Which of the following compounds will turn a piece of red litmus paper to a bluish color? 
a) HI                                         b) C₆H₅NH₂                               c) CH₃COOH                            d) HOCN

• A. HI
• B. C₆H₅NH₂
• C. CH₃COOH
• D. HOCN

Answer: (B)

Verified step by step guidance

1. Understand the concept of litmus paper: Red litmus paper turns blue in the presence of a basic substance. Therefore, we need to identify which of the given compounds is basic.
2. Examine each compound: a) HI is hydroiodic acid, a strong acid. b) C6H5NH2 is aniline, an organic compound known to be a weak base. c) CH3COOH is acetic acid, a weak acid. d) HOCN is cyanic acid, also an acid.
3. Identify the basic compound: From the examination, C6H5NH2 (aniline) is the only compound that is basic.
4. Explain why C6H5NH2 is basic: Aniline has a nitrogen atom with a lone pair of electrons, which can accept a proton, making it a weak base.
5. Conclude that C6H5NH2 will turn red litmus paper blue due to its basic nature.