Explain each observation in terms of chemical reactions. (A) Solid Zn(OH)2 will

Question

Explain each observation in terms of chemical reactions. (A) Solid Zn(OH)2 will dissolve in concentrated HC1 or in concentrated NaOH. (B) Bubbles and heat are generated when sodium metal is added to water at room temperature but neither is observed when aluminum is added to room temperature water. (C) A dilute solution of Cu(NO3)2 is a light blue color but addition of a solution of ammonia causes a dark blue solution. (D) A sample of solid CuSO4. 5H2O is blue initially but turns white after being heated in a crucible for five minutes over a Bunsen burner.

Explanation / Answer

(A) This is because Zn(OH)2 is a an amphoteric compound, which means it can react with both an acid as well as a base

2 HCl + Zn(OH)2 (s) ----> ZnCl2 + 2 H2O

2 NaOH (aq) + Zn(OH)2 (s)----> Na2ZnO2 (aq) + 2H2O (l)

With NaOH, the compound formed is called sodium zincate

(B)

This is because Na is a highly reactive metal as compared to Al, and it can react with water at room temperature, but Al can only react with hot water or steam

2Na + 2H2O ----> 2NaOH + H2(g)

(C)

This is because on addition of ammonia, a complex gets fomed called as which is dark blue in color

Cu2+(aq) + 4 NH3 (aq) -----> Cu(NH3)42+(aq) complex

(D)

This is because on heating, the hydrated form gets converted to the dehydrated form. The color of CuSO4.5H2O is due to the 5 molecules of water which are attached to it. On heating, they are lost, so blue color is also lost.

CuSO4.5H2O -------heat----> CuSO4 + 5H2O

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