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Naked Eggs: Osmosis

Osmosis is a process in which water moves through a membrane. The natural movement of water is from the side of the membrane with a high concentration of water to the side with a low concentration of water. The term osmosis is particular to solutions (dissolved mixtures) seperated by a membrane. Diffusion is similar but refers to any substance spreading from a higher concentration to a lower concentration any doesn't involve a membrane.

After dissolving the eggshell, we are left with a membrane that holds the insides of the egg. This membrane is selectively permeable. This means that it lets some molecules move through it and blocks out other molecules. Water moves through the membrane easily. Bigger molecules, like the sugar molecules in the corn syrup, do not pass through the membrane.

You may have noticed that the egg expanded in the initial vinegar solution when you dissolved the shell. This is because the vinegar has a higher concentration of water than the inside of the egg. To reach equilibrium, water molecules move from the vinegar into the egg through the semi-permeable membrane. If the membrane were completely permeable, water molecules would move in and protein would move out until both solutions were the same concentration. Since the egg membrane is semi-permeable, water can move in but proteins cannot move out.

When a naked egg is placed in the corn syrup the egg will shrink. This is also due to osmosis, but in the opposite direction. The corn syrup is mostly sugar. It has a lower concentration of water (25% water) than the egg (90% water). To reach equilibrium, osmosis causes the water molecules to move out of the egg and into the corn syrup until both solutions have the same concentration of water. The outward movement of water causes the egg to shrivel.

Objectives

  • Describe osmosis.

  • Determine the direction of water movement based on solution concentrations.

  • Describe the function of a semi-permeable membrane.

Materials

  • Scale (optional)

  • Per group of 3–4 students:
    “naked” (shell-less) egg from Naked Eggs: Acid-Base Reaction activity
    jar or bowl slightly larger than the egg
    big spoon
    water
    corn syrup (or sugar solution) – enough to cover each group’s egg

Key Questions

  • Why is your naked egg that was soaked in vinegar bigger than a shelled egg?
  • Why does the egg in corn syrup change shape and weight?
  • Does the egg soaked in water change shape and weight?
  • What could you do to return the egg to its original form?

What To Do

Prior Experiment – make a Naked Egg

Preparation

  1. Designate a “corn syrup pouring station” at your desk so that you can monitor the amount of corn syrup students are using (to avoid wasting).
  2. Place a naked egg in a jar of plain water to use as a “control”. Treat it the same way as the corn syrup-covered egg.

Activity

  1. Weigh your egg and note the measurement.
  2. Put your naked egg in a jar and add enough corn syrup to cover the egg.
  3. Store the egg in a refrigerator (or somewhere cool) for 24 hours.
  4. After 24 hours, scoop out the egg and observe the changes.
  5. Weigh the egg again and note the measurement.

Extensions

  • Draw a diagram of your egg in the corn syrup. In what direction is osmosis occurring (the movement of water molecules across the membrane)?
  • Return the corn syrup-covered shriveled egg to its non-flabby former shape! Carefully lift the flabby egg from the corn syrup and place it in a container of water. Leave the egg in the water for 24 hours. Osmosis will occur; that is, the water will migrate from the side of the membrane where water molecules are abundant (i.e. outside the egg) to the side where water molecules are less abundant (inside the egg). After 24 hours, the egg will be plump again!
  • Experiment with naked eggs by soaking them in other solutions. What happens if you put the egg in water with food colouring? Or salty water?

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