Question

The compound that is used in smoke screens is:

• A. \(\mathrm{Mg_3N_2}\)
• B. \(\mathrm{MgO}\)
• C. \(\mathrm{NaN_3}\)
• D. \(\mathrm{Ca_3P_2}\)

Hint:

Calcium phosphide produces phosphine gas on reaction with moisture, and phosphine forms dense smoke on burning in air.

Answer 1

The Correct Option is D

Step 1: Identify the compound used in smoke screens.
Smoke screens require a substance that reacts with moisture in air to produce a dense, persistent cloud. Calcium phosphide (Ca₃P₂) is the compound used for this purpose.
Step 2: Write the reaction with water.
Ca₃P₂ reacts vigorously with water (atmospheric moisture): \[ Ca_3P_2 + 6H_2O \rightarrow 3Ca(OH)_2 + 2PH_3 \]
Step 3: Properties of phosphine (PH₃) produced.
Phosphine (PH₃) is a highly toxic, flammable gas that undergoes spontaneous ignition in air: \[ PH_3 + 2O_2 \rightarrow H_3PO_4 \] The combustion produces dense white fumes of phosphoric acid mist, creating the smoke screen effect.
Step 4: Why Ca₃P₂ is effective for smoke screens.
The spontaneous ignition of PH₃ (due to trace P₂H₄ impurity) creates immediate, dense white smoke without requiring an external ignition source. This makes Ca₃P₂ ideal for military smoke screens.
Step 5: Eliminate other options.
Other calcium compounds like CaCO₃, CaO, or Ca₃N₂ do not produce self-igniting, smoke-generating gases with moisture. Only Ca₃P₂ has this property.
Step 6: Conclusion.
Calcium phosphide (Ca₃P₂) is used in smoke screens because it reacts with moisture to produce PH₃, which spontaneously ignites to produce dense white smoke.
\[ \boxed{Ca_3P_2 \text{ (Calcium phosphide) - option 4}} \]