點解唔可以用HCl去acidified substances

It's because HCl would react with KMnO4 because KMnO4 is a stronger oxidizing agent than Cl2. Cl^- in HCl therefore would be oxidized to chlorine.
10Cl^-(aq) + 2MnO4^-(aq) + 16H+(aq) → 5Cl2(g) + 2Mn^2+(aq) + 8H2O(l)
　　　　　　　　　　　　　　　　　　　　　　　　　　
However, HCl can be used to acidify K2Cr2O7 as Cl2 is a stronger oxidizing agent than K2Cr2O7. Thus, K2Cr2O7 cannot oxidize Cl^- in HCl to chlorine.
　　　　　　　　　　　　　　　　　　　　　　　　　　
Diluted sulphuric acid can be used to acidify both of them because diluted sulphuric acid itself is not an oxidizing agent and the anion given to the mixture, i.e. SO4^2-, also would not affect the two oxidizing agent in the reaction with other reducing agents.
　　　　　　　　　　　　　　　　　　　　　　　　　　
Besides, HNO3(aq) is also not used to acidify them since it is a strong oxidizing agent itself which may compete with them to react with reacting agents during reactions.

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