Question

A substance (P) releases a gas (Q) on reaction with ${{\text{H}}_{\text{2}}}\text{O}$, (Q) decolourises bromine water, (P) maybe?
A.$\text{Be}{{\text{C}}_{2}}$
B.$\text{B}{{\text{e}}_{2}}\text{C}$
C.$\text{A}{{\text{l}}_{4}}{{\text{C}}_{\text{3}}}$
D.$\text{M}{{\text{g}}_{2}}{{\text{C}}_{\text{3}}}$

Answer 1

Hint:To solve this question, knowledge on the chemical reactions and properties of the organic compounds is required. The compound (P) is a carbide of an alkali earth metal.

Complete step by step solution:
The bromine water is a highly oxidising intense yellow to red mixture containing diatomic bromine dissolved in water. It is generally used as reagent for the determination of unsaturated carbon compounds having one or more than one double or triple carbon- carbon bonds. The most common compounds that react with bromine water are phenols, alkenes, enols, the acetyl group, aniline and glucose.
Now, according to the question, the substance (Q) is a gas which means that any unsaturated carbon compound must have been formed that is of low molecular weight because compounds containing more than 4 carbon atoms are generally liquid.
Among the given options, aluminium carbide or $\text{A}{{\text{l}}_{4}}{{\text{C}}_{\text{3}}}$ reacts with water to liberate methane which is a saturated hydrocarbon hence it will not react with bromine water. Beryllium carbide also reacts with water to liberate methane.
Magnesium carbide reacts with water to form the propyne gas. The propyne gas has a carbon-carbon triple bond which will react with bromine water turning it colourless.

Hence, the correct answer is option D.

Note:
Carbides are compounds composed of carbon and are less electronegative than carbon and they are distinguished by ionic and covalent bonding. The compounds are prepared from metals or metal oxides at high temperatures (1500$^{0}\text{C}$) by combining the metal with carbon.