|HCl with Zn|
When hydrochloric acid of different concentration is added to equal amounts of zinc metal, hydrogen liberates from the flask, and vigorous reaction follows the equation:
Secure rubber balloons to bottle necks and balloons begin to inflate at different speed – the reaction is faster where acid concentration is greater, so the balloon secured to the bottle with more concentrated hydrochloric acid is inflated faster.
Being a strong reducing agent zinc reduces hydrogen ions to molecular hydrogen and forces it out of solution.
Zinc is being consumed in this reaction while hydrogen is being generated, so when the bubbling ceases it indicates the end of the reaction. Hydrogen balloons being easier than ones filled with air will be able to float.