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LESSON L7 · EXPLAINER
Compulsory Practical 2: Diffusion and Osmosis
The potato-strip osmosis practical is the syllabus’s own example for Manipulation & Measurement. Master it.
Narration uses your device’s built-in voice. No sound? Switch to “Captions only” — the captions advance on their own.
Step 2 · Read
Key notes
Copy these into your notebook in your own words — that is how they stick.
Method keys
Equal-sized peeled strips; fully immersed; same time; blot the same way; measure precisely.
Osmosis: water moves from dilute to concentrated solution across partially permeable membranes.
Visking tubing
Glucose passes, starch does not → model of absorption.
Precautions
Same potato, cut accurately, same blotting, same time, repeat & average.
EXAM TIPS & COMMON MISTAKES
Use percentage change in mass to compare strips of slightly different starting mass.
Step 2 · Learn from examples
Worked examples & study resources
The kind of support a teacher gives after the lesson: worked examples to attempt first, a board illustration, key terms, common misconceptions and memory aids.
Worked example 1
Percentage change in mass and the “no change” point
Potato cylinders were weighed before and after 1 hour in sucrose solutions.
Sucrose (mol dm⁻³)
0.0
0.2
0.4
0.6
Initial mass (g)
2.00
2.10
1.90
2.00
Final mass (g)
2.30
2.20
1.85
1.70
(a) Calculate the percentage change in mass for each. (b) Why use percentage change? (c) Estimate the concentration with no change.
Try it in your notebook first, then reveal the solution one step at a time.
(b) The cylinders had different starting masses, so percentage change allows a fair comparison.
(c) The change goes from +4.8% (0.2) to −2.6% (0.4). Zero lies about two-thirds of the way: roughly 0.33 mol dm⁻³. Plotting the graph and reading where the line crosses zero gives a better estimate.
Answer. (a) +15.0%, +4.8%, −2.6%, −15.0%. (b) Different starting masses. (c) About 0.33 mol dm⁻³ — where the cells’ sap has the same concentration as the solution.
Examiner’s tip. Blot every cylinder dry in the same way before weighing — surface liquid is the biggest source of error in this practical.
Worked example 2
The Visking tubing gut model
A Visking tubing bag containing starch solution and glucose solution was placed in a beaker of water at 37 °C. After 30 minutes the water outside was tested. Predict the results and explain what the model shows.
Try it in your notebook first, then reveal the solution one step at a time.
Water outside + Benedict’s, heated → brick-red: glucose molecules are small enough to diffuse through the pores in the tubing.
Water outside + iodine → stays yellow-brown: starch molecules are too large to pass through.
The tubing acts like the wall of the small intestine: only small, soluble molecules can be absorbed.
This shows why starch must be digested into glucose before it can be absorbed into the blood.
Answer. Glucose found outside (Benedict’s positive), starch not (iodine negative). Only small molecules pass the partially permeable membrane — digestion is needed before absorption.
Examiner’s tip. Include a control: test the water outside at the start, to show there was no glucose there before.
On the board · Variables in the potato osmosis practical
Variable type
In this practical
Manipulated
Concentration of the sucrose (or salt) solution
Responding
Percentage change in mass (or length) of the potato
Controlled
Same potato; same size cylinders; same volume of solution; same time; same temperature
Control
Cylinders in distilled water (0.0 mol dm⁻³)
Reliability
Three cylinders per concentration; calculate the mean
Setting out the variables like this is worth marks in both P&D and ORR.
Key terms
Osmosis
Water moving from a dilute to a concentrated solution through a partially permeable membrane.
Visking tubing
Partially permeable tubing used to model cell membranes or the gut wall.
Cork borer
Tool that cuts potato cylinders of equal diameter.
Blotting
Removing surface liquid with paper towel before weighing.
Percentage change
(final − initial) ÷ initial × 100.
Common misconceptions
✗ The potato gained mass because it absorbed sugar.✓ It gained mass because water entered by osmosis; sugar molecules are too large to enter freely.
✗ Change in mass is enough — percentages are optional.✓ Percentage change is needed when starting masses differ.
✗ Peel does not matter.✓ Peel slows water movement; remove it, or make sure every cylinder is treated the same.
Memory aid
Potato in water → gains; potato in strong sugar → drains.
Real-life link
Farmers know that heavy use of salty water or too much fertiliser around roots makes crops wilt — the soil water becomes more concentrated than the root cells, so water leaves the roots by osmosis.
Try this
Put one raisin in water and one in strong sugar solution overnight. Compare their sizes next day and explain the difference.
Play & practise
Games
4 quick games on this lesson. Earn XP and badges as you play — your best score for each game is saved. Games are for practice and do not change your exam readiness.
Step 3 · Practise
Paper 1 practice — multiple choice
6 items modelled on real CSEC Paper 1 questions. Choose one answer for each, then check. Aim for 80% or better.
Step 4 · Apply
Practical-style questions
Write full answers, then reveal the mark scheme and tick the points you earned (7 marks in total). Your answers are saved on this device.
Based on CXC syllabus MM example & Specimen P03/2 Q1
Question 1 [7 marks]
Plan the potato-strip osmosis practical.
(a) List FOUR steps that ensure accurate measurement. [4]
MARK SCHEME — (a) (max 4)
(b) Predict and explain the result for strips in distilled water. [3]
MARK SCHEME — (b) (max 3)
Your progress
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Sources
Syllabus sources and past papers
Everything in this lesson is based on these parts of the CXC syllabus and papers.
CXC syllabus — SBA topic 2; MM example: osmosis in living tissue (p. 68); A&I example: carrot tissue (p. 70)