[Music] hi and welcome back to three sighs lessons code okay by the end of this video you should be able to describe how to carry out a practical investigation to explore the effect of concentration on the rate of the reaction I'm going to show you two different methods to carry out this practical and you need to learn both of them now before we start looking at the practical we need to look at a key idea in science which is a hypothesis a hypothesis is a proposal that could explain a fact on observation and in science a hypothesis must be testable okay so we're going to start by lucky an experiment called the disappearing cross reaction we're going to use this to explore the effect of concentration on the rate of a reaction if we react sodium thiosulfate solution with hydrochloric acid then one of the products is sulfur and sulfur is a solid the sulfur makes a solution go cloudy scientists call as cloudiness turbidity we can use this to see how long the reaction takes to finish so let's take a look at the method first we use a measuring cylinder to put 10 centimeters cubed of sodium thiosulfate solution into a conical flask we then placed the conical flask on to a printed black cross next we have 10 centimeters cube with hydrochloric acid into the conical flask now we swirl the solution and start a stopwatch we now look down through the top of the flask after a certain time the solution will turn cloudy we stop the clock when we can no longer see the cross we then carry out the experiment again using lower concentrations of sodium thiosulfate solution finally we repeat the whole experiment and calculate mean values for each concentration of sodium thiosulfate solution remember that we do not include any anomalous results when calculating a mean now there is one important idea linked to this practical which you need to learn and that is reproducibility a measurement is reproducible if it can be repeated by another person or using a different technique of equipment and still get the same result now the problem with the disappearing cross experiment is that different people have different eye sites that means that some people can see the cross for longer than others so they may not get the same results however because all the students use the same size printed cross hopefully this problem won't be too great okay we're going to look now at the second method for this required practical we're going to measure the volume of the gas produced by a reaction take a look at this reaction here where we act in magnesium with hydrochloric acid this reaction produces hydrogen gas and this allows us to measure the volume of hydrogen gas produced so let's look at the method first we use a measuring cylinder to place 50 centimeters cube with hydrochloric acid into a conical flask we then attach the conical flask to a bong and a delivery tube like this now we place a delivery tube into a container filled with water we then place an upturned measuring cylinder also filled with water over the delivery tube we then add a three centimeter strip of magnesium to the hydrochloric acid and we start a stopwatch the reaction produces hydrogen gas and that's trapped in the measuring cylinder every 10 seconds we measure the volume of hydrogen gas in the measuring cylinder and we continue until normal hydrogen is given off we now repeat the experiment using different concentrations of hydrochloric acid now both of these experiments show us that the greater the concentration of a chemical in a reaction the faster the reaction takes place so because this result is shown by two different experiments we can say that this finding is reproducible remember you'll find plenty of questions on this required practical and my vision workbook you can get up by clicking on the link above okay so hopefully now you should be able to describe how to carry out a practical investigation to explore the effect of concentration on the way to the reaction [Music]