Fair Test:
To make the experiment a fair test, I will make the hydrogen peroxide the independent variable. I will do this by changing its concentration throughout the experiment. I have to make this the independent variable because if it was the same all the way through, the amount of oxygen produced would stay the same. My dependent variable will be the amount of oxygen that is produced, and I will measure the volume of this each time.
I will also make my experiment a fair test by controlling variables to stay the same throughout the experiment, such as the volume of the hydrogen peroxide solution (substrate), the surface area of potato used and maybe the temperature/pH value of the solution. If I didn’t control these variables, then my experiment wouldn’t be fair, as it would result in different amounts of oxygen being able to be produced each time. I need to control the surface area of the potato because the larger the surface area, the more enzymes on the surface, therefore the reaction would be faster and this would be unfair. I need to control the temperature because enzymes are proteins which are damaged at temperatures above 40 C, and become denatured. I could control the temperature by using a water bath, with the temperature set at a constant 37 C. For similar reasons, I need to try and control the pH of the substrate, because enzymes can also be denatured or damaged by very acidic or very alkaline conditions, so a pH of around 7 should be adequate. I can keep the pH as neutral as possible by frequently checking the pH using Universal Indicator.
In my main experiment, not my preliminary, I will repeat each separate experiment three times, then work out an average amount of oxygen produced for each concentration. This should hopefully make it fairer and stop anomalous results from being a problem. Also, in the main experiment, I will start off with 50ml of water in the burette each time.
Preliminary Experiments:
I am going to do a few preliminary experiments to find out the measurements of my controlled variables, and to see which amounts work the best. The first experiment will use 2cm of potato, cut with potato borer 6. I will also use a 100% concentration of hydrogen peroxide, using 25ml. If the amount of oxygen produced is too small to work with, then my next preliminary experiment will use a larger amount of potato. If the amount of oxygen is too large to work with, then my next preliminary experiment will use a smaller amount of potato. For each experiment I do, I have already decided to measure and record the volume of oxygen produced every minute for five minutes. This is another controlled variable that will not change.
Preliminary Results on Next Page…
Preliminary Experiment Results:
Test 1
100% concentration of hydrogen peroxide (25ml) and 2cm potato (borer number 6).
TOTAL OXYGEN PRODUCED = 5.7cm
This experiment produced a measurement of oxygen too small for me to work with. In my next experiment, I will increase the amount of potato I use in order to produce a larger amount of oxygen in five minutes.
Test 2
100% concentration of hydrogen peroxide (25ml) and 4cm of potato (borer number 6).
TOTAL OXYGEN PRODUCED = 17.4cm
This experiment produced a much better amount of oxygen, which will be easier for me to work with. I will use these measurements for my experiment.
Apparatus:
Large beaker
Burette
Small funnel
Delivery tube and bung
Boiling tube
Conical flask
Stand
Boss
Clamp
Stopwatch
Measuring cylinder
Scalpel
Potato borer number 6.
Diagram:
Method:
Firstly, I shall get out all the necessary equipment and set it all up as in the diagram. I shall put on my safety goggles. I will put exactly 50ml water in the burette. I shall then cut my potato to the correct size – 4cm with potato borer 6. This will go in to the test tube, which I will put upright in the conical flask. The conical flask is only there to help keep the tube upright during the experiment. Hopefully the temperature will stay roughly the same throughout the experiment, to make it a fairer test. The room temperature should stay the same.
When the apparatus is set up, I will measure out 25ml of hydrogen peroxide in to the measuring cylinder, for the first of the four main experiments I will do. When I’m sure that the experiment is ready to begin, I will put the hydrogen peroxide solution in to the tube with the potato, fit the bung and delivery tube on to it and immediately start the stopwatch. The other end of the delivery tube must be underneath the funnel and burette, to make sure any gas produced can be recorded. I will measure the volume of the water in the burette every minute for the next five minutes, recording each measurement. Measuring the volume of water in the burette as it decreases will be an indication of how much oxygen is being produced. All measurements made will be as precise as possible, to keep the experiment accurate and fair.
The first experiment is using a 100% concentration of hydrogen peroxide solution, which is exactly 25ml of hydrogen peroxide. The second uses a 60% concentration, which is 15ml hydrogen peroxide and 10ml water. The third uses a 40% concentration, which is 10ml hydrogen peroxide and 15ml water. The last one is 20%, which is 5ml hydrogen peroxide and 20ml water. After each experiment, I will refill the burette up to 50ml again, cut another identical cylinder of potato, measure out another 25ml of the hydrogen peroxide solution and repeat the process. Each concentration will be repeated three times to ensure I get reliable results, then an average for each concentration will be worked out.
I will try and make sure this experiment is carried out with safety and fairness. I will wear safety goggles constantly throughout the experiment, as hydrogen peroxide may be dangerous if it gets into your eyes. Coats, bags and books will be out of the way to reduce accidents, and I will remain standing throughout. I will make sure all the equipment is secured safely in place before beginning the experiment. The potato will be cut carefully using a scalpel and on a ceramic tile to prevent any cutting.
RESULTS
Table to show the results for the 100% concentration experiment:
Table to show the results for the 60% concentration experiment:
Table to show the results for the 40% concentration experiment:
Table to show the results for the 20% concentration experiment:
ANALYSIS
Table to show the Average Volume of Oxygen produced per Concentration:
This table of average results shows that as the concentration of the substrate got weaker, less oxygen was produced and the rate of reaction slowed down considerably. This confirms what I said in my prediction, because as the concentration of hydrogen peroxide was weakened, there were fewer hydrogen peroxide molecules for the catalyse to break down, therefore less hydrogen peroxide was broken down and less oxygen was produced.
My prediction that the 100% concentration would produce the most oxygen in the shortest time was correct, and I was also correct in thinking that 60% would be next, then 40% and then 20%. There is a pattern to the results, as the rate of reaction decreases when the concentration does.
My results support my original predictions. The higher the concentration of substrate, the quicker the rate of reaction because there are more molecules for the enzyme to break down. As a result of this more oxygen can be produced, and my results, averages and graph show this. The graph also supports this, with the total average volume of oxygen increasing along with the concentration of the substrate.
When the concentration of the substrate doubles, i.e. from 20% to 40%, I expected the amount of oxygen to exactly double with it. In one of the experiments, this did happen. The amount of oxygen in the fifth minute was 0.9cm for the 20% and 1.8cm for the 40%, which shows a clear double in the results. I have highlighted this in light blue. This pattern was not, however, repeated for the other results as I expected.
EVALUATION
The results from my experiment were generally accurate. Some of the averages could have been more precise, as I only rounded to 1 decimal place when needed, and I could have rounded to 2 decimal places to make them more precise. However, they were precise enough to support my prediction and investigate the effect of substrate concentration on an enzyme catalysed reaction.
My experiment was quite reliable, because I didn’t get many anomalous results. Apart from the anomalous results highlighted in one of the tables, I didn’t get any particular outstanding ones. I have highlighted a whole column in red. The results I have highlighted as being anomalous are anomalous because they produce less oxygen each time than the other two repeats after it, but thankfully the averages pulled them up, and they were unnoticeable in my averages table, and my results were reliable. My results may have been unreliable due to a number of reasons. The column that I highlighted in blue may have been too low because gas escaped from the equipment on the particular repeat, giving unfair and unreliable results. I could improve my experiment by maybe using Vaseline in and around the gaps to prevent gas leakage and therefore help prevent unreliable results. Also, the results may have been too low due to inaccurate temperature or pH. Had I investigated temperature and pH, the anomalous results could have been avoided. Maybe the low results were due to my clumsy setting up of the experiment, for example, I may have been careless and inaccurate during the process of adding the substrate concentration to the potato, fitting the bung and then starting the stopwatch all at the same time.
Overall, the experiment was a good one to achieve my aim, basically because I achieved my aim by doing it. My aim was to find out the effect of a substrate concentration on the amount of oxygen produced. I have found it out successfully, with only a few minor anomalous flaws.
If I could do the experiment again, I would firstly change the concentrations. The concentrations I used were 100%, 60%, 40% and 20%. The gap between 100% and 60% was too large, and is very noticeable in the graph. The gap between 40% and 60% turned out to be very close, and in a way these results may be seen as anomalous because they were too similar to give precise results. If I did it again, I would use 100%, 75%, 50% and 25% because they are equally spaced and would probably produce more reliable results. Also, when rounding up the averages, I would round them up more accurately to 2 decimal places rather than one.
Other investigations that could be done could include investigating how the temperature of the substrate would affect the volume of oxygen produced, and also the pH value of the substrate. I couldn’t properly measure the temperature this time, because we did not have the proper facilities available. I just hoped that the substrate would stay at roughly room temperature, assuming that the room temperature was acceptable. I could have kept checking the pH of the substrate, by using universal indicator to measure it, but I didn’t. This could have indicated a pH that was too high, and helped me to eliminate possible anomalous results. Another variable I could have investigated is the number of enzymes. Had I changed this, I would have expected the rate of reaction to differ greatly, as well as the volume of gas produced. Investigating these would provide more evidence for the conclusion. If any of the above had been investigated, I think it would have made my experiment much more accurate and precise. My results would probably have been more reliable, and the risk of anomalous results would have been greatly reduced.
Frances Duffy 10H1
