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A solution of compound '' X '' in water was added to the solution of sodium chloride . It formed a white precipitate of the compound '' Y '' .It was filtered and kept in a china dish in sunlight for drying but it turned grey . Explain the possible reasonfor this . Identify the compounds '' X '' and '' Y '' and write the reaction involved .

Yes your answer is correct.

‘X’ is silver nitrate and it dissolves in water forming silver nitrate solution which on reacting with sodium chloride precipitates white silver chloride ‘Y’.

The reaction is:

  AgNO3 (aq) + NaCl (aq) -----> AgCl (white precipitate) + NaNO3 (aq)

Silver nitrate (X)  Silver chloride (Y)

When the Silver chloride(white precipitate) exposed to sunlight, it quickly darkens (grey or black) by disintegrating into elemental chlorine and metallic silver. This reaction is called photodecomposition reaction.

  Sun light

2AgCl (s) -----------> 2Ag  + Cl2

Silver chloride   silver

(White ppt.)  (grey or black)

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X-Silver Nitrate (AgNO3)

Y-Silver Chloride(AgCl)

Equation-NaCl + AgNO3 -- AgCl + NaNO3

Silver Chloride(AgCl)is the precipitate of the reaction above,which is called a double decomposition or precipitation reaction.

Sunlight carries a good amount of energy, which makes silver chloride (AgCl) decompose in silver (Ag) andchloride ions. The metallic silver appears as very small 'flakes', esentially black, so these tiny black spots evenly distributed in the mass of white silver choride makes it look grey.This is a photo-electric decomposition reaction.

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