Common issues

Potential solutions

Time

With a sixty-minute lesson, it can be challenging to outline the practical, prepare it, leave the potato pieces for enough time for sufficient osmosis to occur and analyse the results. 

If necessary, the potato pieces could be prepared to minimise practical prep within the lesson. However, experience has taught me that it still can prove to be insufficient, especially if the osmosis subject content needs to be recapped. Pupils can also set up the experiment first: blindly without understanding what the investigation is. However, I feel this removes the educational value of a significant chunk of the lesson.

By far, the best solution I have found is for pupils to preload the investigation’s methodology through a flipped learning technique (e.g. viewing and note-taking tutorials and undertaking quizzes on https://www.theeverlearner.com/). This minimises lag time at the start through misunderstanding and also the time needed to be spent on covering osmosis content again.

Control of potato piece surface area

Obviously, if the solution concentration is the independent variable and the mass change of the potato pieces through osmosis is the dependent variable, all other variables must be controlled.

By far, the easiest way to control the surface area is to use a cork borer to produce potato cylinders of uniform diameter. These can then be measured and trimmed to a predetermined length using a ruler and scalpel.

Factors affecting the rate of osmosis

When cutting the potato, it is important to trim any skin from the pieces to be used. The potato skin has a different permeability to water to the internal plant tissues, so a varying presence of skin will cause osmosis to occur at different rates and the results to be less precise. This will need to be trimmed off neatly using a scalpel. It is important to note that this must not affect the total length of the potato piece as this must be controlled between different solution concentrations.

Student mathematical skills

In my experience, students often struggle to calculate percentage change. This can be combated by sufficient teacher modelling and through flipped learning (which also addresses the earlier time issue).

Student graph skills

Again, in my experience, students often struggle to draw graphs which have both positive and negative x-axis values (in this case, percentage mass change). This is worth modelling before and during the required practical lesson itself, and any flipped learning that can be done beforehand (e.g. drawing example graphs or completing worksheets) is also helpful.

1.0mol/dm³ sugar solution

0.75mol/dm³ sugar solution

0.5mol/dm³ sugar solution

0.25mol/dm³ sugar solution

Distilled water

Initial mass (g)

6.08

5.97

6.10

5.92

5.98

Final mass (g)

4.05

3.82

4.00

4.45

6.48

Change in mass (g)

-2.03

-2.15

-2.10

-1.47

0.05

Percentage change in mass

-33.4

-36.0

-34.4

-24.8

8.3