Osmosis required practical

Aim: To investigate how the concentration of a sugar or salt solution affects the change in mass of potato tissue.

Variables

• Independent: Concentration of the sucrose (or salt) solution.
• Dependent: Percentage change in mass of each potato cylinder.
• Control: Length and diameter of the potato cylinders; Variety and source of potato; Volume of solution in each tube; Time left in the solution and temperature

Equipment

• Potato
• Cork borer and white tile
• Scalpel and ruler
• Sucrose solutions of different concentrations (e.g. 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol/dm³)
• Boiling tubes and rack
• Measuring cylinder
• Top-pan balance
• Paper towels
• Labels or marker pen

Method

1. Use a cork borer to cut several potato cylinders of the same diameter.
2. Trim the cylinders with a scalpel so they are all the same length (for example 3 cm) and remove any skin.
3. Label one boiling tube for each concentration and add the same volume of solution to each.
4. Gently blot each cylinder dry and record its starting mass on a balance.
5. Place one cylinder in each tube and note which cylinder went where.
6. Leave all tubes for the same length of time, such as overnight, at room temperature.
7. Remove the cylinders, blot them gently in the same way to remove surface liquid and reweigh.
8. Repeat the whole experiment to obtain at least three results per concentration and calculate means.

Safety

• Cut away from your fingers on a tile when using the cork borer and scalpel.
• Wipe up spilt solutions to avoid slipping.

Results and calculations

Record the initial and final masses in a table and work out percentage change in mass = (final mass − initial mass) ÷ initial mass × 100. Plot percentage change (y-axis, with positive and negative values) against concentration (x-axis), draw a line of best fit, and read off where it crosses zero change to estimate the concentration of the potato cell contents.

Common mistakes and improvements

• Blotting cylinders inconsistently so surface water changes the mass; blot each one the same way for the same time.
• Cylinders of slightly different sizes giving different surface areas; cut accurately with the same borer and measure with a ruler.
• Leaving too short a time for osmosis to reach a clear change; leave for a long, fixed time for every tube.
• Only one result per concentration; repeat and calculate a mean, discarding anomalous results.

Exam tips

• Explain why percentage change is used rather than change in mass: the cylinders start with different masses, so percentage change allows a fair comparison.
• Use the words water potential or dilute and concentrated solutions carefully: water moves by osmosis from a dilute solution to a more concentrated one across a partially permeable membrane.
• Describe and explain the graph: mass increases in dilute solutions as water enters, decreases in concentrated ones as water leaves, and no change means the concentrations are equal.
• Be ready to plot the graph with a negative y-axis and to interpolate the zero-crossing point.
• Suggest improvements such as using more concentrations close to the crossing point.