When a white crystalline compound X is heated with `K_2Cr_2O_7` and concentrated `H_2SO_4` a reddish-brown gas A is evolved. On passing A into caustic soda solution, a yellow-coloured solution B is obtained neutralising the of lead acetate, a yellow precipitate C is obtained. When X is heated with `NaOH` solution a colourless gas is evolved and on passing this gas into `K_2HgI_4` solution a reddish-brown precipitate D is formed. Identify A,B,C,D, and X. Write the equations of the reactions involved.

Explanation

To solve the given problem, we will identify the compounds X, A, B, C, and D step by step, along with the relevant chemical equations.

### Step 1: Identify Compound X
The white crystalline compound X is identified as **ammonium fluoride (NH4F)**.

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

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