An unknown substance turns blue litmus paper to red. which additional test could be used to confirm the substance identity

The additional test is that it should turn methyl orange to red because it is an acid.  

The mentioned substance given in the test is an acid, as the bases turn red litmus paper blue, while an acid turns blue litmus paper red. The acid-base indicators are generally weak bases or acids and they function as a chemical detector for hydronium and hydrogen cations.  

Phenolphthalein is an example of an acid-base indicator, it becomes pink in basic solutions and colorless in acidic solutions. Methyl orange is another example, which becomes yellow in basic solutions and red in acidic solutions.  

methyl orange to red because its an acid.

Explanation:

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