Take cold water, normal water and hot water in three test tubes. Drop a small size of magnesium ribbon in each test tube and add one drop of phenolphthalein in each test tube. In which test tube will pink colour appear sharply?

Test tube with normal water.

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Test tube with cold water.

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Test tube with hot water.

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Test tubes with normal water and with hot water.

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Solution

The correct option is C
Test tube with hot water.

Different metals react differently with water. Metals react with water to form metal hydroxide and hydrogen gas.

$M e t a l (s) + W a t e r (l) \to M e t a l h y d r o x i d e (a q) + H y d r o g e n (g)$

Magnesium does not react with normal or cold water. It reacts with hot water to give magnesium hydroxide and hydrogen gas.

$M g (s) + H_{2} O (l) \to M g (O H)_{2} (a q) + H_{2} (g)$

Magnesium hydroxide is a base which turns phenolphthalein into pink colour. Therefore, pink colour will appear in a test tube having hot water.

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