The part of the table that is highlighted is the most important to me as it is the input variable that I am investigating. It shows that in my investigation I will be investigating the effect of sugar concentration change in potato. This will inevitably change the rate of osmosis that is taking place. Another way to think about it is by the water moving from a less concentrated solution like distilled water, to a more concentrated solution like the sucrose solution.
However, many of the input variables will not be varied, as they are not being investigated. These have been included in my table too but they need explaining more:
- The temperature of the solution – Although the temperature would vary the rate of osmosis, we are not going to be changing this aspect. We will be leaving the test-tubes in a room for a couple of days, which will mean that the temperature will be nearly identical for each test-tube, making it a fair test in that aspect.
- The time the potato chip is left in solution - The longer the potato chip is left in the solution, therefore, the more osmosis will occur, as osmosis is not a very fast process, it needs to a certain extent. Once the water molecules have moved because of the process of osmosis and both substances are equal, then no more osmosis will take place. This is exactly what is happening in our experiment because we are leaving it till it has reached equilibrium, so therefore, time would not affect it.
- Surface area of the potato chip – Surface area would affect osmosis as if there was a bigger surface area then there would be more room for osmosis to take place. A partially permeable membrane surrounds the surface area, and this is where the osmosis takes place, so if there is bigger surface area osmosis can take place at a quicker rate and in more places.
- Mass of the potato chip – Although this is not being investigated, the mass would also affect the rate of osmosis. It would be able to absorb more of the solution for both substances to be equal. This would make the process a lot faster as it is doing more work in less time.
The outcome variables that I monitor are very important and I need to make sure that I use them to my advantage giving me the most accurate results. I believe that the most important outcome variable would be the percentage change in mass of the potato as it is most precise. This will enable us to investigate the osmosis that had taken place and see how much water has transferred from the potato or into the potato, inevitably making the mass increase and decrease accordingly. (It is better than just the change in mass because the change in mass will not show a massive change.) The potato will then be measured after the experiment, which will show us the change in mass, and from this we can then calculate the percentage change in mass. Another outcome variable would be the size of the potato. This will show us, along with the mass of the potato, that osmosis is taking place. It also inevitable that this factor is going to vary as when the mass of the potato increases the size of it will do so accordingly. Likewise, if the mass of the potato decreases, the potato will shrink in size. The change in concentration of the solution will be an outcome, as the concentration will change to make both substances equal. So we can figure out the change in the concentration from before and after the investigation. Finally, the change in volume of the solution will be another outcome as, if water is being absorbed at any for the potato to become equal with the solution, the volume of the solution will go down. It could also happen the other way and therefore the volume of the solution will get bigger.
(The possible outcome variables are highlighted, standing out from the rest of the text)
Out of all these possible outcomes, I will still be looking at the percentage change in mass of the potato chip, as this will be the most accurate and precise result. This is because the change in volume and concentration of sugar will be too difficult to measure. The change in the length of the potato Chip will only be very small and very difficult to determine. The change of mass of the potato chip will not be very precise, as I am not controlling the starting mass.
After discussing in detail above how osmosis happens and looking at the factors that determine the rate of osmosis, I have come to the prediction that the larger the difference in concentration between the two substances, the faster the rate of osmosis or the more the osmosis will take place. The bigger the concentration, the more osmosis is needed to lose water or absorb water and make both substances equal. If the concentration of glucose in the potato chip is higher then water molecules will pass from the solution into the potato chip, therefore, increasing the mass of the potato chip and making it grow in size. However, if there is a higher concentration of glucose in the solution, then water molecules will pass from the potato into the solution, therefore, decreasing the mass of the potato and making it shrink in size. So therefore, I definitely believe that a change in the concentration of the solution will have some kind of effect on the mass of the potato chip, due to osmosis.
Knowing my prediction I am now going to write out a suitable method for my experiment and then carry out this method as accurately as possible:
- Firstly I will prepare the solutions by putting them in test tubes with the concentrations of 0%-25% sugar solution. They must be labelled correctly.
- Then prepare the potato pieces of lengths 30mm by using the potato corer to cut them and using a ruler to measure them.
- After, I will weigh the potato chips to get the starting masses and I will record these results in my table.
- I will then do the steps above another 2 times and then work out an average for the start masses of each solution.
- Now I can put all the chips into each solution at the same time
- To allow time for osmosis to occur, I will leave the potatoes in the solution for 2 days.
- After it has been left, the potatoes and solutions must have reached equilibrium. So I will now weigh the potatoes and find out the end masses.
- I will then finally work out the average start and end masses for the potatoes then work out the average change in mass and the average percentage of change in mass.
In order to get the most out of my investigation and to receive accurate and an analyzable set of results, a preliminary experiment will be carried out. This is very useful to us, as it will help us decide which concentrations to use in our experiment. It will also resolve any doubts that we have about the experiment and to ensure that it will go the way we plan it to. We will also know if we have made any errors in the final experiment because the preliminary investigation will show them as incorrect. More positively, the preliminary experiment can justify and conclude the investigation, as they are likely to agree with these correct conclusions, backed up also with scientific knowledge.
As the computer simulates this experiment, no equipment will be needed to carry it out, only the computer itself and the computer program. It will also be a fair test so there is nothing that you need to do to maintain a fair test.
During the preliminary experiment a potato chip having a start mass of 0.98 grams went into distilled water, (0% sugar solution). After osmosis had taken place it then weighed 1.13 grams, having gained 0.15 grams. This was a fairly big change and acknowledged that osmosis was working correctly with our experiment. It also showed that there was a fairly big change when there was no sugar in the solution.
Another chip, with start mass of 0.99 grams, went into a solution of 25% sugar (by mass); the potato chip’s mass decreased to 0.60 grams due to osmosis acting on it. This is a very big change and showed that the difference in concentration between the 2 substances must have been very different for such a big change in mass to take place.
Overall, to see these changes is very pleasing, as I now know that using concentrations of 0%-25% is a good idea. I know this because in the preliminary experiment it caused a big change and reacted well, which means that I will get a varied and interesting set of results.
I will now be using sugar solutions at 5% intervals from 0% to 25%. I will carry out each experiment 3 times to get an average.
To carry out the experiment I will need the following equipment:
- Eighteen pieces of cylindrically shaped potato. This needs to be enough to carry out the investigation on 6 pieces of potato (0%, 5%, 10%, 15%, 20% & 25%) three times, so that I can get an average on each.
- Eighteen glass test tubes to hold the pieces of potato and so that I can use them at the same time, therefore it is then a fair test.
- Three test tube racks. The test tubes will not stand on their own; therefore they will need to be held by the test tube racks.
- A set of scales must be used, as there is nothing else to weigh the potato chips and weighing the potato chips is a very crucial part of the experiment. It is vital that the potato chips are weighed or I do not have sufficient results to complete the experiment.
- Five different strengths of solution will be used. These are the solutions at strength 5%, 10%, 15%, 20% and 25%, which are all made by adding glucose to distilled water at different strengths.
- A sharp knife so that I can cut the potato accurately to the wanted lengths. The results could be totally affected by the tiny change in length due to a blunt knife.
- An accurate ruler measuring in mm. With such small and detailed measurements involved we need an accurate and completely straight ruler so that the results will not be affected.
- A measuring beaker so that we can accurately measure out the solution and make sure it is perfect so that it doesn’t change the percentages and mess with the results.
Highlighted in red are any anomalous results that stood out and they are not going to be included in the calculation of the average. The results highlighted in orange are the corresponding start masses with the end masses that are anomalies. These were not included in finding the average too. This was done to maintain that my experiment is as accurate as possible, knowing that these particular results looked very odd and out of place.
From looking at my table of results I can see that, as I predicted, the direction and rate of osmosis depended entirely on the strength of concentration. When there was 0%, the potato had the less concentration, so it absorbed water molecules from the solution; therefore, the potato gained more weight. This describes the first change in mass. When 5% concentration was added to the distilled water, there was a smaller increase in change because before osmosis had happened we had added sugar to the solution, making it a smaller difference in concentration between the two substances. However there is still an increase of 0.12 grams.
When there is a concentration of 10% added in the solution this is where you start to see the biggest changes, this time with decreases. There is now a higher concentration in the potato so the potato shrinks in size and loses weight, by transferring the water molecules into the solution, through the partially permeable membrane. When 10% is added the potato loses 0.12 grams. From then on the changes get bigger because we are then just adding to the concentration and making the difference between the two substances even bigger. This means that osmosis has to work at a faster rate or osmosis will have to take place a lot more and therefore, there will be a bigger loss in weight. This is shown as we get a decrease of 0.27 grams with 15%, followed by a decrease of 0.38 grams with 20% concentration and finally the biggest being a 0.41 gram decrease with 25% concentration added in.
The concentration of sugar solution and the average percentage increase are inversely proportional. This means that as the concentration of sugar solution goes up, the average percentage increase goes down. You can see this clearly in my graph:
Looking at my graph I can clearly say that it justifies my prediction about osmosis in our experiment. Like I stated before, it follows a negative correlation and this is what I said in my prediction too; In my prediction I said that the larger the difference in concentration between the two substances, the faster the rate of osmosis or the more the osmosis will take place. This is true with the experiment because when the concentrations got bigger, this added a bigger difference between the two substances and therefore there was a bigger change in mass to transfer the difference in water to the other substance. On the graph you can see that the bigger the concentration, the bigger the decrease in mass, as it is quite proportionally going down in a negative correlation.
Also in my graph you can clearly see the difference between 0% concentration and a 25% concentration sugar solution, when added to the distilled water. When it has 0% there is a 0.15-gram increase, but when there is 25% sugar concentration in the solution there is a 0.41-gram decrease. This means that there is a 0.56-gram difference in the increase and decrease put together, which shows the difference between the two.
