vinegar step.

What happens if you stab the raw egg with a needle after soaking in vinegar?

Why does this work?

Osmosis is the process by which water enters our tissues. All of our cells are surrounded by a membrane that selectively allows in anything the cell needs but prevents unwanted molecules from entering.

This works because the membrane contains lots of tiny holes that will let anything smaller through but obviously stop big molecules from entering the cell. Water is small enough to get through and enters by a process called diffusion - this means that it will travel from an area with a high water concentration to an area with a lower concentration until it is balanced out with both areas having the same concentration (known as isotonic).

An egg also has a membrane surrounding it so we can use it to represent a cell and see how osmosis works.

The first stage of the experiment was to soak the egg in vinegar. You will have seen that the shell completely disappears. Vinegar in fact contains acetic acid and this reacts with the calcium carbonate making up the shell of the egg. This reaction gives off carbon dioxide so if you were paying attention you will have seen bubbles coming off the egg as soon as it was added to the vinegar.

The membrane around the egg also becomes very rubbery during this soaking in vinegar. This occurs because the acid denatures the protein making up the egg white (albumin). Denaturation of proteins can occur through exposure to acids, bases or high temperature. This is what happens when you cook an egg- the clear egg white turns opaque and white as the protein chains become tangled due to the heat.

If you soak a cooked egg in vinegar it also becomes very rubbery - so much so that it will bounce when dropped from a low height! It's not a good idea to try this will an uncooked egg as inside the rubbery membrane the yolk and white are still runny and it will make a big mess if split!

So after soaking in vinegar you should have also noticed that the egg increases a little in size. This is because the water in the vinegar can enter the egg through the membrane, moving from the higher water concentration in vinegar to the lower concentration in the egg.

If you now put the egg in water, it will grow in size a lot more due to the much larger water concentration gradient across the membrane. If you add food coloring to the water you can see the process of osmosis in action as the colored water passes into the egg. Water is known as hypotonic, ie. very dilute and contains more water than the egg.

If instead you place the vinegar soaked egg in corn syrup you will see the opposite. There is a much higher water concentration in the egg than in the syrup so water will pass in the opposite direction. This means that the egg will shrink in size. The corn syrup is a hypertonic liquid, ie. very concentrated with not much water compared to the egg.

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