so this is the first of two videos in which we're going to be looking at how we can measure the rate of a chemical reaction in this video we're going to be looking at the reaction between sodium thiosulfate and hydrochloric acid now when the two solutions react together they produce an insoluble precipitate of sulfur and as that's made the solution will go from being clear and colorless to a kind of yellow cloudy color we're going to time how long it takes for that cloudy yellow color to get so thick that we can't observe across that I have drawn on a piece of paper with a pencil now that cross is going to go underneath the reaction vessel and we're going to simply mix together 50 centimeters cubed of sodium thiosulfate with 10 of hydrochloric acid we're going to vary the concentration of the sodium thiosulfate and see what effect that has on the time it takes for the precipitate to form and for the cross to disappear from view and we'll be changing that concentration by diluting our sodium thiosulfate with some distilled water but to start with I'm going to go for the most concentrated sodium thiosulfate which is 40 grams per decimeter cubed as it comes out of the container here and so I'm going to measure out 50 centimeters cubed of this into my measuring cylinder so as before getting down to eye level so I can see the level of the sodium thiosulfate I fill up my measuring cylinder almost there little bit more until I've got exactly 50 centimeters cube measures out and I'm going to pour that into the beaker I'm now going to measure out just 10 of the acid so I'm using slightly smaller measuring cylinder and again getting down to eye level holding the measuring cylinder firmly so it doesn't fall over and measuring out exactly 10 centimeters cubed you can use a and pipette to do that but I don't now the reaction is going to start as soon as I mix the acid with my sodium thiosulfate so I need to be ready with my timer to start as soon as I add it just a swirl or two to get them well mixed and then I'm going to observe the cross through the solution by looking directly above now after a few seconds you start to see the precipitate forming and the solution turning this cloudy yellow color but I'm going to watch from above until the cross has totally disappeared now and stop the timer and so I've got a time for my first concentration of twenty seven point four four seconds so that was the most concentrated and so now I can vary the concentration by mixing different proportions of the sodium thiosulfate with distilled water but still keeping the total volume as fifty centimeters cubed so I'm going to measure out 40 centimeters cubed this time carefully in slowing down as I get near the 40 mark a little bit more and I'm there I'm gonna pour that into my flask but to dilute it I'm going to add 10 centimeters cubed of the water so keeping the volume constant at 50 but now reducing the number of sodium thiosulfate particles in that total volume so we need to think how is this going to affect the rate of reaction well a reaction occurs when particles collide and that collision has got to have sufficient energy for the collision to be successful by diluting the sodium thiosulfate I've got the same volume but fewer particles so a collision is less likely and so within a minute or a second we're going to have fewer collisions and therefore a lower frequency and in theory a smaller or lower rate of reaction so to start it off i'm going to add my ten centimeters cubed of hydrochloric acid being careful again to get the measurement accurate reset my clock and start again pour the acid in start the clock give it a little swirl and i'm keeping everything else constant i've got the same volume of solution I've got the same cross it's the same person observing it I'm going to watch directly from above until I can no longer see that cross so as you're looking down into the flask take care not to breathe in the sulphur dioxide fumes that do come off because it can make you cough so the precipitate is appearing at a slower rate the time is longer this time and as I watch from above the cross has disappeared this time at 32 seconds so the reaction time is longer so the rate is smaller so I would repeat this now for three more concentrations of sodium thiosulfate so I've got five values in total of my independent variable my concentration and five values for my dependent variable which is the reaction time I can repeat the whole experiment to make sure I can get a reliable result by taking a mean average of those repeats and then I will plot a graph of concentration of sodium thiosulfate against the reaction time and so now I've collected the data for all of my different concentrations so I've got my average time for each of the concentrations that I did and as you can see it's forming a curved shape so our line of best fit won't be a straight line it will be a curve so I'm just going to turn the paper around here and with my pen and you should use pencil obviously with my pen I'm just going to Drew draw a smooth curve through my points so I've drawn the line of best fit and as we can see it's a curve not a straight line and it also shows that as we increase the concentration of the sodium thiosulfate the reaction time decreases we could plot one divided by time which would give us a measurement of rate because obviously as time increases rate decreases but we don't actually need to do that for this particular practical