NDA (II) 2025 General Ability Test · Question 109 of 147

Correct — D, carbon monoxide plus hydrogen. Water gas is made by blowing steam over white-hot coke, C + H₂O → CO + H₂, giving a mixture of about equal volumes of carbon monoxide and hydrogen. Both components burn, so the mixture is a useful industrial fuel and also a feedstock (synthesis gas) for making hydrogen, methanol and ammonia. The name comes from the steam (water) used to make it, not from any water in the gas itself.

• (a)H₂O + CO₂ — Water vapour and carbon dioxide are the fully oxidised products of burning a fuel; they will not burn further, so they cannot be a fuel gas. Water gas contains neither as a defining component.
• (b)NH₃ — Ammonia is a single compound made in the Haber process, used mainly for fertilisers; it is not a mixture and is not water gas.
• (c)H₂ + N₂ — A hydrogen and nitrogen mixture is the feed for ammonia synthesis, not water gas. A nitrogen-diluted fuel gas would resemble producer gas (carbon monoxide with nitrogen), which is a different product altogether.

Water gas is one of the industrial fuel gases obtained from coke. It is produced by passing steam over coke heated to white heat, and consists chiefly of carbon monoxide and hydrogen in roughly equal proportion. Because both gases are combustible, the mixture serves as a fuel and, as synthesis gas, as a starting material for many chemicals.

The common confusions are with the other coke-derived gases. Producer gas, made by passing air (not steam) over hot coke, is carbon monoxide diluted with nitrogen. Coal gas, obtained by destructive distillation of coal, is mainly hydrogen with methane and carbon monoxide. Natural gas is largely methane. Knowing which gas is made from steam versus air, and which contains nitrogen, is enough to separate them.

• Water gas is carbon monoxide plus hydrogen, made by passing steam over red-hot coke.
• The making reaction C + H₂O → CO + H₂ is endothermic, so the coke must be reheated from time to time.
• Producer gas is different — carbon monoxide diluted with nitrogen, made by passing air over hot coke.
• Water gas is also called synthesis gas (syngas) and is used to manufacture hydrogen, methanol and ammonia feedstock.

• Do not confuse water gas (carbon monoxide + hydrogen) with producer gas (carbon monoxide + nitrogen).
• Water gas contains no water; the name refers to the steam used to make it.

Asked for the composition of water gas, producer gas or coal gas, or as a match-the-fuel-gas-with-its-constituents item.

• practice — not a real PYQ

  Producer gas is mainly a mixture of

  • (a)carbon monoxide and hydrogen
  • (b)carbon monoxide and nitrogen
  • (c)hydrogen and nitrogen
  • (d)methane and hydrogen

  Answer(b) carbon monoxide and nitrogen — made by passing air over hot coke.

• practice — not a real PYQ

  Water gas is manufactured by passing which of the following over red-hot coke?

  • (a)Air
  • (b)Steam
  • (c)Carbon dioxide
  • (d)Nitrogen

  Answer(b) Steam — C + H₂O → CO + H₂ gives the carbon monoxide and hydrogen mixture called water gas.