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, students could tell when the reaction was complete by observing the reaction between the magnesium and the hydrochloric acid.
When the magnesium reacts with hydrochloric acid, hydrogen gas is produced as a byproduct. This reaction is exothermic, meaning it releases heat. As the reaction progresses, the magnesium will start to dissolve, and hydrogen gas bubbles will form and be released from the solution.
To determine when the reaction is complete at each concentration of acid, students can observe the reaction mixture and look for the following signs:
The magnesium ribbon stops fizzing: As the reaction progresses, the magnesium ribbon will start to dissolve, and hydrogen gas bubbles will be released from the solution. When the magnesium ribbon stops fizzing, it indicates that the reaction has slowed down significantly or has completely stopped.
The magnesium ribbon has completely disappeared: When the magnesium ribbon has completely dissolved in the hydrochloric acid, it indicates that the reaction is complete.
No more gas bubbles are being produced: As the reaction between magnesium and hydrochloric acid produces hydrogen gas, students can observe the reaction mixture to see if any more gas bubbles are being produced. When no more gas bubbles are being produced, it indicates that the reaction has completed.
By observing these signs, students can determine when the reaction is complete at each concentration of hydrochloric acid and accurately measure the time taken for the reaction to complete.