Dependent variable: (what you are measuring)
Range of independent variable:
- 10% - 0% of concentration of salt solution
Controlled variables:
-Length of potato core
- Amount of salt solution
- Amount of time in salt solution
How they will be controlled:
-Use a cork borer accurately
-Use a syringe carefully
-Use a stopwatch
How it will be measured:
-By weighing the core before and after the experiment
Apparatus:
6 boiling tubes, test tube rack, potato, white tile, stanley knife, 10% salt solution, stopwatch, cork borer, syringe.
Method:
- Gather all the equipment needed.
- Take the potato, and cork borer, and take out 6 cores of potato, using the borer. Once you have done this, check that each core is relatively the same length.
- Now, weigh each core using an electronic weighing scale, and record each mass in a table.
- Then, obtain a test tube rack, followed by six test tubes.
- Next, using a syringe, measure out the correct volume of different concentrations of salt solution, and put in the six test tubes. Look at the table above, to see the different amount of concentrations needed for each test tube.
- Before placing each potato in the test tube- it is recommended that you label each potato core (i.e. 1-6) and record its number in the table, as when you have finished the experiment it would be easier to compare the core to its original weight.
- Place each potato core in of the test tubes, and leave for 30 minutes. - Remember to time on the stop watch!
- After 30 minutes, removed each core, and dry them carefully with a paper towel. Now, weigh each core, and record.
- Compare with previous weight.
- The one which weighs the same as it weighed before being put into the solution is the one with the same concentration of salt in the cell sap of potato, as the salt solution.
Because my results were unreliable, and inaccurate, the teacher provided the class with an accurate set of results, where the experiment had take place for 5 hours. Because of this, the changes in mass for potato cores depending on the amount of salt concentration were more precise.
Table of results, from accurate + reliable experiment:
Analysis
The shape of the graph was a smooth curve sloping downwards. This shows that the lower the concentration of salt solution, the higher percentage of mass change. This ranges from 2.38% change in mass for 0% concentration, and -1.03% for 10% concentration. If the concentration was less than 4.4, the potato core gained weight. This proved that the salt solution was more dilute than the concentration of cell sap in the potato. Thus by osmosis, the water molecules diffused across the semi permeable membrane, then entered the core, increasing the potato’s mass, and making it turgid. If the concentration was above 4.4, the potato core’s weight decreased. This was because there was a more concentrated salt solution compared to the dilute solution in the cell sap of the potato. As a result, through osmosis, the water diffused through the semi permeable membrane, then entered the salt solution. The potato became flaccid.
Evaluation
In my opinion, the data is unreliable as the repeats are varied. For instance, the first change in mass recorded was -3.62%, and the second 1.89%. If the experiment was reliable, these changes in masses would have been similar. Most of my results match my line of best . There are some points that are further from my line of best fit, for example, the results recorded for 10% salt solution. An erroneous result like this may have been caused by human error. To reduce this problem, I would do the following things:
Firstly, when I dried off the excess water on the potato cylinders after the experiment, I used a paper towel. This may have either drained some water out of the potato or left some excess water on the potato. This part of the experiment is difficult to come up with an accurate and fair method, as other ways would also lead to some slight mistakes.
The ranges of concentrations were adequate but I would extend the range of concentrations so that there would be more varied results, i.e. 1, 2, 3, 4, and so on.
Another way of improving the results would have been to leave the experiment running longer, this would have enabled me to find the saturation point (when the potato can no longer take in any more water) and dehydration point (when the potato cannot lose any more water) and therefore attain more accurate results.
The cutting of the potatoes was the most difficult part of the experiment as although I was recording my results by mass, it could have affected the surface area and so the overall rate of osmosis. If I were to repeat the experiment I would have found a machine to cut the potato, as it would ensure that all potato cores would be the same weight and dimensions.
Finally, I would have achieved more precise results through measuring out the solutions more accurately. This could be done using a pipette.
In conclusion, I have been able to draw up a final theory which has proved my prediction to be correct:
As the concentration of salt increases, the mass of the potato decreases.
