Select reaction which is not correct.

Question

TestHub · Accepted Answer

Magnesium reacts with cold water very slowly to form magnesium hydroxide and hydrogen gas. The reaction is noticeable only after a long time. With hot water or steam, the reaction is much more vigorous. Therefore, the reaction shown in option C, implying a significant reaction with cold water, is not entirely accurate under typical conditions.

Detailed analysis of the options:

A. $M g_{3} N_{2} + 6 H_{2} O \to 3 M g (O H)_{2} + 2 N H_{3} ↑$

This reaction is correct. Magnesium nitride reacts with water to produce magnesium hydroxide and ammonia gas. This is a common method for the preparation of ammonia in the laboratory.

B. $M g_{2} C_{3} + 4 H_{2} O \to 2 M g (O H)_{2} + H_{3} C - C \equiv C H ↑$

This reaction is correct. Magnesium carbide ( $M g_{2} C_{3}$ ) reacts with water to produce magnesium hydroxide and propyne (methylacetylene). This is characteristic of carbides that contain the $C_{3}^{4 -}$ ion.

C. $M g + 2 H_{2} O co l d M g (O H)_{2} + H_{2} ↑$

This reaction is NOT entirely correct as written for "cold" water. Magnesium reacts very slowly with cold water due to the formation of a sparingly soluble $M g (O H)_{2}$ layer on its surface, which inhibits further reaction. A more significant reaction occurs with hot water or steam. For example, with steam, the reaction is:

$M g (s) + H_{2} O (g) h e a t M g O (s) + H_{2} (g)$

D. $M g C O_{3} Δ M g O + C O_{2} ↑$

This reaction is correct. Magnesium carbonate decomposes upon heating (indicated by $Δ$ ) to form magnesium oxide and carbon dioxide gas. This is a typical thermal decomposition reaction for metal carbonates.

Therefore, option C is the incorrect statement because magnesium reacts very slowly with cold water, not readily as implied.

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