When a white crystalline compound X is heated with `K_(2)Cr_(2)O_(7)` and concentrated `H_(2)SO_(4)` a reddish brown gas A is evolved On passing A into saustic soda solution ,a yellow coloured solution of B with acetic B is obtained .Neutradiding the solution of B with acetic acid and on obtaijned when X of lead acetate with NaOH solution precipitate C is obtained when X is heated with NaOH solution a coloueless gas is evolved and on passing the gas into `K_(2)HgI_(4)` solution a reddish brown precipitate D is formed identify A,B,C and D and X write the equation of the reaction involved.

Explanation

To solve the given problem step by step, we will identify the compounds A, B, C, D, and X, and write the relevant chemical equations for each reaction.

### Step 1: Identify Compound X
The problem states that a white crystalline compound X is heated with potassium dichromate (K₂Cr₂O₇) and concentrated sulfuric acid (H₂SO₄). A common white crystalline compound that fits this description is **ammonium chloride (NH₄Cl)**.

### Step 2: Reaction of X with K₂Cr₂O₇ and H₂SO₄
When ammonium chloride (X) is heated with potassium dichromate and concentrated sulfuric acid, it produces a reddish-brown gas A, which is **chromyl chloride (CrO₂Cl₂)**.

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