In order to carry out a correct investigation and get accurate results, I will carry out a preliminary experiment first. This should also help me back up my prediction and plan the investigation properly. In this experiment, I will use a salt solution of 1.0 molar and pure water. I will use 75.0cm³ of each substance as I feel this will cover the potato chip fully. The length of the potato chip will be 3.0cm and I will put two of these in each solution, because this will give me an average result, which will be more accurate than one result. I will weigh the potato chips too, so that when they are taken out of the solutions, I will be able to see if there has been a change in the mass or not. The potato chips will be left in the two solutions for half an hour and this should give a clear result. The diagram below shows how I will carry out this experiment.
The following table shows the results from this experiment:
The table shows that the salt solution of 1 molar caused the potato chip to increase in length and mass. This means that the salt solution had a higher concentration than the potato chip, as the solution moved from an area of higher concentration to an area of lower concentration. The water caused the potato chip to decrease in length and mass. This proves that the potato had a higher concentration than the water surrounding it.
Now that I have done some preliminary work to show me how to conduct the main experiment, I have decided that I will increase the length of the potato chip to 4 cm as I feel it would be easier to record results from this. In this experiment I used two potato chips in each solution, this time I will put three in each solution, as it will give a better average. I will use the molar solutions of 0.5m, 1.0m, 1.5m, 2.0m and 2.5m. I will use the same amount of solution in each, 75cm³, as this covers the whole chip and is a sufficient volume. In the main experiment I will only look at the varying mass, as it will be easier to concentrate on just one and the mass will be easier to look at.
Fair test:
To make sure this experiment is a fair test, I will be using the same amount of salt solution for each molar throughout the experiment. This will be 75cm³. By varying the amounts of solution, I hope to get a varied set of results from which I will be able to make a reasonable conclusion. The concentration of the salt solution is the only variable. I will have to keep the length of the potato the same, as it is a non-variable. The other non-variables are the size of the beaker, the number of chips in a beaker, the length of time the potato is left in the solution, the temperature, the measuring cylinder and the type of potato.
The key factors that affect the mass are:
- The temperature that osmosis takes place in. If there is a higher temperature, there will be a higher rate of osmosis. Therefore I will keep the temperature the same as the room.
- The time that the potato chips are left in the solutions. To make sure this is the same for all the chips, I will place them in the beakers at the same time and take them out at the same time. I will keep a record of when they go in and when they are taken out.
- The type of potato. To keep this experiment a fair test I will make sure that all the potatoes are of the same type. They will all have to have salt solution in them, as this is what I will be using in the experiment. I will also have to make sure the length and surface area of the potato are kept the same. This will be done with the use of a potato borer. To keep the potatoes the same length, I will use the same ruler.
- The type and size of the container. As I will be using five different solutions, I will need the same size beakers to fit three potato chips each of the same size.
- The type of solution that I use. A sugar solution would give different results to a salt solution. Therefore as I am using potatoes with salt solution in them, I will be using a salt solution too.
Equipment:
- A large potato
- A measuring cylinder
- A ruler
- Weighing scales
- 5 beakers
- A potato borer
- Salt solution- 0.5m, 1.0m, 1.5m, 2.0m, 2.5m
- Stop clock
- Knife
- White cutting tile
- Paper towels
This is a diagram of the experiment I am carrying out.
Safety:
To keep myself safe during this experiment, I will make sure that the area I am working in is clean and safe, that there are not bags on the tables or on the floor which could cause an accident. When using the knife to cut potatoes, I will keep my fingers away from the blade. After I have used the solutions, I will wash my hands so that any excess solution will be washed away.
Plan:
- Collect equipment and place in a safe and clear working area.
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Measure 75cm³ of each solution using the measuring cylinder (0.5m, 1.0m, 1.5m, 2.0m, 2.5m) and pour into the beakers.
- Use the potato borer to cut out 15 potato chips. Cut them to 4cm each using a knife, ruler and white tile.
- Weigh each potato chip with the same weighing scales. Record the results.
- Place three potato chips in each of the five solutions at the same time and record the time. By placing three chips in each of the five solutions, I can carry out three experiments at the same time, which will save time.
- Leave in the concentrations for roughly 24hours, but to have the exact time, record the time to the minute when the potatoes are placed in and taken out.
- When potatoes are taken out of the solutions record the time and work out how long they have been in the solutions for.
- Dry the potatoes slightly with a paper towel and weigh each one again. Record the results and work out the average mass for each solution.
Table of results:
This table of results shows the mass for each potato and the average mass for each solution. I have worked out the average mass in the following way:
Average mass = mass 1 + mass 2 + mass 3
3
I have worked out the difference in the average mass and original mass by doing the following:
Difference = average mass – original mass
In the table of results I have also included the percentage change. This has been worked out using the following method:
(%)Percentage change = average mass – original mass
Original mass x 100
The total amount of time the potatoes were left in their solutions was 25 hours and 10 minutes. They were placed in at 12:32 and taken out at 13:42 the next day.
The average mass can be seen on the graph on the next page. I have also drawn a graph to show the percentage change, which can be seen on the page after.
Conclusion:
The main prediction for this experiment was that a higher concentration solution would move to an area of low concentration of solution. This prediction has been proved correct as the higher the concentration of salt solution, the smaller the average mass of the potato. For example, when the concentration of the solution was 1.0 molar, the average mass of the potato increased to 1.17g. This means that there was a higher concentration in the solution and this moved into the potato as it had a power concentration. This proves that osmosis was occurring in the potato chip and solution. At 2.5 molar, the mass of the potato chip decreased to and average of 0.92 g. This means that the concentration of the potato chip was higher than the concentration of the salt solution. Now that I know this I can see that I have proved another prediction correct which was “ At a higher concentration of salt solution, the potato will decrease in size as the salt solution in it will move into the salt solution surrounding it”
I have drawn a graph, which shows the average mass against the concentration of the salt solution. I have drawn a line of best fit, which shows that the mass is inversely proportional to the concentration of the salt solution. I have circled a point on the graph, which I feel is anomalous as it is the furthest away from the line of best fit. This point is at (2.5, 0.92). This is the only result, which decreased in mass, which means that between 2.0 and 2.5 there should be an isotonic point, where the concentration in the potato and salt solution are exactly the same.
To find this isotonic point, I have drawn a graph showing the percentage change. The line of best fit on this graph crosses the x-axis at 2.24molar. This means that the isotonic point of the concentration of the salt solution and potato chip are equal at 2.24molar. The GCSE Science book says “Water will continue to move into an area of lower mater concentration until the two concentrations are equal (and equilibrium is reached)”.
In this experiment, equilibrium is reached at 2.24 molar. This is where no osmosis is taking place. The concentration in the potato is not equal to the concentration in the salt solution at any point above or below 2.24 molar. This proves that any concentration after 2.24 molar should decrease the size of the potato chip and this is true, as at 2.5 molar, the percentage change was –8%. However, I feel that this result at point (2.5, -8) is anomalous as it is still the furthest away from the line of best fit. From this graph I now know that the concentration of the potato will be higher when the concentration of the salt solution is above 2.24 molar. Any solutions below 2.24 molar will increase the mass of the potato; therefore will have a higher concentration than the potato.
Evaluation:
I feel that this experiment has gone very well. I have obtained a good set of results from which I have drawn two detailed graphs. The time that I left the potato chips in the salt solution for was sufficient as clear results were obtained from the experiment. The aim for this investigation was to investigate into osmosis using a peeled potato and different concentrations of salt solution. I have done this as I have results showing what happens to the potato chip when it is placed in a concentration of salt solution.
The evidence obtained was generally very accurate, as I could see a distinctive pattern in my results. However, as I explained in the conclusion, I do have an anomalous result. This is when the concentration of the salt solution was 2.5 molar. It was the only concentration that caused the potato to decrease in mass and was not as close to the line of best fit as the other results. This result may have been anomalous because of the room temperature. The temperature in the room could have varied and therefore affecting the result. Another reason for this anomaly could be because of human error. A mistake when measuring the potato could have occurred. The result could have been a mechanical error too, as when the potatoes were weighed; extra weight could have accidentally got onto the scales. The other results obtained were fairly accurate as I weighed the potatoes correct to 2 decimal places.
If I were to carry out his experiment again, the main changes I would make would be to use a varied range of salt concentrations. For example 0.25m, 0.50m, 0.75m, 1.00m, 1.25m etc. This would give me accurate results and I would be able to find the isotonic point more accurately too. I would also use better equipment, such as a computerised weighing scale, which would give more accurate results. Instead of results 0.0g I would have results 0.000g.
I would also try to find a machine that would cut the potatoes into exactly the same size chips. This would save time and make the experiment very accurate. I found that cutting the potatoes was the most time consuming. Another change I would make is to experiment with potatoes of a different type. These would have a different concentration to the potatoes used in this experiment and therefore would give me different results which I would be able to compare with the results from this experiment. Another variable I could investigate into would be the amount of solution used. Instead of using 75cm³, I would use more, such as 100cm³. This would tell me whether the amount of concentration used in the experiment varied the results or not.
However, I am pleased with the set of results that I obtained and feel that this experiment has gone well.