I will be recording the length and weight of the potato samples before and after leaving them in the sodium chloride solution for the set amount of time. I will observe the changes in mass and length. I will do this by using a ruler to cut the potato samples to the correct length, followed by scales to weigh the pieces. I will copy this data into a table, work out the average and then proceed with the experiment.
To make sure that my results will be reliable, I will be carrying out the experiment 3 times. This will allow me to see if any anomalous results are present. I will also be working out the averages; this would also allow me to cancel out any abnormalities.
I will make the investigation valid by:
- Making sure all samples are of the same length;
- Making sure the samples are as close to the same mass as possible ;
- Keeping the overall volume of the solution the same for each test;
- Keeping the time spent in the solution the same for all tests;
- Using the same potato for each test.
There are factors which cannot be controlled, but may affect the investigation. These are:
- The room temperature. Although the surrounding temperature should stay around 21ºC, it may differ slightly and subsequently have an affect on the outcome.
- The mass. Despite the fact that the size of the samples will be the same, it is not possible for the mass to be precisely the same.
I have chosen to use five different concentrations of sodium chloride solution;
- 20%- 12ml NaCl + 48ml Water;
- 40%- 24ml NaCl + 36ml Water;
- 60%- 36ml NaCl + 24ml Water;
- 80%- 48ml NaCl + 12ml Water;
- 100%- 60ml NaCl + 0ml Water.
I have chosen these concentrations because they will give me a good range of results. I will do three attempts of test to make sure I get accurate results.
Preliminary
I will do a preliminary test to make sure my method is correct and work out whether my timing is correct, or whether I should leave the samples in the solution for a longer or shorter amount of time.
These results showed me that my method is correct, and if I carry it out properly I can obtain valid results.
Results
I do not need to repeat any of my tests as my results are clear an accurate.
Conclusion
My results tell me that my prediction is accurate. I predicted that the potato in the stronger sodium chloride would drop in size and mass more than the samples in the less concentrated solution.
To make analyzing my results easier, I will use the averages.
(Note: a minus indicates a decrease in mass.)
I will now work out the percentage increase in mass to make plotting my graph easier.
Analysis
From my table I can see that my prediction seems to be almost correct; the potato lost more mass in the high concentrate salt solutions, but I noticed that the amount of mass lost seemed to decrease between 80% and 100% salt solution. This shows that the salt solution was hypertonic.
The points on my graph seem to lie in a basically straight line of a negative gradient. This would suggest that the concentration of the solution is inversely proportional to the % increase in the mass of the potato. This means that as the concentration of the solution increases the % change in the mass of the potato increases up to a certain point.
I got these results because osmosis occurred in the cells of the potato. This happened because the cells are surrounded by a semi-permeable membrane which allows the movement of water, but not of larger molecules. In a hypertonic solution (strong salt) most change occurred because the most water had to diffuse out of the cells to reach equilibrium with the solution on the outside. This caused the biggest change in the mass of the potato as the most water was lost from the cells. As the concentration of the salt solution decreased, the % change in the mass of the potato also decreased. This is because as the solutions got progressively less concentrated, less water needed to be lost to reach equilibrium so the cells lost less mass. In pure water the potato gained mass as the solution outside had a greater concentration of water. This caused water to diffuse into the cells making them gain mass.
Evaluation
I think the investigation generally went quite well as the results seemed to be in keeping with what was expected considering the background theory. There didn't seem to be any anomalous results as the graph had a very strong correlation which suggests that the experiment was successful as the results seem reliable.
However, as I chose to record the collective masses of the 3 samples at each concentration, I have no real way of knowing if the results are accurate or if it's just a coincidence that they lay on that particular line. I think it's unlikely that it's a coincidence because the correlation is so strong, but it's always possible. For this reason, if I did the experiment again and had more time I would record each of the 15 results separately. This would allow me to easily spot any anomalies and I could also plot all the points on a graph to get a more accurate line of best fit.
Also, if I did the experiment again I would use 0% concentrate, distilled water. Another way to make my results more accurate in terms of finding the isotonic solution would be to do the experiment again but concentrate the investigation on a smaller area. This would give me more detailed results, so I could estimate the concentration of the isotonic solution.
I could also investigate osmosis further by looking at how the volume of the potato changes in different concentrations of solution. This would look at the idea that cells should go turgid in pure water as they gain water, and flaccid in strong salt solution as they lose water. I would conduct this experiment in the same way except I would record the volume of the potato cylinders instead of the mass. I'd do this by filling a container with water and then putting the potato in. I'd collect the water that is displaced and the volume of this water would be the same as the volume of the potato. I'd take these measurements before and after osmosis and use the results in exactly the same way as I have when investigating mass.
If I could change one thing about my experiment is the timing. My specified time was one hour, but due to circumstances the samples were actually left for twenty-four hours. This probably affected my results to some degree, but the end result is still as I expected.
