Required practical 3: Production of a dilution series of a solute to…

The idea

Produce a dilution series of a solute, calculating volumes of stock solution and distilled water so every tube has the same final volume. Cut potato cylinders with a cork borer for equal diameter and length, blot them and record their initial mass. Place one cylinder in each concentration for the same time at a constant temperature. Remove them, blot gently to remove surface liquid, and reweigh. Calculate the percentage change in mass for each cylinder, as starting masses vary, and plot this against concentration. Where the line crosses zero, there is no net osmosis. The water potential of that solution equals the water potential of the potato cells. Read off the concentration and convert it to a water potential.

What earns the marks

• A correctly calculated dilution series in which each tube has the same final volume.
• Recording the initial and final mass of each cylinder while controlling time, temperature and volume of solution.
• Blotting the surface dry before reweighing so that surface liquid is not included in the mass.
• Calculating percentage change in mass rather than change in mass, so cylinders of different starting mass can be compared.
• Identifying the concentration at which the percentage change is zero as the point where the external water potential equals that of the tissue.
• Controlling cylinder diameter by cutting with a cork borer, so that diameter is a controlled variable rather than a measured dependent variable.

Common mistakes and how to put them right

• Plotting change in mass rather than percentage change, so cylinders of different starting sizes cannot be compared.

• Recording size as the dependent variable, which is often rejected.

  Put it right: Mass or length are required, while diameter is controlled by the cork borer.

• Assuming a gain in mass means the solution had the lower water potential, when it means the solution had the higher, less negative water potential.

• Quoting a water potential as a positive value, when the water potential of any solution is negative.

• Cutting cylinders of different sizes, or leaving them in the solutions for different times.

Exam tips

• State the variables you controlled, not just the one you changed; time, temperature, volume of solution and size of tissue all earn credit.
• An increase in mass or length shows that water has entered by osmosis, so the cells had the lower water potential; write that reasoning out rather than assuming it.
• The same method appears in unfamiliar contexts such as root tissue in seawater, so learn the steps as a transferable procedure.