What process would coal undergo to give coal gas, coal tar, ammoniacal liquor and coke?

Step 1: Analyze the question The question asks which process breaks coal down into four products: coal gas, coal tar, ammoniacal liquor, and coke.

Step 2: The concept When coal is heated strongly in the absence of air (in a closed retort or oven), it does not burn. Instead, it decomposes into simpler substances. This is called destructive distillation. The word "destructive" refers to the fact that the complex coal structure is broken down (destroyed) into new products.

Step 3: Products of destructive distillation of coal

• Coal gas: a mixture of hydrogen, methane, and carbon monoxide, used as fuel.
• Coal tar: a thick black liquid, a source of benzene, phenol, naphthalene, and similar compounds.
• Ammoniacal liquor: a watery solution containing ammonia, used to make fertilizers like ammonium sulphate.
• Coke: the solid residue, almost pure carbon, used as a fuel and as a reducing agent in metallurgy (e.g. extracting iron in the blast furnace).

Step 4: Check the other options

• A. Steam distillation: separates volatile, water-insoluble substances (like essential oils) using steam. It doesn't decompose coal.
• C. Liquefaction: converts coal into liquid fuels, and doesn't give this set of four products.
• D. Hydrolysis: decomposition by reaction with water, which is not what happens here.

Step 5: Answer The process is destructive distillation, which is option B.

Exam tip: Remember "coal + heat + no air = destructive distillation", and the four products as G-T-A-C: Gas, Tar, Ammoniacal liquor, Coke. The same process applied to wood gives charcoal, wood tar, pyroligneous acid, and wood gas.