When Calcium ethynide is decomposed by water, the gas produced is

Analysis

The question asks for the product formed when calcium ethynide (also known as calcium carbide) reacts with water. This is a classic reaction in organic chemistry used for the industrial preparation of a specific hydrocarbon.

Concept

Calcium ethynide has the chemical formula CaC₂. It is an ionic carbide containing the acetylide ion ($C_2^{2-}$). When it reacts with water, a double decomposition reaction occurs where the calcium ion ($Ca^{2+}$) combines with the hydroxide ion ($OH^-$) to form calcium hydroxide, and the acetylide ion combines with hydrogen ions to form ethyne (acetylene).

Solution

1. Write the chemical equation: $CaC_2(s) + 2H_2O(l) \rightarrow Ca(OH)_2(aq) + C_2H_2(g)$

2. Identify the products: • $Ca(OH)_2$: Calcium hydroxide (slaked lime) • $C_2H_2$: Ethyne (acetylene gas)

3. Match with options: • A. CO (Carbon monoxide) - Incorrect • B. H₂ (Hydrogen) - Incorrect • C. C₂H₂ (Ethyne) - Correct • D. CH₄ (Methane) - Incorrect (Methane is produced when Aluminium carbide reacts with water)

Conclusion

The gas produced is ethyne, which is represented by the formula $C_2H_2$.

Exam Tip

To avoid confusion in the exam hall, remember these two common carbide reactions:

• Calcium Carbide ($CaC_2$) + Water $\rightarrow$ Ethyne ($C_2H_2$)
• Aluminium Carbide ($Al_4C_3$) + Water $\rightarrow$ Methane ($CH_4$)