Question

What is the reaction involved in Holmes signal?

(a) Ca3P2 + 6H2O → 2PH3 + 3Ca(OH) 2

(b) Ca3P2 + 6H2O → 2PH3 → P2H4 + P4O6 and CaC2 + H2O → C2H2

(c) Ca3P2 + 6HCl → 2PH3 + 3CaCl2

(d) P4 + 3NaOH + 3H2O → PH3 + 3NaH2PO2

I got this question in exam.

This key question is from P-Block Elements in portion P-Block Elements of Chemistry – Class 12

Answer 1

Right option is (b) Ca3P2 + 6H2O → 2PH3 → P2H4 + P4O6 and CaC2 + H2O → C2H2

Easiest explanation: The reactions involved in Holmes signal are:

Ca3P2 + 6H2O → 2PH3 → P2H4 + P4O6 and CaC2 + H2O → C2H2

Calcium carbide and calcium phosphide both react with water to give acetylene and phosphine respectively. These gases burn to give the signal.