hydrochloric acid and marble chips

From looking at my results and graph I am able to conclude that the more concentrated the acid the quicker the reaction happened and the more gas was produced.
us From looking at each graph and comparing the straight line that my results produce, with the line of best fit that I have included, it is obvious to the human eye that my results are not completely true to my prediction. On each graph there is one or more plots that are not on the line of best fit (albeit only very marginally in all cases). The shape of each graph is nearly a perfect straight line, however the fact that they are not perfectly straight tells us that the method I used was not totally accurate.
In the case of every graph I have produced, the pattern my results show is that the more concentrated the acid, the higher the volume of gas recorded at each time-station. For example, below are the average volumes recorded at the second time station (30 seconds into the duration of the reaction), for each different c...