Final answer:
When all hydrochloric acid has reacted in the reaction with zinc oxide, the student can watch for when the bubbling of hydrogen gas stops, indicating all acid has been consumed.
Step-by-step explanation:
To determine when all the hydrochloric acid has reacted in the reaction where zinc oxide powder is mixed with hydrochloric acid to produce zinc chloride and hydrogen gas, a student can observe the reaction for the cessation of bubbles. The bubbling indicates the evolution of hydrogen gas. Once the production of this gas stops, it is likely that all the hydrochloric acid has been consumed since it was the limiting reactant.
The chemical equation for the reaction is: ZnO (s) + 2HCl (aq) → ZnCl2 (aq) + H2 (g). In a scenario where hydrochloric acid is the limiting reactant, once all the HCl is used up, no more hydrogen gas will be produced, hence no more bubbling will occur.