A metal was burnt in air, the substance formed was dissolved in water and tested with blue and red litmus paper. What results do you expect?

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Red litmus changes to green

No change in red litmus paper

Blue litmus changes to red

Red litmus changes to blue

answer is B.

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Detailed Solution

Metals when burnt in air react with atmospheric oxygen to give metal oxides.

For example, magnesium on burning in air gives following reaction:
$Mg + O_{2} \to MgO$
Metallic oxides are basic in nature therefore, by dissolving in water they form metal hydroxides i.e., alkali or bases.
$MgO + H_{2} O \to Mg {(OH)}_{2}$
Since metal oxides are basic the solution turns red litmus blue. which confirms the basic nature of metal oxides.

Therefore, a metal when burnt in air and substance formed was dissolved in water on testing with blue litmus paper, no change was observed and on testing with red, it changes to blue.

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