Procedure:
Safety and precaution:
Safety is important in any scientific experiment. We were working with sucrose, water, glass and scalpels. In order to remain safe, I followed these instructions:
- Hold the scalpel downwards when walking with it.
- Be careful when working with sucrose, as it is sticky; if you spill it on the table wipe it down immediately.
- When using the scalpel to cut the potatoes, make sure fingers are not near the blade as they are very sharp.
- Do not allow any liquid to be near electrical equipment.
- Make sure you don’t allow sucrose solution to go onto the floor, as it is a safety hazard and could lead to someone slipping on it and falling over.
I followed all of these procedures and came across no problems involving safety when carrying out my experiment.
Diagram:
Apparatus:
-1x Tweezers
-1x Scalpel
-6x Boiling tube
-1x Tube rack
-18x Coloured drawing pins (6x red, 6x blue and 6x yellow)
-18x potato chips (20mm/2cm)
-1x Scales (sensitivity to be 2 d.p.)
-1x Cutting tile
-1x Beaker with 150ml3 of water
-1x Beaker with 150ml3 of sucrose solution
-1x Pipette for water
-1x Pipette for sucrose
-1x Cork borer size 4
-1x Cork borer size 5
-1x Ruler
-1x Results table
-1x Calculator
-2x Kitchen towel
-12x White labels (6 for labelling boiling tubes, 6 for labelling bundles of potato chips)
-Clingfilm
-2x measuring cylinder (0-25ml3)
Method:
- Get the potato measuring equipment for the potato (cutting tile, scalpel, cork borer etc), including potato. Lay the potato on cutting tile and use the cork borer to get over 18 potato chips (6 solutions x 3 repeats= 18 chips). Over 18 chips is the best number to have as some chips may be misshapen.
- Remove useless potato chips and line remaining ones up. Measure chips; cut them down using scalpel to 2cm (20mm). Then measure again by lining them all up and seeing if they are all identical sizes. Group them in 3’s, cover in cling film and label using white label with their future sucrose concentration.
- Carry them to the scales, unwrap one at a time. Weigh one and record the answer in your results table under the column mass before. To be able to identify the chip, you must create a drawing pin scheme: firstly, get coloured pins and find 3 colours which there are 6 of.
- Stick one colour (e.g. red) into the first potato chip after it being weighed, and then place it back into the cling film bundle. Get another colour (e.g. yellow) and put it into the second after being weighed repeat this with the last colour (e.g. blue).
- All three will now be in the cling film bundle. Put this aside and continue with all the other bundles. For example if the first was for the 0.0m potato chips, then repeat for 0.2, 0.4 etc. Put these on your working desk and now collect equipment for preparing the solution
- Put 6 boiling tubes into the rack; label them with their future solutions. Get the two beakers with 150ml of either water or sucrose solution
- Start with 0.0m solution. Because no sucrose is involved, use the pipette to get 30ml of only water. Pour this into the boiling tube labelled 0.0m. Place Clingfilm over the top in order to stop evaporation. For 0.2m, measure 24ml of water and 6ml of sucrose. Pour these into the boiling tube marked 0.2m and put cling film over top. For 0.4m, measure 18ml of water and 12ml of sucrose. Pour into boiling tube marked 0.4m and put cling film over top. For 0.6m, measure 12ml of water and 18ml of sucrose. Pour into boiling tube marked 0.6m and put cling film over top. For 0.8m, measure 6ml of water and 24ml of sucrose. Pour into boiling tube marked 0.8m and put cling film over top. For 1.0m, measure 0ml of water and 30ml of sucrose. Pour into boiling tube marked 1.0m and put cling film over top.
- This tables shows how to make the various solutions
- Lift up cling film and place the 3 appropriate potato chips in their boiling tube. The bundle marked 0.0m contains 3 chips that will go into the 0.0m solution. After all the chips are in the right places, record the time and leave them in their solutions.
- Whilst waiting for the reactions to take place, clear up unnecessary apparatus and wipe down the benches. Remember to leave the chips in their solutions for over an hour.
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After an hour, remove the pins from each potato chip. To do this you remove cling film and stand over sink, using tweezers to remove the potatoes one by one and pouring the solution down the sink.
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Be careful you do it one at a time otherwise results will be confused. When you have the potato chip, look at the colour of the pin and the solution the boiling tube was labelled in. This means you now know where its mass result can be recorded under. Remove drawing pin and blotted the chip with kitchen towel so excess solution is removed. Then weigh its mass, write it down under column ‘mass after’.
- Repeat this with every potato until your ‘mass after’ column is completed. Then dispose of potato chips. Place boiling tubes upside down on the large rack near the kitchen sink so they can be cleaned later when you have finished attaining all your results.
- Calculate the Mass change by subtracting the ‘mass before’ from the ‘mass after’. Write down the results in the ‘mass change’ column of the results table. Now you can calculate the percentage mass change, dividing the mass before by the mass change, then multiplying it by 100, does this.
- When the percentage mass change is found for each potato chip for every solution, then find the average for every solution. Adding the three percentage changes together then dividing it by 3, giving the average mass change, do this. Add the results to your results table.
- The table and experiment is complete but there is no graph, plot a graph of the results which is titled ‘a graph to show the concentration of a sucrose solution affects the mass of a potato chip’
I had some choice when picking my equipment. For example, there were measuring cylinders of 25ml3 or 50ml3. I decided to use the 25ml3 cylinder because it gave more accurate readings and had1ml3 divisions. However, the 50ml3 cylinders had 2ml3 divisions so reading information off it was difficult and not as precise as I would have wanted when measuring out my solutions.
I had a choice between using boiling tubes or test tubes. I chose to use boiling tubes, as they are larger and allowed the 3 chips to have most of their surface area available for osmosis to take place, unlike test tubes, which are smaller.
Precise and reliable measurements:
- Apparatus and testing procedure stayed constant, which reduces random errors from unknown influences.
- I used simple apparatus that I have been familiar with since year 7. This meant I did not have any problem reading information off measuring cylinders and knew the difference between boiling tubes and test tubes.
- The scales I used were sensitive enough to be able to detect changes of 2 decimal points, which increased precision and accuracy of results.
- By getting repeated measurements (the 3 different chips) I reduced the effects of random errors because I made an average of the 3.
- I was careful around my work and made sure not to knock over any of my boiling tubes.
- I had 6 values for my independent variable, which gave me a wide range of results and made it easier for me to see the relationship between sucrose solution and mass change in a potato.
Obtaining evidence:
Changes to my experiment:
When it came to the actual experiment, I rendered my potato useless after taking only 4 chips from it. I therefore had to use a different potato and discard my first 4 chips.
I realised there should be step 7 in ‘precise and reliable measurements’, this is
7. Cover the whole chip when blotting, as by blotting only one side not all excess moisture is lost making the results not as reliable as they could be.
Results:
A table to show the effect osmosis has on the mass of potato chips left in different concentrations of sucrose solutions for one hour
Analysing and considering evidence:
I took my results and then generated an average percentage change in mass for each sucrose solution.
This is how I did it:
% Change in mass = change in mass of potato
Original mass of potato
In order to calculate my average percentage change in mass, I had to find the average of the three repeats. I followed this simple formula:
Repeat 1 + repeat 2 + repeat 2
3
All of my results are to 2 decimal places as this allows precision when plotting a graph and also the scales were to 2 d.p. And I wanted my results to have a consistent number of decimal places.
My graph is attached at the back. It shows a curve.
Statement conclusion:
At lower concentrations, mass was gained because the water outside the chip moves into areas of high potential- inside the potato. At higher concentration, mass is lost because there is higher water potential outside the potato so water moves out of the potato.
I know that osmosis is the net movement of water across a partially permeable membrane from areas of high water potential (low concentration) to areas of low water potential (high concentration) until equilibrium is reached. I also know that at different sucrose concentrations there is a different rate and direction of osmosis, this is reflected by my results. At 0.0m when sucrose concentration increases, the percentage mass gain is very large 20.34%. This falls to an increase of only 12.12 %at the 0.2Molar concentration. At 0.4m, the mass change is only 1.04%, showing that equilibrium is nearly reached. After this point, the mass begins to decrease. At a concentration of 0.6Molar, there is a mass decrease of -11.04%, very high. The decrease continues to fall but not as rapidly as it was at the lower concentrations and at 0.8m; the mass change is -16.63%, continuing onto -18.86% at 1.0m.
At 0.0Molar, there is no sucrose present because the solution is just 30ml3 of tap water. This means the water concentration is higher outside of the potato chip so water goes into the potato chips cells through the partially permeable membrane. This means mass is gained inside the chip and the cells become turgid because water makes the cell swell up and pushes against the cell wall until the cell cannot hold any more. I know the cells became turgid because when I touched the potato chips from solutions 0.0m-0.4m they were stiff to touch, especially 0.0m.
At 0.2Molar, mass was also gained, showed by the result of a 12.12% increase. Because there is 6ml of sucrose in the solution, there is not as high a concentration outside of the potato and though mass is gained, is not as much as at 0.0Molar where there was no sucrose present as all. The potato cells still became turgid and I know this because when I touched the potato chip, it was hard.
At the 0.4 Molar solution, there was 12ml3 of sucrose solution and 18ml3 of water so there was a dramatically smaller mass increase, only 1.04%, this shows that equilibrium was nearly reached, or an isotonic. This makes sense, as there is only a 6ml3 difference between the two solutions the potato chip is in so there is not much of a movement of water from the higher concentration outside of the potato to inside the potato chip. When I felt the potato, it was not as turgid as the others but was still pretty stable. This means that the water was not pushing as hard against the cell wall.
The first 3 solutions, 0.0m-0.4m had osmosis going in the same direction and I know this because they all gained in mass. This is shown on the graph by all three’s average mass change being plotted above 0. At 0.42 Molar, there is equilibrium. There is no net movement from outside the potato chip and inside the chip. I assume the solutions are equal on either side of the membrane, which explains the fact there is no change in mass.
Between the solutions of 0.6m-1.0m, osmosis was also going in the same direction and this is shown by their negative position on the graph. This means that they all lost mass. Mass is lost by the chips because the concentration of water inside the chip’s cells is greater than that outside of the cell, so water is moved out of the cell by osmosis. The chips become plasmolysed, leaving them flaccid and soft, because of their water loss. Though you cannot show plasmolysis with my results (because it can only be measured using a microscope) I knew it occurred because they were physically soft to touch, the 1.0Molar chips being very soft indeed.
At 1.0Molar, there is no water potential as the whole solution is made up of sucrose. Therefore water is transferred out of the potato chip and it loses mass. This shows me that the higher the concentration of sucrose, the lower the percentage increase.
The trend my graph shows is that as the sucrose concentration increases, mass is firstly gained, this gain decreases until it becomes mass loss, this mass loss increases until it evens out at the end, giving me a ‘s-shaped’ curve, called a ‘sigmoid’.
My conclusion matches my prediction well as I stated that mass is firstly gained and this gain decreases until equilibrium is reached and then mass is lost. I correctly assumed that mass increase would be the highest at 0.0m, where there is the lowest concentration and highest water potential, and that mass loss would be highest at 1.0m, where there is no water potential and where the concentration of sucrose is at its highest. I also predicted that between 0.4m and 0.6 equilibrium would be reached and there will be no net movement of water. I was correct in my prediction because at 0.42m, equilibrium was indeed reached and the concentration outside and inside the potato cell was isotonic. My point of equilibrium was closer to 0.4m that I had expected though, as I thought it would be around 0.5Molar as this is the middle point between 0.0 and 1.0 Molar and where I would have thought the concentrations were isotonic.
However, there is only a partial match between my predicted graph and thee graph I produced from my actual results. There are several similarities however, for example they both show the trend of there being a mass gain, a point of equilibrium and then a mass loss. I had expected all my results to be in a straight line, showing a directly proportional relationship between sucrose concentration and average percentage mass increase. When writing my prediction, I knew that the less sucrose in the solution, the larger the change in mass because of osmosis the water moves across a partially permeable membrane from areas of high water potential (inside the potato chip cells) to areas of low water potential (high concentration and outside the potato chip cells) until equilibrium is reached. However, my predicted graph did not take plasmolysis into account and this explains its straight line instead of a curve. Plasmolysis is when a cell membrane is drawn away from its cell wall because water is taken out of the cell (because of osmosis). The net movement of water happens across the partially permeable membrane, and moves from an area of high concentration to an area of low concentration. At 0.8m, I think the cells become plasmolysed because between 0.8m and 1.0m there is only a small increase in percentage mass loss. This is because after plasmolysis, the cell is not capable to retain water and therefore osmosis cannot occur. This leads to a curved shape in my graph.
Evaluation:
Anomalies:
Due to the presence of anomalies in my data, the data cannot be classed as reliable. Though I used 3 repeats and then generated an average in order to reduce random errors, there were still some present and this is reflected in my graph. For example, in 0.4Molar solution, on my first and second repeats I got percentage mass changes of 0.00 and -0.81, which showed my solution was very close to equilibrium. However, on my third repeat I got a percentage mass change of 3.94, which was over 2% higher than the other repeats. When I found the average of the 3, it was 1.04, though it should have been far closer to 0. This anomaly is an ‘inflated’ anomaly because it increases the mass percentage increase and therefore changes the way the line of best fit is drawn on the graph. However, I decided to include this result in my data because otherwise my results would not reflect my experiment properly.
There are several reasons as to why I have anomalies in my data. Firstly, when carrying out my experiment I altered my methods of blotting excess liquid away from the chips. On some chips I covered the whole chip with the kitchen towel, thus draining away all excess solution but I occasionally only touched one side of a chip when blotting it. This means the measurements of mass change are not reliable, as I did not stick to a constant method. The presence of excess water means there is more mass. This could explain the inflated anomaly in 0.4m, which is 0.05g heavier than the other two repeats. I most probably blotted the first 2 repeats on both sides but the 3rd repeat on one side. If I were to carry out the experiment again, I would most definitely remember to blot both sides of my potato chip as this careless mistake has made deemed my results unreliable.
Another reason for anomalies may be that in every boiling tube there were 3 potato chips. This could have affected the rate of osmosis because all the potatoes were touching, leaving some surface area covered and therefore unable for osmosis to take place. In lower concentrations, where there was mass gain, the chips became large and affected by turgor making them swell and take up more room thus there was more surface area available and when the chip was larger, osmosis was greater. However, in the larger chips, there was more clustering in the boiling tubes and some surface area was covered which hindered osmosis as it cannot take place in covered areas. The chips in low concentrations may also have results affected by surface area. Unlike in higher concentrations, there was mass loss and so the potatoes decreased in size, which decreased the surface area for osmosis to work.
There was a slight discrepancy between the two scales situated in the lab where I carried out my experiment. I unfortunately used one scale to measure my chips’ mass before they were put into solutions and another to measure their mass after. This means that there may be a roughly 0.1g difference between the two results.
Another reason for anomalies may be the cutting of the potato chips. It was very hard to get all my chips an exact size and there was limited time to cut them all the to same size of 20mm. I had limited equipment to use when cutting- a cork borer, scalpel and ruler. In some cases, the potatoes had various dents on them and smooth areas, which could have increased their surface area, and therefore osmosis on those potatoes was greater. When thinking back to my experiment, I may not have removed all the potato skin off some chips, leaving them with different textures so they would have all had different rates of osmosis. If time was available, I would have been more precise in making my potato chips identical; this would have given me more accurate results.
Procedure:
I think my results are accurate to an extent but not as much as I had hoped they would have been. My graph fits with the scientific knowledge I predicted but I may have obtained some wrong results- anomalies- whilst carrying out my experiment. The equipment I used was not very accurate for example the ruler I used was worn down and many of the millimetre measurings had been worn off. . Measuring my chips to 20mm precisely was very difficult and it would have been useful if the chips were cut and measured by a machine of some sort as this would have led the all the potatoes having the same surface area and mass. I took 3 repeats and took an average of the 3 which was used to cancel out random errors but I feel if I took 5 repeats, random errors could be completely eliminated. My range of readings was sufficient and any more would have been too time consuming and unnecessary. Also, boiling tube racks only hold 6 boiling tubes so it would be inconvenient to use any more. If I had followed all of my control variables meticulously then I feel I could indeed have carried out a fair test. However, I had a limited amount of time and instead of spending 20 minutes cutting my potatoes and checking them I only spent about 7. If I had more time I could have carried out a very fair test and produced good, accurate results.
Reliability of evidence:
I had 6 values for my independent variable and I used 3 repeats for each solution. The inclusion of repeats was in order to try and eliminate random errors made but I feel in future I should use 5 repeats in order to fully get rid of any anomalous results and ensure accuracy. My repeats were enough to be able to draw an average, which was very useful. My graph shows the effect that the concentration of sucrose has on the mass change of a potato chip. The graph is an ‘s-shaped’ curved, also known as a sigmoid. The curve almost flattens between 0.8Molar and 1.0Molar and this is because of the effects of plasmolysis on the chips’ cells’. There are several anomalies within my results but they are not clear to see on my graph as the repeats have cancelled the inflated or deflated anomalous readings out.
In several cases, there is a close agreement within repeats but this is not constant throughout my results. For example, if you look to my results table, at 0.6Molar the first and third repeat are exactly the same. This shows that at points my method of getting results was precise and accurate.
My graph shows a clear sigmoid s-shape curve. All the points fit together well and there aren’t any large outstanding anomalies. My entire results lie either on my curve of best fit, or very close to it. Because the points I have plotted on my graph align with my best-fit curve, there is a strong correlation between points.
Further work:
If I wanted to make a more general conclusion about the effect sucrose concentration has, I would have to do further work and make some changes to the way I carried out my experiment. Even though I got some fairly accurate results and a strongly correlated graph, I produced various anomalies and therefore rendered my results ‘unreliable’. There were certain aspects of my experiment I feel I really could have improved on.
Because I have a clear s-shape graph, I could maybe create more sucrose concentrations and use my current graph as a prediction as to where the mass change of a chip would be. This would further my knowledge because I could see if the results I collected were actually correct or if I should actually have done a straight line of best fit. If I had solutions with a difference of 0.1Molar I would see even more clearly the effects of sucrose concentration on percentage mass change of a potato chip.
Firstly, I should have been more careful and precise when cutting and measuring potato chips. Limited time and a small range of equipment- cork borers, ruler and a scalpel- led to the chips not being exact in size as lining them all up was a very time consuming task and I had to move on to making my solutions. When cutting the chips, some had obvious dents in them and when I first tried cutting the potato it wouldn’t let me take 18 chips from it and so I had to start again. Some chips were deformed and others were so long that they were halved. All in all, my chips were not a uniform size and had different surface areas and masses. This meant that my results were not as accurate as I had hoped for because the larger a surface area a chip has, the more room for osmosis to occur. If I were to redo the experiment, I would definitely spend a lot more time cutting my potatoes as having a uniform surface area and size is very important when trying to get reliable and accurate results. It would have been very useful if there were pre-made sets of 18 potato chips (all being from the same potato of course as different potatoes contain different starch levels and therefore different rates of osmosis) that we could work on immediately.
Another area I could improve on is drying my chips after using them. My method of blotting varied immensely throughout the duration of my experiment and this may be a cause of several of my anomalies. I did not have a constant procedure and sometimes only dried one side of the chip with the kitchen towel but on other occasions dried both sides in order to drain all excess liquid. This meant that in some chips the excess water remained and so the mass measurement was not accurate and the effects of osmosis were not properly documented. If I were to repeat the experiment, I would have a constant method I would follow throughout my experiment and therefore I would blot my chips in the exact same manner.
The slight discrepancy between the two scales in the lab acted to my disadvantage. I used both, one for calculating the mass before the experiment, and one for after. This meant there was a difference of about 0.1 of a gram between my results and therefore they were not precise. In future, I intend on only using one scale therefore if there is a random influence on my results it will be constant in all of them.
The fact that in every boiling tube there were 3 potato chips may have affected the rate of osmosis because all the potatoes were touching, leaving some surface area covered and therefore unable for osmosis to take place. In lower concentrations, where there was mass gain, the chips became large and affected by turgor making them swell and take up more room thus there was more surface area available and when the chip was larger, osmosis was greater. However, in the larger chips, there was more clustering in the boiling tubes and some surface area was covered which hindered osmosis as it cannot take place in covered areas. The chips in low concentrations may also have results affected by surface area. Unlike in higher concentrations, there was mass loss and so the potatoes decreased in size, which decreased the surface area for osmosis to work. If I were to redo my experiment, I would have 18 different boiling tubes with every one chip in one boiling tube so that they had the maximum surface area on display. However, in higher concentrations, the chips floated and so the surface area was limited for osmosis to work on. Not all of the chip was in contact with the water and this may have explained why in 0.8Molar and 1.0Molar there is such a small difference in mass loss between the two. If I were to redo the experiment I would try and find a way of stopping the chips from floating and making all their surface area be in contact with the water.
In order to expand my knowledge and further my work I could investigate other things that affect osmosis, not just the concentration of a solution. I could investigate the time a chip is left in a solution of x Molar and then see what the relationship is between time and mass change. I could also look at temperatures because in this investigation the temperature was constantly at room temperature. Other areas I could possibly focus on are surface area of the chips because I know that the larger the surface area, the more osmosis takes place. I could also maybe use a salt solution and see its effects when the chips are put in it and then compare it to the sucrose solution.
