When measuring the liquids, lower eye level to the same level as measuring cylinder to see the miniscus properly, and therefore make more accurate measurements.
- Sort the pieces of potato into groups of three. Take one group at a time and weigh each piece of potato. Record the measurements, before putting all three pieces into a boiling tube; noting which tube the group of potatoes have been placed in (so that in the end, you end up with 3 pieces of potato in each tube, and each piece of potato has been separately weighed). Record the weights in the following table:
- Use paper towels to seal the tops of the boiling tubes, to stop any solution from evaporating, and then rinse and clear away the equipment that is no longer needed.
- Store the test tubes racks somewhere they can be left for 24-48 hours. The reason that they need to be left at least 24 hours is so that osmosis has a chance to take place. The apparatus cannot be left for much longer than 48 hours though, as the potatoes will start to break down and will no longer be any use, as they would no longer be able to be weighed accurately.
- Once the experiment has been left for about 48 hours, take a test tube and pour out as much of the solution into the sink as possible, and the tip the potato onto a paper towel. Gently soak up the excess water on the outside of the potato, and then weigh and record the masses of each piece individually, in a similar table to the first. Do the same for each of the other boiling tubes.
- Throw away the potato, rinse out the boiling tubes, and clear everything away, making sure there is no sucrose solution left on the work surface.
- Find the average start and finish weight of each group of potatoes, and work out the differences, in the following results table:
Safety
To ensure the safety of myself and others around me I will follow general lab rules, e.g. Put bags out of the way, and stools under the bench, to avoid tripping over them.
Also, when dealing with sucrose solution, although goggles are not necessary as it is not harmful, wearing a lab coat is advisable, as the solution will be sticky and messy if it gets onto clothing.
Fair Testing
Most of the fair testing for this investigation is mentioned in the ‘controlled variables’ section; if these variables are kept the same each time the experiment is performed, it ensures a fair test.
Also, the fact that 3 pieces of potato are being used for each concentration and the average will be found means that the results are more accurate.
Prediction
‘Osmosis is the movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration’.
Using this definition of osmosis, I predict that a strong sucrose solution will mean a decrease in the mass of a potato; the stronger the solution, the greater the decrease in mass of the potato.
This is because if the sucrose solution is strong, the concentration of water molecules will be low in relation to the concentration in the potato, and so water molecules will move from the potato to the solution outside, decreasing the mass of the potato. The weaker the concentration of sucrose in the solution, the more water molecules there will be moving out the solution, increasing the difference in start and finish masses of potato.
Results
This table shows the starting weight of each piece of potato individually, and the average weight for each group.
This table shows the end weight of each individual piece of potato, and the average weight for the group.
This table shows the average start and end weights of the potatoes, and the difference between the two.
Because the starting weights were all slightly different, I found the percentage difference between the start and finish mass of the potatoes, instead of just using the actual difference in mass. Then, I drew a graph for visual representation of the data (see separate sheet).
Conclusion
The graph has a negative curve, which shows that the higher the concentration of sucrose solution, the greater the percentage differences in the mass of the potato.
My prediction was ‘I predict that a strong sucrose solution will mean a decrease in the mass of a potato; the stronger the solution, the greater the decrease in mass of the potato’, and my results support this.
The reason stronger solutions mean greater decrease in mass is because in a strong solution of sucrose, there are less water molecules, and so more have to leave the potato before equilibrium is established and in a weaker solution.
However, we did not try very weak solutions of sucrose; we only went down to 60% sucrose. With weaker solutions, I think the difference in mass will just as great as with strong solutions, but I think they would be positive differences, i.e. the potato will increase in mass rather than decrease. I think this would be true because in a weak sucrose solution, there is a lot more water outside the potato, and so a lot of water will have to be absorbed by the potato for equilibrium to be achieved, increasing the weight of the potato. This would be a point to investigate further, given more time.
Evaluation
Reliability of Results
I think the results of the experiment are fairly reliable, as I measured all the liquids and the lengths of potato as accurately as I could, and I did everything I could to keep it a fair test.
The only variable that we couldn’t control was the temperature. However, the lab is usually kept at a steady temperature, and we kept the boiling tubes in the same place in the lab for the whole length of time we left them to make it as fair as possible. Other than that though, all variables were kept the same, making it overall a reasonably fair test, and any temperature changes had no obvious effects on the results, and so I feel that the results are reliable enough to support my conclusion.
Anomalous Results
There was one anomalous result, for the solution with the 90% sucrose concentration, but other than that, the results followed a smooth curve, and the odd result was still very close to the line. I think that this one irregular result was due to an error such as slightly inaccurate measuring of liquids, as they are such small amounts, or perhaps too much liquid was soaked up from the potato before weighing the final mass.
Another source of error was that the potato wasn’t very big, and so it was hard to get 15 full cylinders from it; maybe the cork borer didn’t go straight through the potato, but instead went through at an angle, cutting through a hole made previously. This would have meant that the shape was not quite a cylinder, and this may have affected the result.
Also, the fact that due to lack of space, I had to put 3 pieces of potato into a single boiling tube may have affected results, as it meant that some surfaces on the potato pieces were touching each other. This meant that they were not in contact with the water, which means that osmosis would not have been taking place in that area.
Future Improvements
Improvements for this experiment could be:
- Use a larger potato, so that there will definitely be no overlaps when using the cork borer
- Make up more solution of each concentration than needed, so that it is easier to be more accurate, and the use a pipette to measure out correct volumes
- Use different boiling tubes for each piece of potato, so that there aren’t any surfaces not in contact with the water
- Soak up the excess water on the potatoes using a different, lighter material, such as tissue paper instead of a paper towel. Tissue paper is absorbent enough to soak up excess water, without soaking up water from inside the potato, which is what the paper towel does if not enough care is taken.
