Practical skill 2: Osmosis using dialysis tubing

Aim

To investigate how the concentration of a sucrose solution inside dialysis tubing affects the movement of water into the tubing by osmosis.

Background

Water diffuses through partially permeable membranes by osmosis. In Extended terms, osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane. A partially permeable membrane lets small molecules such as water through but not larger molecules such as sucrose.

Dialysis tubing (also called Visking tubing) is a model of a cell membrane. It has tiny pores. Water molecules pass through the pores easily. Sucrose molecules pass through very much more slowly (a little may leak out over many hours), so during a 30-minute experiment the tubing behaves as if sucrose cannot cross it. This lets us see the effect of osmosis on the mass of a tubing 'bag' without using living cells.

If the solution inside the bag is more concentrated than the water outside, there is a net movement of water into the bag and its mass increases. If the solution outside is more concentrated, water leaves the bag and its mass decreases. If both have the same concentration, there is no net movement and the mass stays the same.

Hypothesis

The more concentrated the sucrose solution inside the tubing, the greater the percentage increase in mass of the bag, because there will be a greater net movement of water into the bag by osmosis.

Variables

Equipment

• Dialysis (Visking) tubing, 5 pieces each about 15 cm long, soaked in water
• Thread or tubing clips; scissors
• Sucrose solutions of 0.0 (distilled water), 0.2, 0.4, 0.6 and 0.8 mol/dm³, about 50 cm³ of each (enough for three repeats)
• 5 beakers, 250 cm³, each with 150 cm³ distilled water, labelled
• 2 syringes, 10 cm³ (or 5 cm³), or a funnel, for filling the bags
• Balance reading to 0.1 g or better; paper towels; stop-clock; marker pen
• Extension: glucose and starch solution mixed; Benedict's solution, iodine solution, test-tubes, hot water bath, eye protection

Risk assessment

Method

1. Soak five pieces of dialysis tubing in water for a few minutes until they are soft and can be opened by rubbing the end between your fingers.
2. Tie a tight knot (or fix a clip) at one end of each piece so that it makes a bag with a closed end.
3. Label five beakers 0.0, 0.2, 0.4, 0.6 and 0.8 and put 150 cm³ distilled water in each.
4. Use a syringe to put 15 cm³ of sucrose solution into each bag, using a different solution for each bag. Use clean syringes or rinse between solutions, working from 0.0 upwards.
5. Squeeze out most of the air and tie the open end tightly (or fix a clip), leaving a short length of empty tubing.
6. Rinse each bag under the tap and blot it dry gently with a paper towel. Use the same method on every bag.
7. Measure the mass of each bag to 0.1 g and record it as the initial mass.
8. Put each bag in its labelled beaker, making sure it is covered by water. Start the stop-clock.
9. Leave the bags for 30 minutes at room temperature.
10. Remove each bag in the same order as it went in. Blot each dry in the same way and measure its mass to 0.1 g. Record it as the final mass.
11. Calculate the change in mass and the percentage change in mass: (final mass − initial mass) ÷ initial mass × 100.
12. Extension: fill a bag with glucose and starch solution and place it in distilled water for 20 minutes. Test samples of the water outside with Benedict's solution (heat in a water bath) and with iodine solution.

Results

Fill this table in as you go. Print the PDF for a copy to write on.

Drawing the graph

Plot change in mass / % (y-axis) against concentration of sucrose solution in bag / mol/dm³ (x-axis). Mark each point with a cross (×), use more than half of the grid in both directions, and draw a single smooth best-fit curve. Use a line graph because the concentration of sucrose is a continuous variable.