I am going to do an investigation to find how changing the volume and the concentration of Hydrochloric acid effects the time taken for a solution of Sodium Thiosulphate and Hydrochloric acid to go cloudy and obscure a cross.

Equipment

100ml Conical flask, two 25ml measuring cylinders, two 250ml beakers, timing device, paper.

Method

1. Draw a cross on a piece of paper
2. Add 20cm3 Sodium Thiosulphate into a conical flask
3. Place the conical flask over the cross
4. Add Hydrochloric acid to the conical flask
5. Time how long it takes for the cross to be obscured
6. Repeat experiment for the different volumes of HCl and concentrations
7. Record results

Add dilute acid and

start timing

                        Conical flask

                            Sodium                                               Cross

                           Thiosulphate                                       drawn on

                           Solution                                               paper

Fair Testing

                To make this a fair test I am going to use 20cm3 Sodium Thiosulphate every time and always keep the solutions at room temperature. I will change the volume and concentration of Hydrochloric acid but do the same volumes for each concentration. I will also use separate measuring cylinders for the Hydrochloric acid and Sodium Thiosulphate.

Safe Testing

                To make sure the testing is safe I will always wear safety goggles.

Prediction

                I predict that the higher concentration of Hydrochloric acid added the sooner a precipitate will be created and the cross obscured.

I believe this because the higher the concentration the more often the particles bump into each other causing the reaction to happen quicker so a precipitate will be created sooner.

                I predict that the higher the volume of Hydrochloric acid added the slower a precipitate would be formed.

                I believe this because there will be a greater volume of solution to go cloudy so it would take longer.        

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Observations

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Analysis

The more Hydrochloric acid that was added to the solution, the longer it took for a precipitate to be formed. The rate of reaction increased with less Hydrochloric acid in the solution. This was proved when 10ml of Hydrochloric acid took 71 seconds to form a precipitate and 30ml of Hydrochloric acid took 86 seconds.

From the graph I noticed that initially the gradient of the curve on the graphs up to 10cm3 is quite steep which shows that it is reacting fast. It then starts to smooth into about a 35° angle which indicates the rate of reaction has slowed down as the volume of Hydrochloric acid is increased.

These results prove that I correctly predicted that as the concentration increases the time taken for a precipitate to form decreases. It also proves I was right in predicting the greater volume of Hydrochloric acid the longer the reaction takes to form a precipitate.  

The results prove that the more Hydrochloric acid particles in the solution bumping into the Sodium Thiosulphate cells the sooner a precipitate will form.

        

           Dilute Solution

         Concentrated Solution

Evaluation

        I thought the investigation was very successful. My predictions that the higher the concentration the quicker a precipitate forms, and the greater volume of Hydrochloric acid the longer the reaction takes to form a precipitate, were both found to be correct. This shows that to form a precipitate quickly then you need more Hydrochloric acid particles to bump into the Sodium thiosulphate particles. All of my results showed this to be true.

        If I had to repeat this experiment again I would keep the equipment the same because I believe the equipment I used was right for the experiment, and I would keep the method the same because I believe it worked well and couldn’t be improved. I would however do more of each test so my results were more reliable and accurate, as it would allow for any anomalous results. I believe it was a fair test as I only changed two parts (the concentration and volume of Hydrochloric acid) of the experiment and only changed one per test.