The variable which I have decided to investigate is the concentration of the acid. I think it will be the easiest to measure as you can change the concentration by adding easily-measured amounts of water to the acid to create different concentrations.
Every 5 seconds of the experiment until 40 seconds I will record how much hydrogen is collected. I will do this because I predict that there will be a faster reaction at the beginning of the experiment and it will slow down towards the end as the magnesium dissolves.
I predict that as I decrease the concentration of the acid, there will be less hydrogen collected in 40 seconds as the rate of reaction will be much slower and it will take a longer time to collect a substantial amount of gas.
Diagram to show equipment used in experiment
I will measure the rate of reaction by taking each concentration of acid, simultaneously dropping a 0.5g piece of magnesium into it and starting the stopwatch. I will then measure the amount of hydrogen collected every 5 seconds, and recording the results, until the stopwatch reaches 40 seconds.
The different acid concentrations that I will be using are:
2m, 1.6m, 1.2m, 0.8m, 0.4m and 0m acid
These are the ratios of acid: water which I will be using to create the different concentrations of acid:
2m – 5:0
1.6m - 4:1
1.2m – 3:2
0.8m – 2:3
0.4m – 1:4
0m – 0:5
Each representative number is equal to 10cm³, e.g. 2m acid- 50cm³:0cm³
To make sure my experiment is a fair test I will always keep the other variables (not concentration) the same. This is to ensure that nothing other than the concentration can have an effect on my results.
All the pieces of magnesium used will be the same size (0.5g) and the amount of liquid will always stay the same (50ml), whatever the concentration. I will use a conical flask measured in 5ml to maintain this volume of liquid accurately for each experiment. I will ensure that no air gets into the conical flask before the experiment as this might make the results less accurate as it would be hard to see which gas is collected from the experiment.
To make sure that my results are reliable I will do each experiment 3 times, using my own judgement to choose the most sensible result for each concentration of acid. This means that I can allow for anomalies, without them making my results inaccurate.
Obtaining Evidence
The numbers highlighted in red are those which I have chosen to use as my data. I have used my common sense to choose the most appropriate results.
