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Al salts on heating with Na(2)CO(3) in c...

`Al` salts on heating with `Na_(2)CO_(3)` in charcoal cavity gives a white infusible residue. On moistening the same with a drop of cobalt nitrate, it changes to ____due to the formation of cobalt meta-aluminate (Thenard's blue)

A

Brown

B

Yellow

C

Blue

D

Lemon

Text Solution

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The correct Answer is:
To solve the question step by step, we will analyze the reaction of aluminum salts with sodium carbonate and the subsequent reaction with cobalt nitrate. ### Step 1: Understanding the Reaction of Aluminum Salts with Sodium Carbonate When aluminum salts (like aluminum sulfate) are heated with sodium carbonate (Na2CO3) in a charcoal cavity, they undergo a reaction that produces sodium aluminate and carbon dioxide. **Chemical Reaction:** \[ \text{Al}_2(\text{SO}_4)_3 + 3 \text{Na}_2\text{CO}_3 \rightarrow 2 \text{NaAlO}_2 + 3 \text{CO}_2 + 3 \text{Na}_2\text{SO}_4 \] ### Step 2: Identifying the White Infusible Residue The product of this reaction is sodium aluminate (NaAlO2), which is a white infusible residue. This residue does not melt upon heating and remains solid. ### Step 3: Reaction with Cobalt Nitrate When the white infusible residue (sodium aluminate) is moistened with a drop of cobalt nitrate (Co(NO3)2), it reacts to form cobalt meta-aluminate. **Chemical Reaction:** \[ \text{NaAlO}_2 + \text{Co(NO}_3)_2 \rightarrow \text{CoAlO}_2 + \text{NaNO}_3 \] ### Step 4: Color Change to Blue Cobalt meta-aluminate (CoAlO2) is known for its blue color, often referred to as Thenard's blue. Thus, when the residue is moistened with cobalt nitrate, it changes color to blue. ### Final Answer The white infusible residue changes to **blue** due to the formation of cobalt meta-aluminate (Thenard's blue). ---
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