Investigating how increasing the amount of potato mixed with a fixed amount of hydrogen peroxide affects the volume of froth produced

Aim: To investigate how increasing weights of potato affect the volume/ml² of froth produced over a fixed period of time, when mixed with a fixed amount of hydrogen peroxide.

Prediction: There should be more ml2 of froth in direct proportion to the increase in the amount of potato (i.e. if there is a greater weight of potato, the amount of froth ml² should also increase) as this will show that the increased concentration of potato has reacted with the hydrogen peroxide to generate a proportionate increase in the amount of froth or gas given off by the reaction.

Hydrogen peroxide is a chemical compound of hydrogen and oxygen (H2O2) and is used as an oxidising or reducing agent in many industrial processes. This is because the extra oxygen molecule (compared to water – H2O, which is often used to dilute hydrogen peroxide to make it less reactive and therefore safer to use) is highly reactive when it comes into contact with other substances. Because it is so reactive, pure or undiluted (anhydrous) hydrogen peroxide is also used as a source of oxygen in the fuel mixture for rockets.

When hydrogen peroxide is mixed with potato a froth is produced, which indicates that a chemical reaction is taking place that is generating a gas as one of the products of the reaction.

A chemical reaction occurs when reacting particles collide with each other with sufficient energy. This can be affected by a number of different factors.

The speed or rate of a chemical reaction should increase if the temperature goes up because this will cause the speed of the reacting particles to be increased, making it more likely that they will collide with sufficient energy to react. If there is a greater concentration of any of the reacting particles the chances of a collision will also be increased, and if solid reactants are in smaller pieces (so that they have greater surface area) the same applies. A catalyst is a substance that can be added to a mixture to increase the rate of a chemical reaction, but without being used up by the reaction. Our experiment does not use a catalyst, but all of the other factors (temperature, concentration and size of solid reacting particles) could have an effect on the result.

In carrying out this experiment we will be increasing the concentration of one of the reactants (the potato) to see how this affects the rate of the reaction. It will therefore be important to make sure that the only element that changes is the amount of potato that is being used, to avoid the results being affected by any of the other factors that might change the rate of the reaction and therefore the amount of froth produced in the fixed time we have chosen.

Apparatus: three pipettes, five large measuring cylinders, 150g of potato, 100ml of ...

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Apparatus: three pipettes, five large measuring cylinders, 150g of potato, 100ml of hydrogen peroxide, a stopwatch, weighing scales and a knife.

Method: First cut the potato into five separate weights (13g, 11g, 9g, 7g and 5g). Then mash each amount of potato separately and place each separate weight of potato into a measuring cylinder and add 6ml2 to each separate cylinder. Then time two minutes for the reaction to take place. After two minutes, measure the froth that has been produced.

We tried to make sure the test was fair by making sure we carefully cut up and mashed the potato pieces separately, We mashed the potato (instead of just cutting the potato up) to make sure that there was an equal surface area per gram of potato for the hydrogen peroxide to come into contact with for each different weight of potato. We used the same bottle of hydrogen peroxide throughout the experiment as other bottles of hydrogen peroxides may have had slightly different concentrations. We used the same pipettes as others may have different measurements and used the same weighing scales as others may have different forms of calibration. We used identical measuring cylinders to make it easier to take comparative measurements.

Before testing with 6ml2 of hydrogen peroxide, we carried out a preliminary test with 4ml2. The preliminary test gave expected results except for one. My prediction was that as the weight of potato increased, the ml² of froth produced would be greater. This expectation was met in the results apart from the measurement taken with 5g of potato, which gave us froth measuring 49ml². This result must have been incorrect as the measurement with 7g of potato had only produced 47.5ml² of froth. I think that the preliminary test produced an unclear result because of a lack of precision. This was probably caused by adding slightly too much hydrogen peroxide or because we had not cut and weighed the 5g of potato accurately enough.

As well as the anomaly caused by the 5g result, we also felt that by using only 4ml of hydrogen peroxide, we didn’t get very much froth, so we decided to increase the amount to 6ml for the retest. When we reran the test we got more accurate results, but to secure those results we did the test a final time.

How did we make it a fair test?:

Made sure that the room stayed at a constant temperature, as reactions work more quickly as the temperature increases.
Used the same pipettes with the same measurements for each different weight of potato in case other pipettes have different forms of calibration. If pipettes have different forms of calibration then more or less hydrogen peroxide might be added than expected.
Thoroughly cleaned the pipette so no extra hydrogen peroxide is present, this will make sure that the exact ml² of hydrogen peroxide is added to each different amount of potato to give an exact result
Used the same bottle of hydrogen peroxide throughout the experiment as other bottles of hydrogen peroxides may have had slightly different concentrations which would affect the rate of the reaction and the amount of froth produced.
Mashed the potato (instead of just chopping it up into different sized pieces) to make sure there will be an equal surface area per gram of potato for the hydrogen peroxide to come into contact with, however much potato is used. This should ensure that the concentration of potato in the mixture is proportionate to its weight.
Weighed the potato after mashing up to make sure no weight was lost, because if weight was lost then the results would be varied as the ml² of hydrogen peroxide being used would be reacting with less potato than expected.
Washed the knife after cutting, to wash the excess potato away. This way if there was potato on the knife, unnecessary weight would be added to the next measure of potato, causing an inaccurate result for the next weight of potato.
Wiped away any left over potato from the weighing scales to make sure no extra weight was added onto the next measure of potato, causing an inaccurate result.
Used the same measuring cylinders with the same diameter to make it easier to take comparative measurements.
Used the same scales in case different scales had different forms of calibration. If different scales had different forms of calibration, then the final results would be inaccurate.
Used the same stopwatch throughout for accuracy of timekeeping.
Made sure we used the same kind of potato with the same texture in case this affected the final results. Different types of potato might not have the same texture when mashed.

Things we kept the same:

Temperature
Maximum surface area (mm2) by mashing the potato.
Ml² of hydrogen peroxide used.
Type and diameter of measuring cylinders.
Concentration of hydrogen peroxide.
The pipettes to make sure no hydrogen peroxide is gained or lost.
The scales in case different scales have slightly different calibrations.
Type and texture of potato.
Stopwatch

Things we changed:

The weight (g) of the potato

Results table

Amount of froth produced in ml²

Analysis

The results graph shows that the ml2 of froth produced did not increase in the same proportion as the increase in the weight of the potato used at each stage of the test.

The weight of the potato increased by the same amount (2g) at each stage, but the average increase in the amount of froth produced goes up from 3.75ml2 between 5g and 7g to 4.5ml2 between 7g and9g, it is then 11ml2 between 9g and 11g and 13.5ml2 between 11g and 13g.

From analysing my graph I saw an obvious increase in the amount of froth as the weight of the mashed potato increased. My results overall did agree with my previous scientific prediction that an increase in the concentration of the potato in the mixture would show an increase in the rate of reaction by producing more froth in a fixed time, but only on a basic level (e.g. amount of froth increasing with the amount of potato added). On a more detailed scientific level, my experiment did not meet my hypothesis, because the rate of increase was not proportionate across all of the values used. This was may have been because of a lack of precision in cutting and weighing the potato and in measuring the amounts of hydrogen peroxide being added at each stage.

The line of best fit for the results plotted on the graph suggests that the results for the 11g and 13g weights were anomalies for being too high.

Evaluation

After completing this experiment, I can say that overall I am fairly pleased with the outcome and general procedure of this experiment; although I do believe that certain aspects of the experiment can be improved upon, in order to achieve a set of results that scientifically agree with the hypothesis.

The overall results were less proportionate than I initially expected. I think this may have happened because the amount of hydrogen peroxide used was able to react with all of the available potato when lower weights of potato were used. However, if the test was extended to even greater weights (say 15g and 17g) the rate of increase in the amount of froth might start to decrease as there would be insufficient hydrogen peroxide to react with all of the available potato. This might produce an S shaped line of best fit that would be scientifically consistent with the reaction being observed.

When I made the decision to use 4ml of hydrogen peroxide in the preliminary test, I thought this would be a sufficient amount to get a valid result. Instead it didn’t produce a sufficient amount of froth to make a detailed analysis, so when doing my 1st and 2nd tests, I changed the amount of hydrogen peroxide used to 6ml, hoping this would give me a more valid result. Although it gave me a more detailed result than when using 4ml of hydrogen peroxide, I still think that I should have increased the amount of hydrogen peroxide used to more than 6ml, as this still didn’t give me the consistent results I was hoping for when conducting my 1st and 2nd tests.

My experiment was fairly reliable except for the unexpected result in the preliminary test (this helped my realise that accuracy was an important factor when carrying out my experiment). My tests were fairly carried out, but I feel I could have tried harder so that all aspects of the experiment were kept consistent. I could have changed the texture and type of potato to see if this made any difference to the final results. Another aspect I could have changed was the amount of potato I was using. To get higher and more varied results to analyse, I could have used potatoes with weights greater than 13g. By doing this I could have discovered if the amount of froth produced would continue at the same rate of eventually slow down as all the hydrogen peroxide was consumed.