Chemistry Practical

A glass rod is also called :
A white crystalline solid 'A' on boiling with caustic soda solution gives a gas 'B', which on passing through an alkaline solution of potassium tetraiodomercurate (II) solution gives a brown ppt. The substance 'A' on heating evolves a neutral gas 'C', which is inert at room temperature and reactive is presence of catalyst and does not give brown fumes with nitric oxide.
You have a mixture of $$\displaystyle { CO }_{ 2 },{ SO }_{ 2 }$$ and $$\displaystyle { O }_{ 2 }$$ gas. You have been provided following reagents for testing 
(A) lime water
(B) acidified potassium dichromate solution
(C) pyrogallol
A colourless mixture of two salts (A) and (B) (excess) is soluble in $$\displaystyle { H }_{ 2 }{ O }$$. Separately (A) turns blue litmus red and (B) turns red litmus blue. (A) gives white ppt. with (B),which dissolves in excess of (B) forming (C). (A) when placed in moist air gives fumes and can form dimer. (A) gives white ppt. with $$\displaystyle { NH }_{ 4 }{ Cl }$$ and $$\displaystyle { NH }_{ 4 }{ OH }$$ soluble in (B). (A) also gives white precipitate with $$\displaystyle { AgNO }_{ 3 }$$ soluble in $$\displaystyle { NH }_{ 4 }{ OH }$$.
Read the above analysis report and answer the following questions.
In alkaline solution, the litmus paper turns from :