Question

why do HCL not show acidic behaviour

Answer 1

until HCl is not in contact with water it will not show acidic behaviour because acid has H positive ions , which are due to the presence of water.
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Answer 2

This is because it is present in the inner lining of the stomach which may help to kill the unwanted microbes come inside the body through food. It doesn't show any acidic reaction in the body because of pepsin and mucus present in the stomach.

Though in the aqueous solution it shows acidic reaction.

Hope it helps...