Osmosis Demonstration – Osmosis and the Mighty Potato

Osmosis and the Mighty Potato – Classroom Inquiry and Demonstration

Osmosis is simply the diffusion of water molecules across a semi-permeable membrane. As is the case with diffusion, water molecules like to go from an area of high concentration to an area of low concentration.

This demonstration shows quantitively and qualitatively, the effects of osmosis. At the bottom you’ll find the results of the experiment I did in my class and trust me, the results are significant.

Materials needed:
• A potato (or multiple if you students will also be doing the experiment)
• A knife
• A balance
• 2 plates with lips to avoid spills
• Tap Water (fresh water)
• Salt

The Activity:
1. Have your students draw this chart.

2. Ask your students what they think will happen when you place a cut potato to two different environments. One in fresh water (tap water) and the other in salt water. Have them write it down in the ‘My Opinion’ column including why they think so.
3. Have your students share their answers with the person beside them. Each student should then write the combined opinion in the ‘Group Opinion’ column as they did before.
4. Lastly, have each group share their results with another group and write the consensus opinion in the ‘Shared Opinion’ column.
5. Weigh both cut potatoes and record the masses.

6. Perform the demonstration by placing one cut potato into the dish with fresh water and the other into the dish with salt water. Make sure you saturate the salt water plate, and then add more just to make sure. You’ll need to wait about four hours so now is a good time to do something else. It can get pretty boring looking at a pair of potatoes for four hours.

7. Once the time is up, take the potatoes out of the water, dry them, and place them on the scale. Compare their masses before and after.

8. Have your students complete the row ‘Describe what happened’
9. Explain what occurred.

Explanation:
Water moves from high to low concentration in order to dilute any solutes (substances in the water). Salt is a solute and when you place the potato in the salt solution, you are exposing the potato to a much lower water concentration (high solute concentration) due to the solutes present. Although water moves into and out of the potato, there’s a net movement of water from inside the potato into the salt-water solution. Less water in the potato means less mass. When the potato is placed into the fresh water dish, there’s no difference in water concentration between the potato and the surrounding water environment, therefore, there’s no net movement of water. This leaves the potato with an equal mass before and after the experiment.

10. Have your students complete the last row using the information you provide in the explanation.

My Experimental Results:
Trial #1 – Fresh water Potato

Initial mass – 288 g

Final mass – 298 g

Difference = 10 g increase = 3.4% INCREASE in mass

Trial #2 – Salt Water Potato

Initial mass – 305 g

Final mass -272 g

Difference = 33 g decrease = 11% LOSS in mass

As shown above, there was a significant difference in the two trials. The fresh water potato gained some mass (small amount of water absorbed), while the salt water potato lost 11% of its mass. The loss in mass was due to water leaving the potato to balance the water concentrations as the environment was rich with salt.