Investigating Osmosis Using Visking Tubing

Aim

This experiment shows that osmosis occurs across visking tubing, an artificial partially permeable membrane, by setting up a simple osmometer and observing the change in the level of solution in a glass tube over time.

Variables

• Manipulated variable, the concentration difference across the membrane, set up by using a concentrated sucrose solution inside the visking tubing and distilled water outside it in the beaker.
• Responding variable, the change in the level of solution in the glass or capillary tube, measured in centimetres over time.
• Controlled variables, the temperature of the surroundings, the duration of the experiment, the size (length and diameter) of the visking tubing, and the volume of solution used.

Materials and Apparatus

• visking tubing
• a glass tube or capillary tube
• concentrated sucrose solution
• distilled water
• a beaker
• a retort stand and clamp
• thread or a rubber band
• a ruler
• a marker pen
• a stopwatch

Procedure

1. Soak a length of visking tubing in water until it becomes soft and easy to open.
2. Tie one end of the tubing tightly with thread to seal it.
3. Fill the tubing with concentrated sucrose solution using a funnel or by pouring carefully.
4. Insert a glass tube into the open end of the tubing and tie the tubing tightly around it with thread so the join is airtight.
5. Mark the initial level of the sucrose solution in the glass tube with a marker pen.
6. Clamp the tubing upright on a retort stand and immerse it in a beaker of distilled water, so the water level in the beaker is above the tied end.
7. Mark or measure the level of solution in the glass tube every 10 minutes for 30 to 40 minutes.
8. Record the level of solution in the glass tube at each time interval.

Expected Results

The level of solution in the glass tube is expected to rise steadily as the experiment proceeds, since water continues to move into the tubing by osmosis.

Conclusion

Water moves into the visking tubing by osmosis, from the dilute distilled water outside to the more concentrated sucrose solution inside, across the partially permeable membrane. This net movement of water raises the level of solution in the glass tube, supporting the aim of showing that osmosis occurs across visking tubing.

Paper 3-style questions

These original questions follow the style of Paper 3 and use the science process skills examined with this apparatus.

Question 1 (hypothesis). A student sets up three osmometers with 10%, 30% and 50% sucrose solution inside the visking tubing, each standing in distilled water. Write a hypothesis.

Model answer. The higher the concentration of sucrose solution inside the tubing, the greater the rise in the level of solution in the glass tube over a fixed time, because the steeper water potential gradient draws water in by osmosis more quickly.

Question 2 (variables). State the manipulated, responding and one controlled variable, and say how the controlled variable is kept constant. Model answer. Manipulated: the concentration of sucrose solution inside the tubing.

Responding: the rise in the level of solution in the glass tube. Controlled: the temperature of the surrounding water, kept constant by running every set-up under the same laboratory conditions at room temperature.

Question 3 (tabulation and graph). Readings of the level in the glass tube are taken every 10 minutes for 40 minutes. Describe how to display the data and the expected shape of the graph.

Model answer. Plot a graph of level of solution (cm) on the y-axis against time (min) on the x-axis, and draw a smooth curve of best fit. The curve rises steeply at first and then flattens, showing that the rate of osmosis slows over time.

Question 4 (inference). After 40 minutes the level stops rising. Explain this observation.

Model answer. As water enters, the sucrose solution inside becomes more dilute, so the water potential difference across the membrane falls; the rate of net osmosis decreases until it approaches zero, and the level stops rising.

Safety

• Insert the glass tube into the visking tubing gently, holding it close to the end, because glass tubing can snap and cut the hand if it is forced.
• Wipe up any spilt water or sucrose solution at once so the bench and floor do not become slippery.
• Report and clear away any broken glass safely, rather than picking up the pieces with bare hands.
• Wash your hands after handling the apparatus and clean the glassware before storing it.

Common Mistakes

Source:SRC-DSKP-EN

Frequently asked questions