Investigate osmosis using materials such as dialysis tubing.

3.2 Osmosis

Objective

Investigate osmosis using materials such as dialysis tubing. Understand how water moves across semi‑permeable membranes and how this process is important for living cells.

What is Osmosis?

Osmosis is the passive movement of a solvent (usually water) from an area of low solute concentration to an area of high solute concentration through a semi‑permeable membrane.

Key Terms

• Solute – the substance dissolved in a solvent.
• Solvent – the liquid in which a solute dissolves.
• Concentration – amount of solute per unit volume of solvent.
• Osmotic Pressure – the pressure required to stop the net flow of water.
• Dialysis Tubing – a semi‑permeable membrane used in experiments.

Analogy: Water as Traffic 🚗

Imagine a city where cars (solute) want to move from a crowded area to a less crowded one. The roads (membrane) allow cars to pass only if they are small enough. Water is like the traffic flow that moves from the less crowded side to the crowded side to balance the number of cars on each side.

Experiment Setup

You will use dialysis tubing to explore how water moves between two solutions.

1. Fill one end of the tubing with a 0.5 M glucose solution.
2. Seal both ends with a small piece of tape.
3. Submerge the tubing in a beaker of pure water.
4. Weigh the tubing before and after 30 minutes.
5. Record any changes in weight and note the appearance of the tubing.

Observations & Calculations

After the experiment, you might see that the tubing has gained weight, indicating water has moved into the tube.

Use the following formula to estimate the osmotic pressure:

$$P_{osm} = iCRT$$

Where i = number of particles the solute dissociates into (for glucose, i = 1), C = molar concentration, R = 0.082 L·atm·K⁻¹·mol⁻¹, and T = temperature in Kelvin.

Data Table

Exam Tips 📚

Remember:
• Osmosis is a type of diffusion – water moves from low to high solute concentration.
• The semi‑permeable membrane allows only the solvent to pass.
• Osmotic pressure can be calculated using the formula $P_{osm} = iCRT$.
• In questions, look for clues like “water moves into the cell” or “cell swells”.
• Use diagrams to show the direction of water flow.
• Practice converting units (e.g., mM to M, °C to K).
• When given data, calculate the expected change in weight or volume.
• Don’t forget to state assumptions (e.g., temperature constant).

Further Exploration

Try varying the concentration of the glucose solution (e.g., 0.1 M, 1.0 M) and observe how the rate of water movement changes. Compare the results with the theoretical osmotic pressure.