School Science/Osmosis demonstration

In biology, the osmosis effect can be demonstrated in a number of ways. In schools, there is a standard apparatus for demonstrating osmosis with visking tubing. However, osmosis can also be demonstrated using household ingredients, as described below.

Demonstrating with eggs

An egg contains a semipermeable membrane underneath the shell, which can be used to demonstrate osmosis. The eggshell is mostly made of calcium carbonate that will dissolve in acid. Vinegar or hydrochloric acid are suitable. Stronger acids will dissolve the shell faster, but are more corrosive. Vinegar takes several days.

Method

• Place three eggs in a beaker, cover them with acid and weigh them down so they don't float above the surface.
• Allow them to remain in the acid until the shells completely dissolve.
• Then place each of the eggs in a different liquid:
  • Place one in water
  • Place one in an isotonic sucrose solution, [about 0.3M]
  • Place one in a 1M sucrose solution
• Leave overnight.
• The next day remove the eggs and compare their size.

You will find that the egg in water will have swollen considerably. If you carefully pierce the membrane with a needle a jet of water will shoot into the air. The egg in an isotonic solution will be approximately the same size, while the egg in 1M solution will have shrunk.

Demonstrating Osmosis with Potato Slices

There are a number of variations on this demonstration. Potato slices can be used, as also can raw potato 'chips' (English) or 'French fries' (North American) Consistency of sample can be ensured by using a commercial potato chipper, or by using a cork borer of selected diameter. Other vegetables or fruit may, of course, be used. The changing dependent variable may be weight or length.

Materials

• Potatoes
• Knife, cork borer, commercial chipper
• Cutting board
• Dilutions of sugar (sucrose), suggested range 0 - 0.6M
• Suitable containers (beakers or test tubes or potato cylinders) to match selected dilution range
• Weighing balance / millimetre ruler
• Paper towel
• Marking pen/pencil
• Forceps

Solutions of sodium chloride can be used instead of sugar. Sucrose solutions ranging from 0 to 3.0M give a more convincing range of results.

Method

• Cut suitably sized slices or cylinders of potato. As far as is practical, all pieces should be the same length, width, and thickness, the actual size depending on the chosen container (beaker or test tube).
• Mark each container, pour in the appropriate dilution of sucrose.
• Pat each piece of potato dry, and a) weigh it or b) measure its longest length. Note the measurement.
• When the piece has been measured, immerse it in one of your solutions.
• Leave the potato in the sugar solutions for at least half an hour.
• Use the forceps take out each chip in turn, carefully blot it dry without squeezing, and remeasure the piece.
• Plot a graph of either absolute change or percentage change in weight/length v molarity of the sugar dilution.

See also

• Apparatus for demonstrating osmosis