Osmosis - Demonstration

Osmosis

Osmosis is the movement of water molecules through a semi-permeable mem-brane from an area of low solute concentration to an area of high solute concentration.

Learning Objectives

To demonstrate osmosis.

Materials

Irish potatoes, 4 beakers* or petri dishes*, sugar, water, knife, kerosene stove, and a cooking pot

Preparation Procedure

1.           Dissolve 10 table spoons of sugar in about 100 mL of water. This solution should be very concentrated and thick.

Fill a cooking pot half way with water and heat it to boiling on a stove.

Activity Procedure

1.           Boil one potato and leave the other uncooked.

2.           Peel the potatoes and cut them into two halves.

3.           Make a shallow hole in the four halves of the potatoes. Each cut potato should look like a bowl.

4.           Put a small amount of water in each beaker.

5.           Put one carved potato in each beaker. The water should not spill into the inside of the potato bowl.

6.           Put sugar solution in the centre of 1 raw potato and 1 boiled potatoes. The other two potatoes will act as controls.

7.           Set the experiment aside for an hour.

8.            After one hour examine the potatoes and write down what you observe.

Results and Conclusion

After an hour the level of water in the raw potato with sugar will rise while in the boiled potato with sugar there will be no change. Boiling kills the cells, therefore the cell membrane loses its permeability. The potatoes without sugar should show no change.

Clean Up Procedure

1.           Collect all the used materials, cleaning and storing items that will be used later. No special waste disposal is required. The used potatoes should not be eaten.

Notes

Osmosis is a special case of Diffusion - the movement of a substance from an area of high concentration to an area of low concentration. The area of low solute concentration has relatively high water concentration whereas the area of high solute concentration has relatively low water concentration. Therefore water moves from the area of high water concentration to the area of low water concentration, or from the area of low solute concentration to the area of high solute concentration. The solute cannot pass through the semi-permeable membrane.