Preliminary work
Before carrying out this investigation on how the concentration of salt solution affects osmosis I had carried out a previous experiment on osmosis. I carried out that experiment to see what the affects of osmosis were.
The experiment entailed ribena solution being placed in some visking tubing and then placing the visking tubing inside a beaker of water. Also there was a measuring tube placed inside the visking tubing. The experiment was set up as the diagram below shows:
What I found out from the experiment was that the water molecules had osmosed from the beaker into the ribena solution through the visking tubing. I could tell that this had happened as the ribena solution had risen up the measuring tube. Through this experiment I found out that as the concentration of water was higher in the beaker than the concentration of water in the ribena solution, the water molecules in the beaker diffused through the semi-permeable membrane which was the visking tubing to the ribena solution. The preliminary experiment helped me understand what osmosis is and also it helped me to plan my investigation and make my prediction of what I would expect to find out.
Prediction
I predict that when the concentration of the salt solution is most dilute the potato chip will increase in mass but as the concentration of the salt solution increases, the mass of the potato chip will decrease.
Hypothesis
I have used what I learnt from my preliminary work and background knowledge to my prediction into what I predict is going to be the outcome of this investigation. The reason for my prediction is that when the salt solution concentration is low, the water concentration outside the potato chip cells will be greater than the concentration of water inside the cells of the potato chip. This will lead to the water molecules outside the potato chip to move through the cell membrane and into the cells, therefore the potato chip will gain mass. So as the concentration of salt solution increases the water concentration outside the potato chip becomes less then that it is inside the cells of the potato chip. This causes the water to move out of the cell membrane of the potato chip and into the salt solution, and so causing the mass of the potato chip to fall.
Equipment
I need require various pieces of equipment to carry out his experiment, these are as follows:
- Cutting tile
- Scalpel
- Tweezers
- Fiver boiling tubes
- Weighing scales (accurate to 2 decimal places)
- Potato
- Test tube rack
Diagram
I will set my experiment how I have illustrated in the diagram below.
Fair test
In order to keep this experiment as fair possible I will need to keep the other factors that might affect osmosis as constant as possible. This is because I am only investigating one variable of osmosis in this investigation and can’t let the other factors affect my results. The only factor that I will allow to vary is the controlled variable which is the concentration of salt solution.
Type of potato
I will use the potato chips from the same potato spud to ensure that the potato chips have the same genetic make and have the same semi-permeable membrane. Also I cut the potato chips to the same dimensions so that the surface area of the potato chips does not affect the reading I take of the potato chips.
Volume of salt solution
I will use the same volume of salt solution at each separate concentration of the experiment. This will ensure that the vole of solution does not affect osmosis in this experiment.
Temperature
The temperature will be controlled in this experiment. I control the temperature by doing all of the experiment at room temperature. All five tubes will be put in the same area as they will be on the same test tube rack and so will be exposed to the same temperatures.
Time
Also I will keep the potato chips in each solution for the same amount of time, to prevent time contributing to osmosis in this experiment.
Through keeping all these factors constant in this experiment I aim to make this experiment as scientifically accurate as I possibly can.
Plan
- Before I can start this experiment I will have to carry out tasks of preparation to have items I require for experiment ready for use. The first of which is to make up the concentrations of solution. I will make up the concentrations 20%, 40%, 60%, 80% and 100%. The way I did this is shown through the table below.
I have chosen to go up in concentration of the salt solution by 20% each time I believe that this interval is sufficient enough to achieve reliable result.
- I need to cut out the potato chips to equal sizes, which is 20mm x 30mm. I will do this using a scalpel and cutting on top the cutting tile. I will cut out ten potato chips so that I can put 2 in each tube. After I have cut the potato chips I will weigh each one separately and record it into my result table.
- I will then put five boiling tubes onto a test tube rack. Then I will pour the concentrations of salt solution in their separate tubes and label them accordingly.
- After that I will place two potato chips in each tube. I will leave the experiment for 24 hours for osmosis to take place in that time. After that time I will com back and take out the potato chips out of the tubes individually and weigh them for any change in mass and I will record this change in mass in my result table.
- then I will calculate the percentage change in mass of the potato chips using this formula,
Change in mass (g)
Original mass (g)
When weighing the potato chip before and after the experiment I will record the mass of the potato chip to two decimal places as in order to get accurate results I need to note the mass as accurately as possibly.
I will also record these percentages changes in the mass of the potato chips to two decimal places into my result table. As I am using two potato chips in each boiling tube I can calculate an average percentage change in the mass of the potato chips which will mean that I will get more reliable and accurate results.
Method
I carried out the experiment as a have already have already specified in my plan but I just changed a few things. The things that I changed are as follows. I didn’t use boiling tubes as they were not available for use at the time, so I had to use smaller test tubes to carry out this experiment. As the test tubes were smaller than the boiling tubes I couldn’t pour 20ml of solution into each beaker so I had to the amount of solution to 10ml instead. Also another change that I had to carry out was to only keep the potato chips in each solution for one hour as opposed to keeping them in the solutions for twenty four hours. I did this as is was not possible me to take down results after that time and I found out that I would get accurate after this time any way.
Results
In this table I recorded my results of what the mass of the potato chips were originally and how they changed after they had been placed in the solutions. The table also shows the percentage change in the mass of each potato chip.
To measure the mass of the potato chips accurately I used a sensitive weighing scale that would give the mass of the potato chips to two decimal places at each point they were weighed. Also the percentage change in mass is to two decimal places.
First set of results
Second set of results
From the two sets of results I made a third table of results which was of the average potato mass and the average percentage change in mass.
Table of average results
Analysis
As I have calculated an average set of result which make my result more accurate I have chosen to analyse that set of results by plotting graph 3. Graph 3 (Page 14) shows that the rate of osmosis and percentage change in mass are not linked proportionally but still it show that as the concentration of the salt solution increases the percentage gain of the potato begins to slow and then decrease in mass.
What this means is that as the concentration of salt solution increases the percentage change in mass decreases. The results prove this as the graph shows that when the salt concentration was at 20% the percentage change in mass was an increase of 7.08 on average. This fell dramatically to a decrease in mass of the potato chips as the concentration increased to 40%. At 40% of salt solution concentration the mass of the potato chips decreased by -2.11% on average. This decrease continued as the salt concentration was 60% where the mass of the potato chips fell by -4.53% on average. The mass decreased even further as the salt concentration increased even more. At 80% salt concentration the percentage change was -12.74% in the mass of the potato chips on average. The change at salt concentration 100% slowed down but still was decreasing to a percentage of -14.52% of average.
Conclusion
At 20% concentration of salt solution the concentration of water in that solution is much higher than the water concentration in the cells of the potato. So the water molecules moved from the salt solution into the cells of there of the potato chip. As the water had omosed into the cells of the potato, the potato chip gained in mass by a percentage increase of 7.08%.
From looking at the average percentage change in mass graph (page 14), I can see that when concentration of the salt solution was at 33% the percentage change in mass was 0.0. The reason for this is because at 33% the concentration of water in the salt solution was the as the water concentration inside the cells of the potato. So no molecules of water entered the cells of the potato and no water molecules left the cells of the potato, meaning that at this point the mass was lost or gained by potato chip.
As the concentration of the salt solution increased from 40 % to 100% the potato chip lost mass. The potato chip lost mass as the concentration of water in the potato chip cells was greater than the concentration of water in the salt solution because of this the water molecules inside the potato chip cells moved out of the cells and into the less concentrated salt solution. As the potato cells had lost water from their cells at these concentrations of salt solution, the cells were not turgid no more and potato chip became soft.
The trends in my graphs tell me that potato chips gained mass first, when the concentration of salt solution was 20%. Then when the concentration of salt solution increased from 20% the potato chips began to lose mass quite rapidly, but near the end the rate of the mass being lost was beginning to slow downed as the line tailed off.
From looking at all three graphs all graphs I can conclude that my prediction of what would happen in this investigation was correct, as the outcome of the investigation which I have already explained matched what I said in my prediction (page 5) and the background knowledge (page 4) that I had gathered.
Evaluation
Overall I think that my investigation into osmosis went very well, as the experiment went smoothly and I didn’t have any major problems. The problems that did arise were easily solved. The results that I got from this experiment I think are accurate as what I found out through my background knowledge matched my results and as I had performed repeats to check the consistency of the results. The pattern of results that I predicted so too did show in my results. As all three graphs are very similar this very strongly means that the experiment was fair and accurate. I found that on both experiments there was an anomalous result at 60% salt concentration as at this point on both experiments the result gathered was slightly off and did not fit into the trend of the other results. This may have occurred because at this point when weighing the potato chips a bit of excess water had been left on the chip and so this could have tampered with the results. Even though three was a slight anomaly it dint really affect the findings as it still shown what the overall trend was very clearly.
Improvements
I could have made improvements to the experiments by using potato chips that were same weight instead of using chips of just the same surface area. This would have meant that the change in the mass of the potato chips was very clear to see even without the percentage and would have made this investigation even more accurate. Another thing I would have improved would be three potato chips instead of just two. This would have meant that I could have got I third set of results to compare and to get a better average as some of the results even though very accurate were still quite far apart.
Further work
Further work that could do into investigating osmosis is to use another variable and study how this variable affects osmosis in potato chips. The variable I would investigate would be how the size of a potato chip affects osmosis.
For this experiment I would have the size of the potato chips as my controlled variable. The variables that I would keep constant would be:
- Temperature
- Volume of water
- Concentration of solution
There would be five different sized potato chips which were from the same type of potato. The sizes of the potato chips would be 10mm x 10mm, 10m x 20mm, 10mm x 30mm, 10mm x 40mm and 10mm x 50mm. Each potato chip would be placed in different boiling tubes of water. To keep the variable constant the volume of water in each boiling tube would be 20ml. The potato chips would be left in the boiling tubes of water for one hour each. Each potato chip would be weighed before and after being placed inside the boiling tubes to measure the percentage change in mass foe each different sized potato chip. I would then record the results in a results table.
Each boiling tube would be set up as shown in the diagram below:
In this experiment I would expect to see that the smallest potato chip to have the greatest percentage gain in mass and the large potato chip to have the least percentage gain. This is because the small potato chip has a lower concentration of water compared to the large potato chip, so more water molecules would diffuse from the high concentration of water in the beaker into the potato chip. I could then compare that set of results with the result from this investigation to see if they both have similar outcomes.
