Question
What is destructive distillation of coal? Name the main products obtained and state their important uses.
Solution — Step by Step
Destructive distillation is the process of heating coal in the absence of air (oxygen) at very high temperatures (around 1000°C). Because there is no oxygen, coal does not burn — instead, it decomposes into several useful products. The word “destructive” refers to the fact that coal is chemically broken down (destroyed) rather than simply separated.
This process is also called carbonisation and has been industrially important since the 19th century.
Heating coal in the absence of air gives four distinct products:
- Coke — the solid residue remaining in the vessel after distillation
- Coal tar — a thick, dark, oily liquid that condenses from the volatile gases
- Ammoniacal liquor (coal liquor) — aqueous liquid containing ammonia
- Coal gas — the remaining gaseous mixture that passes off as vapour
Coke is almost pure carbon — impurities have been driven off. Its uses:
- Metallurgy: used as a reducing agent and fuel in blast furnaces to extract iron from iron ore (most important industrial use)
- Fuel: burns at very high temperatures — used in industrial furnaces
- Making water gas and producer gas: coke reacts with steam or air to form these fuel gases
Coal tar is a complex mixture of over 200 organic compounds including benzene, toluene, phenol, naphthalene, and anthracene. Its uses:
- Raw material for synthetic dyes, drugs, explosives, perfumes, and plastics — many early synthetic organic chemicals came from coal tar fractions
- Road surfacing (bitumen is derived from it)
- Naphthalene balls (moth repellent) come from coal tar
- Antiseptics: carbolic acid (phenol) obtained from coal tar was the first surgical antiseptic used by Lister
Coal gas is a mixture of hydrogen, methane, and carbon monoxide. Historically it was used as a fuel for street lighting (before natural gas) and in homes. It has a high calorific value.
Ammoniacal liquor contains dissolved ammonia and ammonium salts. It is used as a source of ammonia for manufacturing fertilisers (ammonium sulphate).
Why This Works
Coal is a complex organic material — mainly carbon, with hydrogen, nitrogen, oxygen, and sulphur atoms in various compounds. When heated strongly in the absence of air, the weaker chemical bonds break. Volatile compounds (gases, oils, water) escape from the solid mass. The remaining solid (coke) is mostly carbon because C–C bonds and graphite-like structures are very stable at high temperatures.
The absence of air is critical — with air present, coal simply combusts to CO₂ and water, giving no useful products. The closed vessel traps all volatiles and allows them to be condensed and collected separately.
Alternative Method — Classification by State
You can organise the products by physical state at room temperature:
- Solid: Coke
- Liquid: Coal tar, ammoniacal liquor
- Gas: Coal gas
This classification helps in exam answers — state each product, its state, and one key use in three clean rows.
Common Mistake
Students often confuse coke (from coal, nearly pure carbon) with charcoal (from wood, also nearly pure carbon) and coal itself (raw fossil fuel, not pure carbon). In exam answers, be specific: coke is the product of destructive distillation of coal.
Also, do not write that coal tar is used directly as fuel — it is primarily a source of organic chemicals. Coal gas is the fuel among the products. Mixing up coal tar and coal gas uses is a common 1-mark error.
For board exams, remember this mnemonic for the four products: C-C-A-G — Coke, Coal tar, Ammoniacal liquor, Gas. And for each: Coke → blast furnace; Coal tar → dyes/drugs/naphthalene; Ammoniacal liquor → fertilisers; Coal gas → fuel.