Question

an unknown substance turns blue litmus paper to red. which additional test could be used to confirm the substance’s identity?
○ it should turn phenolphthalein from colorless to pink because it is a base
○ it should turn phenolphthalein from pink to colorless because it is a base
○ it should turn methyl orange to orange because it is an acid.
○ it should turn methyl orange to red because it is an acid

Explanation:

1. First, analyze the property of the unknown substance: Turning blue litmus paper red indicates the substance is an acid (acids turn blue litmus red, bases turn red litmus blue).
2. Then, analyze each option:

• Option 1 and 2: Phenolphthalein is colorless in acid and pink in base. Since the substance is an acid, it should not cause phenolphthalein to turn pink. So these two options are wrong.
• Option 3: Methyl orange shows orange in neutral to weakly acidic conditions and red in strongly acidic conditions. Since the substance is an acid, it should turn methyl orange to red (for strong acid) or at least show a color change towards red (depending on acidity), and the statement "turn methyl orange to orange because it is an acid" is incorrect.
• Option 4: Methyl orange turns red in acidic conditions (especially strong acids), which is consistent with the substance being an acid (identified by blue litmus turning red). So this option is correct.

Answer:

D. It should turn methyl orange to red because it is an acid