Plan an osmosis investigation with dialysis tubing
Use stated membrane permeability, matched conditions and mass measurements.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall osmosis and signed mass change.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why blot the tubing consistently before weighing?
What you will learn
- Use stated membrane permeability, matched conditions and mass measurements.
- Apply the idea to a supplied biological example or investigation.
Define the membrane model
Dialysis tubing provides a classroom membrane model. Its permeability depends on the tubing and particle; this exercise explicitly uses a membrane that permits water but retains the named larger solute. Do not generalise that assumption to every solute or tubing product.
An investigation can put a solution in sealed tubing and immerse it in a different surrounding solution. Measure the sealed bag before and after the same interval. Check seals and leaks, and keep tubing dimensions, immersion time, temperature and blotting method comparable. Practical work requires teacher-approved materials and supervision.
Attribute the mass change carefully
Blot the outside consistently to remove surface liquid before weighing; do not squeeze water from the bag. A gain in bag mass is evidence consistent with net water entry when the solute is retained and there is no leak. A mass change alone is not proof that the membrane retained every solute.
A bag containing the same solution as the surrounding liquid is a useful comparison. Repeat the conditions, calculate changes and discuss measurement variation. Written planning and supplied data do not demonstrate that the learner has performed the practical competently.
Sources for this lesson
Pause and explain
Why check the bag for leaks?
Worked example
A sealed bag gains mass from 12.0 g to 13.8 g. The stated membrane passes water, retains the solute and has no leaks. Interpret the result.
Show the worked solution
- Change = 13.8 − 12.0 = 1.8 g.
- The retained solute cannot explain entry.
- Net water entered under the stated conditions.
Answer A 1.8 g gain, consistent with net water entry by osmosis.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A bag changes from 10.0 g to 11.2 g. Calculate mass change.
Give me a hint
Final minus initial.
Compare my reasoning
- 11.2 − 10.0 = 1.2 g.
- The sign is positive.
+1.2 g.
Look for these in your work
- State gain.
A bag changes from 15.0 g to 13.5 g under the stated retained-solute model. Interpret it.
Give me a hint
Find the signed change.
Compare my reasoning
- 13.5 − 15.0 = −1.5 g.
- With no leak, net water left.
A 1.5 g loss, consistent with net water leaving by osmosis.
Look for these in your work
- Use the assumptions.
The initial bag is dry outside but the final bag drips. Explain why the apparent gain is unreliable.
Give me a hint
Separate attached liquid from internal contents.
Compare my reasoning
- Surface liquid adds final mass.
- Use the same gentle blotting method at both measurements.
Unequal surface liquid confounds mass change; standardise blotting.
Look for these in your work
- Name the measurement error and repair.
Common mistakes
- A wet outer surface can produce a misleading mass gain.
- Check the stated solute permeability rather than guessing.
What can you explain now?
Why check the bag for leaks?
Compare with the explanation
A leak could change mass independently of osmosis.
The interpretation depends on the sealed model.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
List four controls and one leak check for a tubing investigation.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.