Brass School
Why use hydrochloric acid in the brass penny experiment?
Why do you soak the penny in hydrochloric acid before boiling it in zinc and sodium hydroxide when turning the penny to brass? It's a school experiment.
It's called cleaning. Pennies will, over time, accumulate a thin oxide layer (among other by continuouis handling by sweaty hands, being kept in dark wallets and the like).
A brief treatment with dilute HCl will remove the surface stains so you are left with a really clean surface on which you reaction can take place. (this is also a reason for NOT handling the cleaned penny wit your hands afterwards because then you are re-introducing the surface stains)
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