Answer:
In an investigation to determine if the time taken for magnesium to react completely with hydrochloric acid is affected by the concentration of the acid used, the students would have controlled the following variables:
Concentration of Hydrochloric Acid: The concentration of the hydrochloric acid used in each trial would have been controlled. The students would have prepared hydrochloric acid solutions of different concentrations, and ensured that the concentration of the acid was the only variable that changed between the trials.
Mass of Magnesium: The mass of the magnesium used in each trial would have been controlled. The students would have used the same length of magnesium ribbon for each trial, ensuring that the mass of magnesium was the same in each experiment.
Volume of Hydrochloric Acid: The volume of hydrochloric acid used in each trial would have been controlled. The students would have used the same volume of hydrochloric acid for each trial, ensuring that the amount of acid was constant across all experiments.
To ensure consistency and accuracy, the students would have used precise measuring tools to measure the volumes of hydrochloric acid used and the length of the magnesium ribbon. They would have also ensured that the temperature of the acid and the timing method were kept constant throughout the experiment. By controlling these variables, the students would have been able to accurately determine the effect of the concentration of hydrochloric acid on the time taken for magnesium to react completely.