Red cabbage changes colour depending on how [acidic] or [alkaline] a solution is, so we call it an [indicator]. When you added [HCl] (an acid), it turned red; when you added [NaOH] (an alkali) it turned green. In the mystery solution it turned red, so it must be an [acid]. When you made the indicator with tap water, it was blue; so tap water must be a weak [alkali]. .

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