Year 11 ATAR Human Biology Osmosis Investigation

Below is a step-by-step guide on how you might address each section of the Osmosis Investigation write-up in full detail. Note that you should tailor the details (particularly the numerical results in the table and graph) to your actual experimental data.

1. Hypothesis

Sample Hypothesis

In other words, you hypothesize that water will move from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration), causing measurable changes in mass.

2. Variables

1. Independent Variable

   • The concentration of the external glucose solution (e.g. 0 M [distilled water], 0.3 M, 0.6 M, 0.9 M).

2. Dependent Variable

   • The percentage change in mass of the dialysis tubing contents.

   • (You can calculate this using: \text{% mass change} = \frac{\text{(final mass – initial mass)}}{\text{initial mass}} \times 100 \])

3. Controlled Variables

   • Volume of solution inside dialysis tubing (e.g. 10 mL).

   • Size/length of dialysis tubing (same type and approximate surface area).

   • Time for which the tubing is immersed (e.g. 30 minutes).

   • Temperature (conduct all trials at the same room temperature).

   • Equipment (use the same balance for weighing, same measuring cylinders, etc.).

   • Drying method (patting tubing gently with paper towel in the same manner each time before weighing).

By controlling these factors, you ensure that any changes in mass are primarily due to differences in the concentration of the external solution rather than other factors.

3. Results

(a) Draw/Present a Results Table

Your table should include the following columns (an example is shown below—replace the placeholders with your actual data):

(b) Graph Your Results by Hand

1. Axes

   • X-axis: Concentration of external solution (0.0 M, 0.3 M, 0.6 M, 0.9 M).

   • Y-axis: Percentage change in mass (%).

2. Plotting

   • Plot each concentration on the x-axis and its corresponding % mass change on the y-axis.

   • Draw a line or curve of best fit (often a smooth trend or a roughly straight line that might slope downward as concentration increases).

(c) Discuss the Results / Trends

• Typically, you might observe a positive % change at lower external concentrations (or in distilled water), indicating net water movement into the dialysis tubing.

• As the external concentration increases, the % change in mass decreases and often becomes negative, indicating net water movement out of the tubing.

• This trend supports the concept of osmosis: water moves from a region of higher water potential (inside the tubing if it has lower solute concentration) to a region of lower water potential (outside if the solution is more concentrated).

4. Evaluation

(a) Scientific Background Supporting the Results

• Osmosis is the net movement of water molecules across a selectively permeable membrane from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration).

• Dialysis tubing acts as a semipermeable membrane, allowing water to pass but restricting larger solute molecules (e.g., glucose).

• As external solution concentration increases, water is more likely to flow out of the tubing (if the tubing’s internal concentration is lower) to balance the solute concentration on both sides of the membrane.

A simplified diagram might show:

Low Solute (inside)       Semi-permeable Membrane       High Solute (outside)
       ( H2O )  -->           [Dialysis Tubing]               (Glucose)

(b) Problems / Uncontrolled Variables

1. Incomplete Sealing of Dialysis Tubing

   • If the tubing is not sealed tightly, the solution inside might leak out, affecting mass measurements.

2. Inaccurate Measurements

   • Inconsistent use of the balance or not taring (zeroing) the balance properly can cause errors.

   • Not patting the tubing dry in a consistent manner can add extra water droplets to final mass.

3. Temperature Fluctuations

   • If the temperature changes significantly, the rate of osmosis could change.

4. Time Inconsistencies

   • If one bag soaks for longer than another, that could alter the mass change.

5. Variations in Dialysis Tubing

   • If some pieces of tubing are thicker or have slightly different pore sizes, they might allow different rates of water movement.

(c) Improving Validity and Reliability

1. Repeat Trials

   • Perform multiple repeats (e.g., three trials per concentration) and calculate an average % mass change to increase reliability.

2. Use More Concentration Points

   • Adding intermediate concentrations (e.g., 0.2 M, 0.4 M, etc.) would help you see a more detailed trend.

3. Control Temperature

   • Conduct the experiment in a water bath or temperature-controlled environment to reduce temperature fluctuations.

4. Ensure Proper Sealing and Drying

   • Tie off the dialysis tubing carefully and pat it dry consistently before each weighing.

5. Accurate Measuring Equipment

   • Use calibrated pipettes or burettes for measuring solutions and a digital balance with fine precision for mass measurements.

5. Conclusion

• Was the Hypothesis Supported?

  • Based on typical results, yes. As the external glucose concentration increased, the dialysis tubing’s mass tended to decrease (negative % mass change). This supports the hypothesis that water flows out of the tubing into the higher solute concentration solution.

• Evidence

  • The percentage change in mass became more negative with increasing external solute concentration.

  • In the lowest concentration (distilled water or 0.3 M), the mass generally increased (positive % change).

  • In the highest concentration (e.g., 0.9 M), there was a clear loss in mass.

Thus, the results align with the principle of osmosis: water moves from higher water potential (inside the tubing if it’s filled with a more dilute solution) to lower water potential (the external concentrated solution), confirming the initial hypothesis.

Final Tips

• Always cite actual numerical data from your experiment in your discussion to make your conclusion strong (e.g., “At 0.3 M, we observed a +3.5% mass change on average, whereas at 0.9 M, the mass change was –6.2% on average.”).

• Make sure any graphical representation is clear, with axes labeled (including units if applicable), and a descriptive title (e.g., “The Effect of Glucose Concentration on Dialysis Tubing Mass Change”).

• In your evaluation, focus on real-world sources of error that you personally encountered—this is what demonstrates your understanding of the experimental process and how it might be improved.