A solute or chemical is dissolved in a solution or liquid. The amount of solute in the solution is referred to as the concentration. When the concentration of a solute is greater in the solution than in the cell, the solution is hypertonic. Cells placed in a hypertonic solution shrink as water leaves the cell, in an attempt to dilute the concentration of the solute outside the cell. When the concentration of a solute is greater inside a cell than outside the cell, the solution is hypotonic. Cells placed in a hypotonic solution swell as water moves into the cell to dilute the solute concentration inside the cell. When the concentration of a solute is the same both in the solution and inside the cell, the solution is isotonic, and the cell does not change.
By soaking slices of potato in different concentrations of salt and water, students can observe the effects of osmosis on the flexibility and size of potato slices. Work with at least three different salt concentrations and one water-only solution. Before the experiment commences, encourage students to gently bend, but not break, the slices and note the initial flexibility. Instruct the students to trace around each potato slice and label which concentration of salt they go into. This provides a graphic representation of the change in the potato slices after the experiment. Leave one set of slices exposed to the air to provide the students with flexibility reference after the experiment. Soak the slices for 15 to 30 minutes. The longer they soak the more dramatic the results. Avoid using iodized salt with this experiment. Iodine reacts with the carbohydrates in the potato and turns them black. Students may mistake the discoloration as an effect of osmosis.
The unsoaked slices should remain exactly as flexible as before the experiment. Evaporation may cause the slices to become slightly more flexible. Potato slices soaked in only water are less flexible, or completely inflexible. As the concentration of salt increases, potato slices should become visibly smaller and wrinkled. Depending on how much salt you use, students may be able to smash the potato slices between their fingers. The students should blot each of the slices and retrace the shape of the slice to compare its size pre- and post-experiment.
The water-only solution was hypotonic. Water rushed into the potato cells and caused the slice to swell. The salt solutions were hypertonic. Water left the potato cells and caused them to swell. The greater the concentration of salt, the more water was drawn from the potato slice in an attempt to create equilibrium in the solution.