Osmosis is a special case of diffusion in which water passes through
a membrane, but other larger molecules do not. In this exercise,
you will design an experiment in order to learn more about the
factors that caused the changes that you saw in the Elodea
at the beginning of this lesson.
Materials : Osmosis apparatus (J-shaped glass tubing (2), dialysis membrane, plastic tubing), water, starch, iodine solution, buret clamp, ring stand, iodine solution, 50-mL beakers (2).
1. To assemble the osmosis apparatus,
first soak a piece of dialysis tubing (a semipermeable membrane)
in a small amount of distilled water. Cut the tubing along one
side with a pair of scissors to form a flat sheet. Cover one end
of the J-shaped glass tubing with the dialysis membrane. Wet the
piece of plastic tubing and slide it over the dialysis membrane
to hold it in place. Then slide the other piece of glass tubing
into the open side of the plastic tubing to complete the unit.
Finally, clamp the assembly to a buret clamp that is supported
by a ring stand.
2. Prepare the solution you have been
assigned. As an example, to prepare a 5% solution of starch dissolve
5 grams of starch in 95 grams of water. Remember that water has
a density of 1 gram per milliliter, so 95 milliliters of water
is the same as 95 grams of water. This means that there are now
5 grams of starch in 100 grams of solution, which makes a 5% starch
To make the starch solution, measure
both the starch and the water. Add a small portion of the water
to the starch in a container to make a paste. Then slowly add
the rest of the water until there is a solution with no lumps
Solutions of other concentrations or
with other solutes can be made in the same way. Remember that
distilled water is not the same as tap water. Distilled water
is pure water, while tap water has minerals and other molecules
dissolved in it.
3. Obtain two small beakers. Measure
10 mL of the starch solution into one of them. Measure 10 Ml of
water into the other, and add ten drops of iodine solution. Swirl
it to mix, and record the colors of each solution. Pour the contents
of one beaker into the other and swirl to mix. Record your observations.
4. Carefully pour 10 mL of the starch
solution into one side of the osmosis apparatus. Measure 10 mL
of distilled water into a graduated cylinder, add ten drops of
iodine, swirl to mix, and carefully add the water to the other
side of the osmosis apparatus. Use a grease pencil to mark the
initial levels of the starch solution and the iodine solution
in their respective tubes. Record the colors of the solutions.
5. At five-minute intervals, record
any changes in the liquid levels in the osmosis apparatus and
any color changes that you observe. Continue until no change is
observed. Record the final height of the test solution in the
group data table on the board, then clean up your materials.
Prepare a data table in which to record the changes in the height of the water and the test solutions, and any color changes you may have observed. Be sure to record what happened when the iodine-water and the starch were mixed.
Prepare a data table in which to record
the class data for the final heights of the test solutions. Prepare
a histogram (bar graph) for the group data.
Science Education Connection|
Department of Biochemistry
The University of Arizona
Wednesday, February 12, 1997