Sodium Thiosulfate & HCL Produce a Cloudy Precipitate

• This reaction releases sulphur dioxide, so the experiment should be carried out in a well-ventilated place
• 1. These 2 chemicals are both clear solutions. They react together to form a yellow precipitate of sulfur
• 2. Start by adding a set volume of dilute sodium thiosulfate to a conical flask
• 3. Place the flask on a piece of paper with a black cross drawn on it. Add some dilute HCl to the flask and start the stopwatch
• 4. Now watch the black cross disappear through the cloudy sulfur and time how long it takes to go
• 5. The reaction can be repeated with solutions of either reactant at different concentrations. The depth of the liquid must be kept the same each time
• 6. These results show the effect of increasing the concentration of HCl on the rate of reaction, when added to an excess of sodium thiosulfate
• 7. The higher the concentration, the quicker the reaction and therefore the less time it takes for the mark to disappear
• 8. One disadvantage about this reaction is that it doesn't give a set of graphs. All you get is a set of readings of how long it took until the mark disappeared for each concentration
• 9. Although you can draw a graph of concentration against 1/time which will give you an approximate rate