Five of the six treatments in this experiment use a vinegar solution. why is it important to use a water solution at the beginning of the experiment for sample a instead of just starting with sample b?

The sample A, prepared with water instead with vinegar, is called control sample.

It is important to prepare a control sample for comparison purposes.

Using a contral sample the scientist try to discard effects that could drive to erronous conclusions.

The scientific will compare the observations on what happens to the samples with vinegar with the observations on the sample with water. If the sample with water is treated in the same way that the samples with vinegar, and the results are different, the scientific may conclude the effect of the vinegar, but is the sample with water shows a similar behavior to that of the samples with vinegar, the scientific will know that it is not the vinegar but another variable the responsible of the effect.