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 control variables would include:
The temperature of the hydrochloric acid: The temperature of the hydrochloric acid should be kept constant throughout the experiment to ensure that it does not affect the rate of the reaction.
The length of the magnesium ribbon: The length of the magnesium ribbon should be kept the same for each trial to ensure that the same amount of magnesium is used in each experiment.
The volume of hydrochloric acid used: The volume of hydrochloric acid used should be the same for each trial to ensure that the amount of acid is constant throughout the experiment.
The surface area of the magnesium: Using magnesium ribbon ensures a consistent surface area-to-volume ratio for each trial.
The timing method: The method of measuring the time taken for the reaction to complete should be kept constant to ensure consistency across all trials.
By controlling these variables, it is possible to ensure that any differences in the rate of reaction observed are due to the concentration of the hydrochloric acid and not due to any other variables that may affect the reaction.