Investigate Osmosis

Investigate Osmosis

Aim:To investigate osmosis. The student is going to see what happens to the weight of a potato when it is placed into different solutions of sodium chlorides and water.

Prediction: The student predicts that potato placed in distilled water will undergo a gain in weight and potato placed in salt solution will lose weight. Potato placed in mixed concentration will have no change in its weight. This prediction has been made using scientific knowledge of the special type of diffusion known as osmosis. In osmosis, each individual cell is surrounded by a cell membrane. The cell membrane allows small molecules to pass through it, but not large ones. Osmosis is the movement of water molecules from an area of high concentration (weak solution) to an area of low water concentration

(strong solution) through a partially permeable membrane.

Water moves in both directions to try and make the concentrations inside and outside of the cell even. If there is more movement one way, it is said that there is a net movement of water into the area where there is less water.

If the potato is placed into distilled water, the concentration of water outside of the cell will be greater than the concentration of water inside the cell. There will be a net movement of water from the area of high concentration to the area of low concentration and as a result of this the cell will contain more water, it will become turgid and weigh more than it did when it had a smaller concentration of water. If the potato is placed into a salt solution the concentration of water will be greater inside the cell than outside, osmosis will take place and the water will once again move from the area of high concentration (inside the cell) to the area of low concentration

(outside the cell). This will cause the potato to lose weight as it loses its water.

If the potato is placed in a mixed solution then the concentrations inside and outside of the cell should be equal, the water molecules will move by osmosis but the concentrations will remain equal.

Apparatus:

- 125ml of 1ml sodium chloride solution.

- 125ml distilled water.

- 15 cylinders of potato each weighing 5.3g.

- 15 beakers (150ml).

- 2 measuring cylinders (50ml)

- 2 potato borers (one large one small).

- 1 set of digital weighing scales.

- 1 calculator

- 1 small scalpel

- 1 stop clock

Method: The student will use a potato borer to cut out 15 cylinders of potato to the same size and weigh them, the weights will be recorded in the table of results (each piece of potato should be the same