Pipette 3cm3 of sucrose solution with each sucrose concentration into other test tubes and label them. Place one drop of methylene blue in each of these test tubes. This will colour the solution but not affect its water potential. These are to be shared among the class.
Next, using a cork borer and a razor blade, prepare a solid cylinder into thirty discs of approximately equal (2mm) thickness.
Note the time and place four of the potato discs into each of the seven 3 cm3 test tubes of sucrose solutions. Manipulate the discs so that they are all covered with sucrose solution in each tube.
After at least 40 minutes, pour off the fluid from the 3cm3 test tube labelled 0.1 mol dm-3 into a clean test tube. This test tube of decanted solution should be placed in the test tube rack in the position of previously occupied by the tube containing the potato discs immersed in 0.1mol dm-3 sucrose. Do this with all the solutions.
*Using a teat pipette to collect a small amount of blue sucrose solution from the 0.1 mol dm-3 sucrose tube. Now with great care, introduce a single drop of this blue fluid into the tube with the same concentration but without the methylene blue that was decanted from the potato slices. The drop should be released carefully into the centre of the liquid about 5 mm below the surface.
Watch the drop and note whether it remains in the same place, sinks, or rises. Repeat with another drop until you are certain you have made the correct observation about the movement of the drop.
Repeat the steps from * to all of the test tubes and record the observations.
Variables
The variables in this experiment are the different concentrations of sucrose solution used in each test tube for each test.
Results
Table Showing the Movement and Observations of the Drop for each Concentration
Conclusion
As the concentration of the solution increased, the speed of the drop rising decreased until eventually at 0.5 mol dm-3 it remained in the same place and the fell at 1 mol dm-3. At 0.3 mol dm-3 the speed at which the drop rose was extremely slow. When the solution is more concentrated than drop of blue sucrose solution the sucrose solution rises and vice versa when the sucrose is more concentrated than the solution. We have found the isotonic point (when the drop of blue sucrose solution remains still, does not rise nor fall); this is at about 0.5 Mol dm-3.
Evaluation
This experiment was rather successful since the isotonic point was found. However, it could have been repeated more times in order to have more fair results. Then an average could have been found and it would have been more accurate. Another factor is that the potato from which the cylinders are taken could be abnormal - this could be prevented by amalgamating sets of results, for example of a whole class, where each experimenter used a different potato.