In welding and cutting of metals, the organic gas commonly used in the heating process is

Step 1: Analyze the question The question asks which organic gas is used for heating in welding and cutting metals. The key idea is a gas that burns with a very hot flame.

Step 2: The concept Ethyne (acetylene, C₂H₂) is an alkyne with a carbon-carbon triple bond. When it burns in pure oxygen, it produces the oxy-acetylene flame, which reaches about 3,000 °C or more. This is hotter than the flame from any other common hydrocarbon fuel gas.

The combustion reaction is:

2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O + a lot of heat

Why ethyne gives such a hot flame:

• It has a high carbon-to-hydrogen ratio and a strong triple bond.
• Its formation is endothermic, so breaking it down and burning it releases a large amount of energy.
• That energy is concentrated in a small, very hot flame, which is enough to melt and cut steel.

Step 3: Check the options

• A. Methane is the main component of natural gas and is used for cooking and domestic heating. Its flame is not hot enough for welding.
• B. Propene is mainly used to make polypropylene, not as a welding fuel.
• C. Butane is used in cooking gas and lighters. Its flame is cooler than ethyne's.
• D. Ethyne is the standard fuel for oxy-acetylene welding and cutting.

Step 4: Answer The correct option is D. ethyne.

Exam tip: Remember "Welding = Alkyne". Oxy-acetylene torch means ethyne (acetylene). It is also made in the lab and in industry from calcium carbide and water: CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂. JAMB often asks about this reaction too.