A soluble compound of a poisonous element `M`, when heated with `Zn//H_2 SO_4`, gives a colourless and extremely poisonous gaseous compound `N`, which of passing through a heated tube gives a silvery mirror of element `M`. Indentify `M` and `N`.

Explanation

To solve the problem, we need to identify the poisonous element `M` and the gaseous compound `N` produced from the reaction described.

### Step 1: Identify the poisonous element `M`
The question states that `M` is a soluble compound of a poisonous element. Common poisonous elements in chemistry include arsenic (As), lead (Pb), and mercury (Hg). Among these, arsenic is known for forming soluble compounds.

### Step 2: Determine the soluble compound of `M`
Given that `M` is arsenic, a common soluble compound of arsenic is arsenic trichloride (AsCl3). This compound is indeed soluble in water and is poisonous.

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