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What affects how quickly sodium thiosulphate solution goes cloudy?

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Tuesday, 12 November 2002 Candidate Number: Ilford County High School CHEMISTRY SCIENCE 1 INVESTIGATION What affects how quickly sodium thiosulphate solution goes cloudy? When an amount of acid is added to a solution of sodium thiosulphate, the mixture reacts. As a precipitate forms, the mixture turns from being translucent into a cloudy, yellow colour. This reaction can be timed quite easily, by measuring how long it takes the precipitate to make the mixture opaque. The equation for this reaction is: S2O32-(aq) + 2H+(aq) --> H2O + SO2(g) + S(s) Several factors could be changed to either accelerate or slow down the speed of the reaction. These are: 1. Temperature. The particles in both the acid and thiosulphate will begin to move more quickly, thereby causing more collisions (as the particles have increased their speed, the rate at which they collide also increases) with more energy. 2. Concentration. Since there are more particles present in the mixture, there is a greater chance that they will collide. 3. Catalysts. Using enzymes etc., catalysts can change the speed of the reaction. Skill P - Planning My Experiment In my investigation, I will alter the concentration of the liquids. ...read more.


Measure out the amounts of thiosulphate and water necessary for this test. Pour together into a conical flask, gently shaking to ensure that they are wholly mixed. 2. Measure 5ml of hydrochloric acid in the pipette. 3. Squirt acid into conical flask and begin timer. 4. Carefully observe the cross from directly above. As soon as it is completely obscured, stop the timer. 5. Record the results, wash out the conical flask very thoroughly and repeat with the next concentration of thiosulphate. The entire experiment will be repeated, which will allow me to spot any anomalous results. Also, should one experiment be totally wrong, this would leave me with at least some good results. Fair Testing: To keep the experiment fair, in each repetition, the amount of acid will remain constant. Also, all the equipment used will be the same (i.e. beakers, burettes etc.), as some instruments may vary slightly, but crucially, in size. I shall also be measuring the temperature at the beginning and end of the experiment because, as I have already mentioned, increases in temperature can speed up reactions. Finally, I will ensure that the conical flask is washed out thoroughly after each test - even a drop of acid will begin a reaction with the thiosulphate and potentially ruin the results. ...read more.


I can explain this, as washing out the conical flask left a tiny amount of water residing in it, even after drying it somewhat. This would lessen the concentration of the thiosulphate, and affect the speed of the reaction, although minutely. That both sets of results concurred precisely with my prediction (and the findings from my preliminary experiments) shows that they are reliable. Since I was extremely careful while using the pipettes and burettes, and since these are virtually infallible instruments (along with the electronic timer), I believe that I did not conduct any part of the experiment incorrectly (barring the anomaly mentioned above). This proves that I have enough evidence to be sure my conclusion is valid, along with my prediction. If I had the equipment available to me, I would prefer to set up an electronic measuring device. If the solution was to be placed on top of an LDR (light dependent resistor) embedded in a table, the accuracy of the results would be perfected - my eye was, although reliable, not exact, and the reaction time between seeing the cross disappear and stopping the timer would be removed. This could affect my results quite significantly, and it would be interesting to check whether my prediction was still true using this method. ...read more.

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