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Standard XII

Chemistry

Reactivity with Halogens and Hydrogen (Group 2)

What would be...

Question

What would be the observations when aqueous ammonia is added to a solution of zinc nitrate (until in excess) and heated?

White precipitate formed. On heating, a colourless, pungent gas evolved.

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White precipitate formed which dissolved in excess aqueous ammonia to form a colourless solution.

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White precipitate formed which dissolved in excess aqueous ammonia to form a colourless solution. On heating, a colourless, pungent gas evolved.

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Blue precipitate formed which dissolved in excess aqueous ammonia to form a dark blue solution. On heating, a colourless, pungent gas evolved .

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Solution

The correct option is C White precipitate formed which dissolved in excess aqueous ammonia to form a colourless solution. On heating, a colourless, pungent gas evolved.
The ammonia, being a weak base, produces hydroxide ions in the water.
${Z n}^{2 +} (a q) + 2 O H^{-} (a q) ⟶ Z n {(O H)}_{2} (s)$
The zinc hydroxide is the white precipitate.
If you continue to add the ammonia solution the precipitate redissolves because of the formation of the soluble ion $Z n {(N H_{3})}_{4}$ to give a colourless solution. When heated ammonia gas is evolved which is pungent in smell.

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