After carrying out the preliminary investigation I decided that it took too much time to wait until the cross disappeared and so instead decided to record the time of when the first sign of purple appears; as this will make the experiment faster and so will make more time for repeating the experiment. This also means that I will be investigating how the sodium thiosulphate affects the time for the solution to change colour, as in my preliminary work it was shown that as the concentration of the thiosulphate increased the time of that it took for the cross disappear also increased which is different to my prediction, although it can be explained.
Prediction 2: I predict that the thio will slow down the experiment as when its concentration increases there is less iodine being produced, unlike the other substances; when Sodium thiosulphate (Na2S2O3) is used in the experiment it slows down the experiment by decreasing the amount of iodine produced (the iodine is needed to turn the solution purple). This is shown in the equation below. these equations show what will be happening during the experiment. This is why I think that as the concentration of the thio increases the time will also increase, because the thio slows down the experiment. When there is double the concentration of thio I would expect it to take almost double the amount of time, but this cannot be shown through the preliminary results. But you can see that when the concentration of both potassium iodide and ammonium persulphate is doubled the time is halved e.g. When all the substances are the same it took 451 seconds, but when the amount of potassium iodide was doubled the time the experiment took dropped to 145 seconds which is nearly half the time.
Diagram 2:
I would predict the graph made to look something like this:
With the time it takes for the experiment to work gradually increasing, although the reaction would not be able to take place when there is only sodium thiosulphate present and so would take infinity time.
Fair Test: To make sure that I keep the test fair I will keep the following the same throughout the experiments:
- Keep the amounts of Potassium iodide and Ammonium persulphate the same each time as these will affect the speed at which the experiment takes place.
- Same amount of starch
- Same total volume of all substances
- Same person to record the time that it took for the solution to turn a different colour (as people have different opinions and so may think it turns a different colour at different times).
- Use the same size container
List of Apparatus:
- Beaker
- Reactants: Potassium iodide, Sodium thiosulphate, Ammonium persulphate, Starch solution and water.
- Pipette
- Boling tubes X 2
- Stop watch
Method:
- Measure out the reactants to the nearest millimetre using a measuring cylinder and put the potassium iodide, Sodium thiosulphate and starch solution into one test tube, and the ammonium persulphate and water into the other test tube. Use the set amounts of each reactant, making sure to use the same amounts for all the reactants except the thiosulphate which will change each time.
- Set up the equipment as shown in diagram 3, making sure that you start the stopwatch at the same time as putting the two solutions together.
- Watch the experiment until the solution turns a different colour and then record the time that was taken.
- Repeat the experiment changing the concentration of the Sodium thiosulphate each time, and then repeat the investigation again to make sure that there are no anomaly results, and find the average of all the results which can then be plotted onto the graph.
Diagram 3:
Safety:
- Wear safety goggles to make sure you don’t get any of the reactants into your eyes.
- Be careful of spillages; be sure to mop up any spilt liquid as quickly as possible to avoid any accidents.
- When any of the glass is broken be sure to clear it up as soon as possible to stop anyone cutting themselves.
- Make sure that all bags and stools are away before starting the experiment to make sure that no one trips over.
Results:
There was no need to repeat any of the results a third time, since the results all followed a clear pattern and there were no anomaly results; as shown in the results table above. To find the average I added up the results and divided by the amount of results. E.g. 572+608 = 1180/2 = 590 s.
Conclusion: The graph has a positive line to it, it shows that when the concentration of the thiosulphate increases so does the amount of time that it takes for the solution to react, this is what I predicted would happen. The thiosulphate is slowing down the reaction by breaking up the iodine which is the substance that changes a different colour and so when there is not as much being produced it takes longer for the colour to change. There are no totally anomaly results in the graph although they do not stick to a complete pattern. The pattern would carry on increasing in time until you could no longer increase the concentration of the thio, but the experiment could never take any less than 1millisecond because that would not be possible. The graph is similar to the graph that I predicted would form, I did not repeat all of the experiments because there seemed to be no anomaly results and so there was no need. There is enough evidence to prove that as the thiosulphate concentration increases so does the time in which it takes for the solution to turn purple, you can see this by looking at the table of results and the graph above, which has a strong linear pattern to it; which provides reliable evidence to prove my prediction correct.
Evaluation: Although the graph shows a strong linear pattern some of the results are not completely correct; this could have been for many different reasons, such as the temperature being different on the different days that the experiment was done on, using different equipment, or simply an error when recording the results. All of these problems with the experiment could be stopped if I was to repeat the investigation, I could have done it on one day instead of over a couple of days if there was enough time, and taken more care when reading the results. The results could not be completely accurate because we could only tell when the solution started to turn a different colour by our own eyesight; next time I could use a light probe( a piece of equipment developed to measure change in colour instead that stopped timing the experiment automatically when the solution reached a certain colour). This would make the test much fairer, but would cost too much money and so would not be a possibility. Beyond reasonable doubt the results are enough to provide sufficient evidence that my prediction was correct, even though some of these results could be improved upon; as related to above.
When doing the experiment the method was usually well kept to although at time the measuring of the substances could have been more accurate, even though it was difficult to get it perfect because when you moved the solutions from the test tubes some of the solution was left in the base. If I was to re-do the experiment I would use a different way to tell when the experiment had ended or solution had changed colour, even though we kept the person the same each time I still do not think that this would be the most accurate way. I made sure that I kept all the other factors that could have affected the experiment the same throughout the investigation as otherwise the experiment would have not been a fair test. I used the chosen amounts of each substance as these were readily available and kept the experiment running for the right amount of time. Altogether I was pleased how the investigation worked out as I have a reliable set of results that seem to follow a pattern such as the one I predicted.
Extension: If I was to repeat the experiment, or improve upon it, I would change one of the following: temperature of the solution (instead of concentration), use a light probe or different more reliable resources, change the substance to investigate or extend the experiment by taking the concentrations further.
I think that I would change temperature if I was to extend the experiment to see what effect it had on the time it took for the solution to change colour.
Plan for another experiment:
Aim: To investigate how a change in temperature affects the time it takes to change the colour of the solution.
Theory: I already know from further resources that when the temperature of a substance increases the particles in that substance get more energy and so move faster; causing more collisions and so the experiment speeds up; as shown in
Diagram 4:
Prediction: I would predict that as the temperature of the solution increased the time it took to change colour would decrease, as the particles would have more energy to bump into each other and so more reactions would take place. This would mean that the solution would turn a different colour faster.
Method:
- Set up apparatus as shown in diagram 5 for the water bath and then place into large container as shown in diagram 2. Heat each test tube of ammonium persulphate and water until it reaches the correct temperature by putting into a water bath, (only change the temperature of this as it will be too complicated and difficult to get everything at the right temperature.) and checking with thermometer.
- Place all the reactants into the beaker and watch until the solution starts to change colour, as soon as this happens stop the experiment and record the results.
Diagram 5:
Although there are many changes I could have made to the experiment I think that the results that have been produced show reliable evidence to prove my prediction correct and to assume that the theories are in fact true.
Bibliography:
- Co-ordinated Science 1 – for some of the background knowledge.
