In the preliminary investigation I will be testing four different concentrations. The sugar solution is to be made up by dissolving a certain mass of sugar in water and the solution made up to 100ml. Firstly a ‘stock solution is made up by dissolving 10g of white ‘granulated’ sugar. This will be labelled as our 100% sugar or most concentrated solution. The four concentrations to be tested are; These are pure water (this is the 100% water), the 25% sugar solution which will be (25% sugar i.e. 2.5ml of our stock sugar solution and 75% or 7.5ml of water), 50% sugar solution (this is 50% i.e. 5ml sugar from our stock sugar solution and 50% or 5ml of water) and 100% sugar solution (this is 100% sugar i.e. 10ml of our stock sugar solution and 0% or 0ml of water). I will then collect the apparatus and set them up as shown in the diagram on the next page.
To begin my experiment, I will use the cork borer to make up the same size chips from the potato. After this, I will weigh each potato chip on a scale so that each is of similar size and weight. I will then add a potato chip to a particular amount of concentration of sugar solution and water in the test tube. To pour the sugar solution and water into the test tube, I will be using a 10ml measuring cylinder. I will make sure that the chip is of particular temperature, using the thermometer, so that my results are more accurate. I will use a clock to determine the six hours after the potato chip was placed in the test tube of sugar solution and water. I will then take the potato chip out of the test tube and remove excess water with tissue paper and then the test tubes will need to be rinsed out with distilled water and dried before being reused. After this, I will repeat the procedure with different concentrations of sugar solutions.
In the preliminary investigation I will collect eight pieces of data. The experiment will be carried out at four different concentrations of sugar solution and each experiment will be repeated to get an average result. This will increase the reliability of the results. The range of concentrations as mentioned before will be; 0%, 25%, 50% and 100% sugar solutions. The data will be as accurate as possible by ensuring a fair test is carried out. This will be done by ensuring that the following actions are taken;
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The concentration of the sugar solution will be kept constant – this will be done by making a sample stock of sugar solution of a known concentration which will then be used in all the experiments.
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The volume of the sugar solution will need to be kept constant – this will be done by making use of the measuring cylinder so the sugar solution is accurately measured out and poured into the test tube for each experiment.
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The concentration of the potato chips will need to be kept constant – this will be done by using similar types of potato chips.
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The Temperature of the solution mixture will also be kept constant. This will be done by carrying out the experiment in the same environment and carried out at the same time – I will measure the temperature of the reaction mixture using a thermometer to ensure it stays the same.
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The Dimensions/cross-sectional area of potato chips will also be kept constant by using potato chips of same length and width and using the same size borer to make all the potato chips.
Changes to be made:
While doing the investigation I came across a few problems. These were minor problems more to do with carrying out the practical. For instance I found it difficult pouring liquids into the measuring cylinders so needed to make use of a funnel. When I first carried out the experiment the balance that we used was not fit for purpose it was not sensitive enough as it only measured in grams to the nearest 0.5 grammes which we initially thought would be fine but when it came to weighing the potato chips the masses of which were 5 or 6 grams so it was not sensitive enough. To overcome this problem I invested in a more sensitive balance which measured masses in grams and this gave us increased precision up to 0.01grams. What I also realised was that I had to be quick in getting the chip with appropriate dimensions and placing it in the test-tube as moisture was quickly being lost from the chip to the surroundings. Overall the preliminary experiment worked according to the plan and the results obtained were also in line with what I expected.
Method for Main Experiment
Now that the preliminary investigation has been carried out I am in a position to proceed with my main investigation looking at the effect of altering the concentration of sugar solution on osmosis in potato chips. I am now in a position where I am familiar with the apparatus, the chemicals and the practical method. More importantly I now know what a suitable range of concentrations of sugar solution to use in the investigation.
In the main investigation I will be testing six different concentrations of sugar solution. The sugar solution is to be made up by dissolving a certain mass of sugar in water and the solution made up to 100ml. Firstly a ‘stock solution is made up by dissolving 10g of white ‘granulated’ sugar. This will be labelled as our 100% sugar or most concentrated solution. There are six concentrations to be tested; These are pure water (this is the 100% water), the 20% sugar solution which will be (20% sugar i.e. 2ml of our stock sugar solution and 80% or 8ml of water), 40% sugar solution (this is 40% sugar i.e. 4ml sugar from our stock sugar solution and 60% or 6ml of water), 60% sugar solution (this is 60% sugar i.e. 6ml sugar from our stock sugar solution and 40% or 4ml of water), 80% sugar solution (this is 80% sugar i.e. 8ml sugar from our stock sugar solution and 20% or 2ml of water) and 100% sugar solution (this is 100% sugar i.e. 10ml of our stock sugar solution and 0% or 0ml of water). I will then collect the apparatus and set them up as shown in the diagram on the next page.
To begin my experiment, I will use the cork borer to make up the same size chips from the potato. After this, I will weigh each potato chip on a scale so that each is not just of a similar size but also of similar mass. I will then add a potato chip to 10ml of the appropriate sugar concentration solution in the test tube. To pour the sugar solution and water into the test tube, I will be using a 10ml measuring cylinder. I will use a clock to determine the six hours after the potato chip was placed in the test tube of sugar solution and water. I will then take the potato chip out of the test tube and remove excess water with tissue paper and then the potato chip will be placed on the balance again to reweigh it. The final mass will be recorded and then I would work out the change in mass and a % change in mass. The test tubes will need to be rinsed out with distilled water and dried before being reused. After this, I will repeat the procedure with different concentrations of sugar solutions.
In the main investigation I will collect twelve pieces of data. The experiment will be carried out at six different concentrations of sugar solution and each experiment will be repeated to get an average result. This will increase the reliability of the results. The range of concentrations as mentioned before will be; 0%, 20%, 40%, 60%, 80% and 100% sugar solutions. The data will be as accurate as possible by ensuring a fair test is carried out. This will be done by ensuring that the following actions are taken;
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The concentration of the sugar solution will be kept constant – this will be done by making a sample stock of sugar solution of a known concentration which will then be used in all the experiments.
-
The volume of the sugar solution will need to be kept constant – this will be done by making use of the measuring cylinder so the sugar solution is accurately measured out and poured into the test tube for each experiment.
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The concentration of the potato chips will need to be kept constant – this will be done by using similar types of potato chips, both in terms of brand of potato, size and dimensions.
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The Temperature of the solution mixture will also be kept constant. This will be done by carrying out the experiment in the same environment and carried out at the same time – I will measure the temperature of the reaction mixture using a thermometer to ensure it stays the same.
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The Dimensions/cross-sectional area of potato chips will also be kept constant by using potato chips of same length and width and using the same size borer to make all the potato chips.
Each experiment will be repeated so we can work out an average and thus increase the reliability of the method. We will be repeating the experiment three times to increase the reliability of the method.
Once I have collated all my results in a results table I will begin to analyse them by firstly working out the % mass change in the potato chips, this will be done by using the following formula;
I will then work out the average of the % change in mass and plot this on a graph to see any trends/patterns and do a complete analysis.
Y
% concentration of sugar solution
Variables Description: Dependant Variables
The dependant variable is plotted on the y-axis. In this experiment this is the average % change in mass of the potato when it is placed in different concentrations of sugar solution.
Variables Description: Independent Variables
The independent variable is plotted on the x axis and is what I change in the experiment. In this experiment it is the % concentration of the sugar solution.
Glossary
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Conclusion
Concentration is a measure of the number of particles of a particular solute in a given volume of solvent. A dilute solution is likely to contain fewer molecules of the solute and more water molecules. A concentrated solution is likely to contain more solute molecules and fewer water molecules in the same total volume of solution. In this investigation I altered the concentration of the HCL solution. By having different concentrations of HCL solution and keeping the other factors constant I could see the effect of altering the concentration on the rate of the reaction.
As you can see from the results table and the graphs plotted that as the concentration of the HCL solution is increased the time taken for the cross to disappear increases. This trend can clearly be seen in graph A for instance when the concentration was 25% HCL solution it took 63 seconds for the cross to disappear and when the concentration of the HCL solution is increased to 70% HCL solution the time taken for the cross to disappear was 33.8 seconds. This decrease in time taken for the cross to disappear was in line with what I expected.
With regards to the rate of reaction; graph B clearly shows that as the concentration of the HCL solution is increased the rate of reaction decreases. When the concentration of the HCL solution was 25% the rate of the reaction was 0.0160 however when the concentration of the HCL solution is increased to 70% the rate of the reaction was much faster at 0.0294. This again was in line with my expectations and scientific thinking according to the collision theory as we know that all particles have some kinetic energy and are thus able to move about and collide with other particles and thus lead to a chemical reaction. As my results show in a dilute HCL solution there will be fewer particles of HCL and more water molecules thus the chances of a collision between sodium thiosulphate particles and HCL particles will be less likely and thus we get a longer time taken for the cross to disappear and a reduced rate of reaction. On the other hand when the concentration of the HCL solution is increased there will be more particles of HCL and fewer water molecules thus the chances of a collision between sodium thiosulphate particles and HCL particles will be more likely and thus we get a shorter time taken for the cross to disappear and a faster rate of reaction. This can also be shown in a diagram as is shown below.
All particles have some kinetic energy and are thus able to move about and collide with other particles. In a dilute sugar solution there will be fewer particles of sugar and more water molecules thus the chances of a collision between water particles and sugar particles will be less likely and so we get a reduced rate of reaction.
The consequence of Osmosis; what happens to plant cells:
Plant cells always have a strong cell wall surrounding them. When they take up water by osmosis they start to swell, but the cell wall prevents them from bursting. Plant cells become ‘turgid’ when they are put in dilute solutions. Turgid means swollen and hard. The pressure inside the cell rises; eventually the internal pressure of the cell is so high that no more water can enter the cell. This liquid or hydrostatic pressure works against osmosis. Turgidity is very important to plants because this is what makes the green parts of the plant ‘stand up’ in the sunlight.
When plants cells are placed in concentrated sugar solutions they lose water by osmosis and they become ‘flaccid’; this is the exact opposite of ‘turgid’. If you put plant cells into concentrated sugar solutions and look at them under a microscope you would see that the contents of the cells have shrunk and pulled away from the cell wall: they are said to be plasmolysed.
Pure water has a water potential of zero. This is the highest water potential value. All other solutions will have a lower water potential or a more negative water potential. A more concentrated sugar solution will have a lower and more negative water potential. Potato chips contain amongst other things sugar molecules and water. The water potential of the potato chips will be low; it will have a negative value. If the potato chip was to be placed in a pure water solution, then I would expect the potato chip to gain mass.
MASS LOSS (concentrated sugar solution)
Potato chip
MASS INCREASE (dilute sugar solution)
Potato chip
The pure water solution will have a higher water potential but the concentrated water solution will have lower water potential. I would expect that the water molecules to move into the potato chip down a concentration gradient and this means that the potato chip will gain mass. The concentrated water solution will have a lower water potential and the water molecules would move away from the potato chip, down the concentration gradient, and this means that the potato chip will lose mass.
Initially I also made a quantitative prediction stating that if the HCL concentration is doubled the rate of reaction will also double and the time taken will be halved. For example, when the concentration was 50% the time taken was 40 seconds. However, when the concentration was 100% the time taken was 25 seconds.
Calculations
50% concentration = 40.00 seconds
100% concentration = 25.00 seconds
If the quantitative prediction was correct, then time taken should have doubled from 100% concentration to 50% concentration. However, my measurements show that from 100% concentration to 50% concentration, the time taken was increased by a factor of 1.6. This is not completely accurate as the time taken should have increased by a factor of 2.0. The results obviously strongly agree with the qualitative prediction I made and follow the expected trend with regards to the quantitative prediction but do not agree completely with it and the reasons for this will need to be looked at.
Evaluation
The aim of this investigation was to find out the effect of altering the concentration of HCL solution on the rate of reaction between the HCL solution and the sodium thiosulphate solution. The method that I selected was one that was relatively straightforward to set up and carry out in the home. It was also one that allowed me to keep all other factors constant except the concentration of HCL(the independent variable) and one in which I was able to measure the outcome; time taken for the cross to become completely obscured (dependent variable). To make it a fair test I made sure that all the variables were kept constant, except the concentration of HCL (the independent variable). To ensure the test was reliable, I repeated the experiment three times. It is clear that my results agree with my prediction, which was, by increasing the concentration of the HCL solution the rate of reaction will increase. My results are also in line with scientific theory. My results show a clear pattern, which are, as the concentration of HCL increases, the time taken decreases. The rate graph also depicts a clear pattern, which is, as the concentration increases, the rate of the reaction also increases. I drew a line of best fit for my concentration graph which follows the same pattern. I also drew a line of best fit for my rate graph which follows the same pattern, except one. The figure obtained at 60% concentration was slightly higher than what we expected, according to the line of best fit. There could be a number of reasons for the slightly anomalous result. One of the reasons could be due to human error. The concentration of the HCL might not have been accurate enough or the time taken for the cross to disappear may not have been entirely precise. Even though this result was an outlier, from the other results, which show an obvious pattern, we can still draw a strong valid conclusion that as the concentration of HCL increases the rate of reaction also increases. Since I repeated the experiment three times and had a wide range from zero to one hundred, we can draw a valid conclusion. Another reason why I am confident in my conclusion is because most of the results had little differences between our repeat measurements so the results were sufficiently reliable. Overall, our method was reliable and easy to carry out. The only weakness in our procedure was the time taken for the cross to be obscured as this relied on human perception which may not always be reliable. Precision is another area in which there is room for improvement, as my method relied upon judging by eye rather than using specialised instrumentation. Precision with regards to the equipment used was acceptable as I used the appropriate measuring cylinders which measured volumes to the nearest millimetre and the stopwatch to the nearest 100th of a second which again was acceptable.
Having evaluated the procedure I carried out and the results that I obtained I believe that my conclusions are valid and in line with scientific theory and I have explained the conclusions obtained with scientific reasoning. I have identified the shortfalls in the method, reliability and precision. There is always room for improvement and I will now discuss how the method could be improved and how I would change things if I was to do the experiment again. Firstly a larger range of concentrations could have been tested with smaller increments i.e. concentration going up in increments of 10% increase rather than the 20% increase in concentration. To increase the reliability the experiment could have been done with more repeats rather than just testing each concentration three times we could have repeated each one five times to obtain a more reliable average. Arguably the most significant weakness in the whole method was the use of the human eye to detect when the cross was completely obscured. This is not a subjective way of doing things and human error creeps in. To overcome this shortfall I suggest making use of ICT. I think that a data-logger can be connected to a light sensor and this would record the change in light intensity as the reaction proceeds and produce real time graphs. This method then removes the human error and is objective. A further advantage is that real time graphs are produced and this means if there is an error or an anomalous result it can be detected straight away discarded and the experiment carried out again which further improves the accuracy and reliability of the results. Below is a diagram which shows how the data-logger could be connected in this experiment.
Prediction
I predict that in a dilute sugar solution, the potato chip will gain mass. This is because, there will be more water particles around the outside of the potato chip so the water particles will be moving in, thus expanding the potato chip. However, in a more concentrated sugar solution the potato chip will shrink. This is because there will be less water particles around the outside of the potato chip and so the water particles will be moving out, hence shrinking the potato chip. Overall, as the concentration increases, I will expect the mass of the potato chip to decrease.
Glossary
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