Question

Does Holme's signal use chemical compounds?
(a) calcium carbide
(b) calcium phosphide
(c) calcium carbide and calcium phosphide
(d) calcium carbide and aluminum carbide.

Answer 1

Hint: Holmes signal is an age-old technique used by sailors for making emergency lighting on the sea surface during night. It was used as a method to guide the ships in the sea where no light sources are available.

Complete step by step solution:
Holme's signals technically use the spontaneous combustion of phosphine, which gives a bright light. Calcium carbide (\[Ca{{C}_{2}}\]) and calcium phosphide (\[C{{a}_{3}}{{P}_{2}}\]) are placed in containers pierced at top and bottom, and thrown into the sea. When these two chemicals react with sea water, gases are evolved which burn and serve as a signal.

Containers which are used have a perforated bottom and a hole at the top. These containers are filled with calcium phosphide and calcium carbide. When thrown into the sea, water enters the containers through the bottom and reacts with the calcium carbide and calcium phosphide present inside. As a result, it gives acetylene (\[{{C}_{2}}{{H}_{2}}\]) and phosphine (\[P{{H}_{3}}\]). When phosphine comes in contact with air, it gets ignited spontaneously and also ignites acetylene. The reaction can be given as:
\[\begin{align}
  & Ca{{C}_{2}}+2{{H}_{2}}O\to Ca{{\left( OH \right)}_{2}}+{{C}_{2}}{{H}_{2}} \\
 & C{{a}_{3}}{{P}_{2}}+6{{H}_{2}}O\to 3Ca{{\left( OH \right)}_{2}}+2P{{H}_{3}}\,or\,{{P}_{2}}{{H}_{4}} \\
\end{align}\]

Therefore, a bright red flame is produced with lots of smoke due to the burning of phosphine, which serves as a signal to the approaching ships.

So, the correct option is (c).

Note: \[{{P}_{2}}{{H}_{4}}\] is also formed as a byproduct of the reaction of calcium phosphide with water. Phosphine reacts with acetylene and catches fire to get smoke which forms the smoke screen, often used in warfare also.