Answer:
To check whether aluminium foil has a protective coating or not, we treated the aluminium foil with a base, KOH, and an acid, HCl.
We observed that the reaction was slow at first. but suddenly afterwards, the reaction became faster and more vigorous.
We can conclude from the above observation that initially the reaction was slow because a protective coating was present. As soon as the protective coating was consumed in the reaction, the reaction turned fast and vigorous.