Question

Which of the following is used as a fire extinguisher, where water cannot be used?

• A. Nitrogen
• B. Carbon dioxide
• C. Hydrogen
• D. Sulphur dioxide

Hint:

Remember the fire triangle: Fuel + Oxygen + Heat. CO2 extinguishes fires primarily by excluding oxygen (smothering). It is the safest option for electrical and chemical fires because it is non-conductive and leaves no residue. Never use water on electrical fires (electrocution risk) or oil fires (spreading risk).

Answer 1

The Correct Option is B

Concept:
For a fire to burn, three elements must be present simultaneously --- this is called the

fire triangle: fuel, heat, and oxygen. A fire extinguisher works by removing one or more of these elements. Water is commonly used to extinguish fires, but it cannot be used when:

• Electrical fires are involved (water conducts electricity and could cause electrocution).

• Flammable liquid/chemical fires are involved (water can spread the burning liquid).
Step 1: Why CO2 is ideal as a fire extinguisher.
Carbon dioxide (CO2) extinguishes fires by two mechanisms:

• Smothering (oxygen displacement): CO2 is denser than air and settles around the fire, displacing oxygen and cutting off the supply needed for combustion.

• Cooling: When CO2 is released from a cylinder, it expands rapidly and cools the surrounding area, lowering the temperature of the fuel below its ignition point.
Step 2: Why CO2 is safe where water cannot be used.

• CO2 is a

non-conductor of electricity, making it safe for electrical fires.

• It does not react with burning chemicals or liquids.

• It leaves no residue (unlike foam or powder extinguishers).

Step 3: Analyse the other options.

• (A) Nitrogen: While nitrogen can in theory displace oxygen and extinguish fires, it is not commonly packaged as a fire extinguisher agent. Industrial use exists in special cases, but CO2 is far more widely used for the purpose described.

• (C) Hydrogen: Hydrogen is extremely flammable (it has a very wide flammability range of 4%--75% in air). Using hydrogen near a fire would be catastrophically dangerous.

• (D) Sulphur dioxide: SO2 is toxic, corrosive, and not used as a fire extinguishing agent. It is a harmful air pollutant.

Step 4: Real-world application.
CO2 fire extinguishers are the standard choice for:

• Electrical fires (server rooms, electrical panels).

• Flammable liquid fires (class B fires: petrol, oil, solvents).

• Situations where residue from other extinguishers would cause damage.