Question

Holmes signals can be given by using:
(A) $$Ca{C}_2+Ca(CN{)}_2$$
(B) $$Ca{C}_2+C{a}_3{P}_2$$
(C) $$Ca{C}_2+CaC{O}_3$$
(D) $$C{a}_3{P}_2+Ca(CN{)}_2$$

Answer 1

Hint: Holmes signal contains a mixture of compounds in which react with water to give fumes. One of the compounds formed during reaction with water directly ignites and another compound has a flammable characteristic.


Complete step by step solution:
- Holmes signal is a bright red colored signal shown on the ocean surface.
- To give a Holmes signal, a container is used which is filled up with a mixture of two compounds which react with water to give a signal.
- Calcium carbide and Calcium phosphide reacts with water as follows.
 $$Ca{C}_2+2H_{2}O\to C_2{H}_2+Ca(OH{)}_2$$
$$C{a}_3{P}_2+6H_{2}O\to 2P{H}_3+3Ca(OH{)}_2$$

- Here Acetylene is produced by reaction of Calcium carbide with water and Calcium phosphide gives Phosphine when reacted with water.
- Now, Phosphine gets ignited immediately when it comes in contact with air and Acetylene produced by Calcium phosphide is flammable and also burns. So, we get bright red colored flames which are also followed by fumes. Hence, their mixture is used as a Holmes signal.
- Calcium cyanide reacts with water to give ammonia which is neither flammable nor visible from a distance. So, it can’t be used to give signals. Hence option (A) and (D) are not correct. Calcium carbonate does not react with water and so it also does not have any importance in giving any signal. So, option (C) is also not correct.

So, the correct answer is (B) $$Ca{C}_2+C{a}_3{P}_2$$.

Additional Information:
To give a Holmes signal, a container filled with a mixture of Calcium carbide and Calcium phosphide is thrown out into the ocean. The container has a hole on its surface and so, water from the ocean can react with the contents of the container. This produces a signal that can be watched from distance clearly.

Note: Note that ammonia is not a flammable gas and it is colorless. Although Phosphine is a very poisonous gas, it is produced at the time of giving Holmes signal. Calcium carbide and Calcium phosphide do not react with each other.