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Holmes signals can be given by using:
(A)- ${\text{Ca}}{{\text{C}}_{\text{2}}}{\text{ + CaC}}{{\text{N}}_{\text{2}}}$
(B)- ${\text{Ca}}{{\text{C}}_{\text{2}}}{\text{ + C}}{{\text{a}}_3}{{\text{P}}_{\text{2}}}$
(C)- ${\text{Ca}}{{\text{C}}_{\text{2}}}{\text{ + CaC}}{{\text{O}}_3}$
(D)- ${\text{C}}{{\text{a}}_3}{{\text{P}}_{\text{2}}} + {\text{CaC}}{{\text{N}}_{\text{2}}}$

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Last updated date: 18th Jun 2024
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Answer
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Hint:. Mixture which shows Holmes signals produces phosphine (${\text{P}}{{\text{H}}_{\text{3}}}$) gas and acetylene (${{\text{C}}_{\text{2}}}{{\text{H}}_2}$) gas, which is used in ships for giving signals in the form of dazzling flames.

Complete step by step answer:
Holmes signal is a type of container in which a mixture of those chemicals is present which produces phosphine and acetylene gas on reacting with water.
-In option (A) mixture of calcium carbide and calcium cyanide ${\text{Ca}}{{\text{C}}_{\text{2}}}{\text{ + CaC}}{{\text{N}}_{\text{2}}}$ is given but this mixture is not used to show Holmes signal, because in this mixture phosphorous is not present through which phosphine (${\text{P}}{{\text{H}}_{\text{3}}}$) gas produced.

-In option (B) mixture of calcium carbide and calcium phosphide ${\text{Ca}}{{\text{C}}_{\text{2}}}{\text{ + C}}{{\text{a}}_3}{{\text{P}}_{\text{2}}}$ is given and this mixture is used to show Holmes signal, because in this mixture phosphorus is present which produces phosphine (${\text{P}}{{\text{H}}_{\text{3}}}$) gas and acetylene (${{\text{C}}_{\text{2}}}{{\text{H}}_2}$) gas on reacting with water.
\[\mathop {{\text{C}}{{\text{a}}_{\text{3}}}{{\text{P}}_{\text{2}}}}\limits_{{\text{(CalciumPhosphide)}}} {\text{ + }}\mathop {{\text{6}}{{\text{H}}_{\text{2}}}{\text{O}}}\limits_{{\text{(Water)}}} \to \mathop {{\text{3Ca}}{{\left( {{\text{OH}}} \right)}_{\text{2}}}}\limits_{{\text{(CalciumHydroxide)}}} {\text{ + }}\mathop {{\text{2P}}{{\text{H}}_{\text{3}}}}\limits_{{\text{(Phosphine)}}} \]
\[\mathop {{\text{Ca}}{{\text{C}}_{\text{2}}}}\limits_{{\text{(CalciumCarbide)}}} {\text{ + }}\mathop {{\text{2}}{{\text{H}}_{\text{2}}}{\text{O}}}\limits_{{\text{(Water)}}} \to \mathop {{\text{Ca}}{{\left( {{\text{OH}}} \right)}_{\text{2}}}}\limits_{{\text{(CalciumHydroxide)}}} {\text{ + }}\mathop {{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{2}}}}\limits_{{\text{(Acetylene)}}} \]
Produced phosphine (${\text{P}}{{\text{H}}_{\text{3}}}$) gas is flammable in nature and catches fire very rapidly and due to this fire, acetylene (${{\text{C}}_{\text{2}}}{{\text{H}}_2}$) gas burns by showing blinding flame because it is also flammable in nature.

-In option (C) mixture of calcium carbide and calcium carbonate ${\text{Ca}}{{\text{C}}_{\text{2}}}{\text{ + CaC}}{{\text{O}}_3}$ is given but this mixture is not used to show Holmes signal, because in this mixture phosphorous is not present through which phosphine (${\text{P}}{{\text{H}}_{\text{3}}}$) gas produced.

-In option (D) mixture of calcium phosphide and calcium cyanide ${\text{C}}{{\text{a}}_3}{{\text{P}}_{\text{2}}} + {\text{CaC}}{{\text{N}}_{\text{2}}}$ is given but this mixture is not used to show Holmes signal, because in this mixture calcium cyanide is present which on reacting with water produces ${\text{HCN}}$ not acetylene.
Hence, Holmes signal can be given by using ${\text{Ca}}{{\text{C}}_{\text{2}}}{\text{ + C}}{{\text{a}}_3}{{\text{P}}_{\text{2}}}$.
So, the correct answer is “Option B”.

Note: Here some of you may consider option (D) is correct by only noticing phosphorus atoms but that will be wrong because calcium cyanide does not produce acetylene gas which is also used for showing signals.