Heating Coal Without Air Gives Four Useful Products at Once
Learn the amorphous forms of carbon and what each is used for, why heating coal without air is so much more valuable than burning it, and how wood, animal and sugar charcoal are prepared and what makes each special.
Why heat coal without air instead of simply burning it?
Because burning it destroys everything except the heat, while heating it without air keeps four useful products.
Burn coal and you get heat, carbon dioxide and ash — and every other substance locked in the coal is gone. Heat the same coal strongly in a closed vessel with no air, and it cannot burn. Instead it breaks down, and the pieces separate out as a solid, two liquids and a gas, each with its own industrial value.
That process is destructive distillation, and it is why coal is worth more as a raw material than as a fuel. This page covers the second part of the ICSE Class 8 Chemistry chapter on carbon: the amorphous forms, destructive distillation, and the three charcoals.
Burn coal and you get heat, carbon dioxide and ash — and every other substance locked in the coal is gone. Heat the same coal strongly in a closed vessel with no air, and it cannot burn. Instead it breaks down, and the pieces separate out as a solid, two liquids and a gas, each with its own industrial value.
That process is destructive distillation, and it is why coal is worth more as a raw material than as a fuel. This page covers the second part of the ICSE Class 8 Chemistry chapter on carbon: the amorphous forms, destructive distillation, and the three charcoals.
What are the amorphous allotropes of carbon used for?
Amorphous means the carbon atoms have no regular, repeating arrangement — unlike the crystalline diamond, graphite and fullerene of the previous part. All amorphous forms are impure and black.
Coal — formed underground from the remains of ancient plants buried under heat and pressure. Its use: a fuel, and the raw material for coke and coal gas.
Coal comes in ranks of increasing carbon content, and the higher the carbon the better the fuel:
- Peat — lowest carbon, about , smoky and poor
- Lignite or brown coal — about
- Bituminous coal — about , the commonest household and industrial coal
- Anthracite — highest carbon, about , hard, and burning almost without smoke
Coke — the grey, porous solid left when coal is destructively distilled. Its use: a reducing agent in the extraction of metals from their ores, especially iron, and a smokeless fuel.
Charcoal — made by destructive distillation of wood, bones or sugar. Its use depends on which kind, which is the third section's business; wood charcoal is used as a fuel and as an adsorbent.
Lampblack — a fine soot made by burning oils, fats or kerosene in a limited supply of air, so combustion is incomplete, and collecting the soot on a cool surface. Its use: printing ink, black paint, shoe polish, carbon paper, and as a filler in rubber tyres.
Gas carbon — a hard deposit left on the inside walls of the retort during the destructive distillation of coal. Its use: electrodes and arc-lamp rods, since it conducts electricity.
The common property of the porous ones. Coke, wood charcoal and animal charcoal are all porous, meaning riddled with tiny holes. That gives them an enormous surface area for their size, and surface area is what makes adsorption possible.
Adsorption, not absorption. A sponge absorbs water into its body. Charcoal adsorbs gas molecules onto its surface, holding them in a thin layer. The two words differ by one letter and mean different things, and examiners test the distinction.
Coal — formed underground from the remains of ancient plants buried under heat and pressure. Its use: a fuel, and the raw material for coke and coal gas.
Coal comes in ranks of increasing carbon content, and the higher the carbon the better the fuel:
- Peat — lowest carbon, about , smoky and poor
- Lignite or brown coal — about
- Bituminous coal — about , the commonest household and industrial coal
- Anthracite — highest carbon, about , hard, and burning almost without smoke
Coke — the grey, porous solid left when coal is destructively distilled. Its use: a reducing agent in the extraction of metals from their ores, especially iron, and a smokeless fuel.
Charcoal — made by destructive distillation of wood, bones or sugar. Its use depends on which kind, which is the third section's business; wood charcoal is used as a fuel and as an adsorbent.
Lampblack — a fine soot made by burning oils, fats or kerosene in a limited supply of air, so combustion is incomplete, and collecting the soot on a cool surface. Its use: printing ink, black paint, shoe polish, carbon paper, and as a filler in rubber tyres.
Gas carbon — a hard deposit left on the inside walls of the retort during the destructive distillation of coal. Its use: electrodes and arc-lamp rods, since it conducts electricity.
The common property of the porous ones. Coke, wood charcoal and animal charcoal are all porous, meaning riddled with tiny holes. That gives them an enormous surface area for their size, and surface area is what makes adsorption possible.
Adsorption, not absorption. A sponge absorbs water into its body. Charcoal adsorbs gas molecules onto its surface, holding them in a thin layer. The two words differ by one letter and mean different things, and examiners test the distinction.
What does destructive distillation of coal produce?
Destructive distillation is the heating of a substance strongly in the absence of air, so that it decomposes instead of burning.
Coal is heated in an airtight iron retort. Since no oxygen can reach it, combustion is impossible, and the coal breaks apart into four products.
1. Coke — the solid residue. A grey, porous, hard solid, nearly pure carbon, left behind in the retort. Used as a reducing agent in metallurgy and as a smokeless fuel.
2. Coal tar — a thick black liquid. It condenses out of the escaping vapours. Coal tar is itself a mixture of many compounds, separated afterwards by fractional distillation into substances used for dyes, medicines, explosives, perfumes, plastics and road surfacing. This is the most chemically valuable product.
3. Ammoniacal liquor — an aqueous liquid. Water containing dissolved ammonia, which condenses along with the tar and is separated from it. Used to manufacture ammonium sulphate, an important nitrogenous fertiliser.
4. Coal gas — the gaseous product. A mixture of hydrogen, methane and carbon monoxide, and a good fuel gas. It was once piped to houses for lighting and cooking.
How the four are separated. The vapours leaving the retort are cooled. The tar and the ammoniacal liquor condense to liquids and settle into two layers, since they do not mix — so a separating funnel parts them, exactly as in the chapter on separation techniques. What refuses to condense is the coal gas, and the coke never left the retort at all.
Why the absence of air is the whole point. Admit air and the coal simply burns: you get carbon dioxide and ash, and the tar, the ammonia and the gas are all destroyed in the flame. The airtight retort is not a detail of the apparatus — it is the reason there are products to collect.
The same process on other substances. Destructive distillation of wood gives wood charcoal, wood tar, pyroligneous acid and wood gas. Destructive distillation of bones gives animal charcoal. So the method is general, and coal is only its most important application.
Coal is heated in an airtight iron retort. Since no oxygen can reach it, combustion is impossible, and the coal breaks apart into four products.
1. Coke — the solid residue. A grey, porous, hard solid, nearly pure carbon, left behind in the retort. Used as a reducing agent in metallurgy and as a smokeless fuel.
2. Coal tar — a thick black liquid. It condenses out of the escaping vapours. Coal tar is itself a mixture of many compounds, separated afterwards by fractional distillation into substances used for dyes, medicines, explosives, perfumes, plastics and road surfacing. This is the most chemically valuable product.
3. Ammoniacal liquor — an aqueous liquid. Water containing dissolved ammonia, which condenses along with the tar and is separated from it. Used to manufacture ammonium sulphate, an important nitrogenous fertiliser.
4. Coal gas — the gaseous product. A mixture of hydrogen, methane and carbon monoxide, and a good fuel gas. It was once piped to houses for lighting and cooking.
How the four are separated. The vapours leaving the retort are cooled. The tar and the ammoniacal liquor condense to liquids and settle into two layers, since they do not mix — so a separating funnel parts them, exactly as in the chapter on separation techniques. What refuses to condense is the coal gas, and the coke never left the retort at all.
Why the absence of air is the whole point. Admit air and the coal simply burns: you get carbon dioxide and ash, and the tar, the ammonia and the gas are all destroyed in the flame. The airtight retort is not a detail of the apparatus — it is the reason there are products to collect.
The same process on other substances. Destructive distillation of wood gives wood charcoal, wood tar, pyroligneous acid and wood gas. Destructive distillation of bones gives animal charcoal. So the method is general, and coal is only its most important application.
How are the three charcoals prepared, and what is special about each?
All three are amorphous carbon, but each is made from a different starting material and each has one property the others lack.
Wood charcoal. Prepared by the destructive distillation of wood — heating wood strongly in the absence of air. Traditionally, wood is stacked, covered with earth and set to smoulder with only a tiny air supply.
Its special property is the adsorption of gases. Being highly porous it has a huge surface area, and gas molecules stick to that surface in large quantities.
This is why it is used in gas masks, where it adsorbs poisonous gases from the air being breathed in, and in water filters, where it removes dissolved gases and the substances that cause taste and smell. It is also used as a fuel — it burns hotter and with less smoke than the wood it came from — and in gunpowder.
Animal charcoal, also called bone black or bone char. Prepared by heating bones strongly in the absence of air.
Its special property is the adsorption of colouring matter. Pass a brown sugar solution through animal charcoal and it comes out colourless, the coloured impurities having stuck to the charcoal's surface.
This is its main industrial use: decolourising sugar solution in a sugar refinery to produce white sugar. It is also used to decolourise other solutions and oils.
Sugar charcoal. Prepared by heating sugar strongly in the absence of air, or by the action of concentrated sulphuric acid on sugar, which dehydrates it and leaves behind a black mass of carbon.
Its special property is that it is the purest form of amorphous carbon, containing almost no mineral impurity — because sugar itself contains only carbon, hydrogen and oxygen, and no ash-forming minerals.
It is used as a reducing agent in the laboratory, and wherever pure carbon is needed for an experiment.
The distinction that gets tested. All three adsorb, but they are asked about differently: wood charcoal for gases, animal charcoal for colour, sugar charcoal for purity. Matching the charcoal to its property is the whole of the question.
And a boundary case. Charcoal's adsorbing power can be restored after use by heating it, which drives off the adsorbed substance and frees the surface again. That reusability confirms nothing chemical happened — the gas or dye had merely been held on the surface, never combined.
Wood charcoal. Prepared by the destructive distillation of wood — heating wood strongly in the absence of air. Traditionally, wood is stacked, covered with earth and set to smoulder with only a tiny air supply.
Its special property is the adsorption of gases. Being highly porous it has a huge surface area, and gas molecules stick to that surface in large quantities.
This is why it is used in gas masks, where it adsorbs poisonous gases from the air being breathed in, and in water filters, where it removes dissolved gases and the substances that cause taste and smell. It is also used as a fuel — it burns hotter and with less smoke than the wood it came from — and in gunpowder.
Animal charcoal, also called bone black or bone char. Prepared by heating bones strongly in the absence of air.
Its special property is the adsorption of colouring matter. Pass a brown sugar solution through animal charcoal and it comes out colourless, the coloured impurities having stuck to the charcoal's surface.
This is its main industrial use: decolourising sugar solution in a sugar refinery to produce white sugar. It is also used to decolourise other solutions and oils.
Sugar charcoal. Prepared by heating sugar strongly in the absence of air, or by the action of concentrated sulphuric acid on sugar, which dehydrates it and leaves behind a black mass of carbon.
Its special property is that it is the purest form of amorphous carbon, containing almost no mineral impurity — because sugar itself contains only carbon, hydrogen and oxygen, and no ash-forming minerals.
It is used as a reducing agent in the laboratory, and wherever pure carbon is needed for an experiment.
The distinction that gets tested. All three adsorb, but they are asked about differently: wood charcoal for gases, animal charcoal for colour, sugar charcoal for purity. Matching the charcoal to its property is the whole of the question.
And a boundary case. Charcoal's adsorbing power can be restored after use by heating it, which drives off the adsorbed substance and frees the surface again. That reusability confirms nothing chemical happened — the gas or dye had merely been held on the surface, never combined.
Exam tip
Exam tip: write in the absence of air every time
The phrase in the absence of air is the definition of destructive distillation and is worth its own mark. Leave it out and the answer describes burning instead.
Name all four products of coal's destructive distillation — coke, coal tar, ammoniacal liquor and coal gas — and give one use of each. Listing three is a common and costly omission.
Use adsorption and not absorption for charcoal, and say the substance is held on the surface.
Match each charcoal to its property: wood charcoal adsorbs gases, animal charcoal adsorbs colouring matter, sugar charcoal is the purest form. These are three separate one-mark answers and they are easy to swap.
Say charcoal is porous and has a large surface area as the reason it adsorbs at all.
Name the coal ranks in increasing carbon order — peat, lignite, bituminous, anthracite — and say the higher the carbon the better the fuel.
For lampblack, state that it is made by burning oil in a limited supply of air, so combustion is incomplete.
And distinguish amorphous from crystalline: amorphous forms have no regular arrangement and are impure and black, while diamond, graphite and fullerene are crystalline.
Name all four products of coal's destructive distillation — coke, coal tar, ammoniacal liquor and coal gas — and give one use of each. Listing three is a common and costly omission.
Use adsorption and not absorption for charcoal, and say the substance is held on the surface.
Match each charcoal to its property: wood charcoal adsorbs gases, animal charcoal adsorbs colouring matter, sugar charcoal is the purest form. These are three separate one-mark answers and they are easy to swap.
Say charcoal is porous and has a large surface area as the reason it adsorbs at all.
Name the coal ranks in increasing carbon order — peat, lignite, bituminous, anthracite — and say the higher the carbon the better the fuel.
For lampblack, state that it is made by burning oil in a limited supply of air, so combustion is incomplete.
And distinguish amorphous from crystalline: amorphous forms have no regular arrangement and are impure and black, while diamond, graphite and fullerene are crystalline.
Did you know
Why does a charcoal filter make water taste better without removing any solid?
Water that has been filtered through cloth is perfectly clear and can still taste and smell unpleasant. Pass the same water through charcoal and the taste goes, though nothing visible has been taken out.
A cloth filter works by particle size — it stops what is too big to pass. The substances that cause taste and smell are dissolved, so they slip through any filter paper as easily as the water does.
Charcoal works on an entirely different principle. Its surface is riddled with pores, and dissolved molecules that happen to stick well to carbon are adsorbed onto that surface as the water flows past. Nothing is strained out; the unwanted molecules are simply held back while the water moves on.
This is why a charcoal filter eventually stops working and has to be replaced. Its pores fill up, the surface is fully occupied, and there is nowhere left for the next molecule to stick — a limit that a sieve-type filter never runs into in the same way.
A cloth filter works by particle size — it stops what is too big to pass. The substances that cause taste and smell are dissolved, so they slip through any filter paper as easily as the water does.
Charcoal works on an entirely different principle. Its surface is riddled with pores, and dissolved molecules that happen to stick well to carbon are adsorbed onto that surface as the water flows past. Nothing is strained out; the unwanted molecules are simply held back while the water moves on.
This is why a charcoal filter eventually stops working and has to be replaced. Its pores fill up, the surface is fully occupied, and there is nowhere left for the next molecule to stick — a limit that a sieve-type filter never runs into in the same way.
Key takeaways
Amorphous carbon and destructive distillation: quick revision
- Amorphous carbon has no regular arrangement of atoms, and all its forms are impure and black.
- Coal — a fuel and the source of coke and coal gas. Ranks by rising carbon: peat (about ), lignite (about ), bituminous (about ), anthracite (about , nearly smokeless).
- Coke — grey, porous, nearly pure carbon; a reducing agent in metallurgy and a smokeless fuel.
- Charcoal — from wood, bones or sugar; a fuel and an adsorbent.
- Lampblack — soot from burning oil in a limited air supply; printing ink, black paint, shoe polish, carbon paper, tyre filler.
- Gas carbon — deposited on retort walls; used for electrodes.
- Coke and the charcoals are porous, giving a large surface area — hence adsorption, which holds molecules on the surface, unlike absorption.
- Destructive distillation is heating strongly in the absence of air, so the substance decomposes instead of burning.
- Coal gives four products: coke (solid residue), coal tar (thick black liquid, split later by fractional distillation into dyes, medicines, explosives, perfumes and plastics), ammoniacal liquor (for ammonium sulphate fertiliser) and coal gas (hydrogen, methane and carbon monoxide, a fuel).
- Tar and ammoniacal liquor are immiscible and parted with a separating funnel.
- Wood charcoal — destructive distillation of wood; adsorbs gases, so gas masks and water filters.
- Animal charcoal — heating bones without air; adsorbs colouring matter, so decolourises sugar solution.
- Sugar charcoal — heating sugar, or concentrated sulphuric acid on sugar; the purest amorphous carbon, used as a reducing agent.
- Heating used charcoal restores its adsorbing power, showing nothing chemical occurred.
Test yourself by naming the four products of coal with a use for each, then matching the three charcoals to their properties — those two lists carry most of the marks in this chapter.
- Coal — a fuel and the source of coke and coal gas. Ranks by rising carbon: peat (about ), lignite (about ), bituminous (about ), anthracite (about , nearly smokeless).
- Coke — grey, porous, nearly pure carbon; a reducing agent in metallurgy and a smokeless fuel.
- Charcoal — from wood, bones or sugar; a fuel and an adsorbent.
- Lampblack — soot from burning oil in a limited air supply; printing ink, black paint, shoe polish, carbon paper, tyre filler.
- Gas carbon — deposited on retort walls; used for electrodes.
- Coke and the charcoals are porous, giving a large surface area — hence adsorption, which holds molecules on the surface, unlike absorption.
- Destructive distillation is heating strongly in the absence of air, so the substance decomposes instead of burning.
- Coal gives four products: coke (solid residue), coal tar (thick black liquid, split later by fractional distillation into dyes, medicines, explosives, perfumes and plastics), ammoniacal liquor (for ammonium sulphate fertiliser) and coal gas (hydrogen, methane and carbon monoxide, a fuel).
- Tar and ammoniacal liquor are immiscible and parted with a separating funnel.
- Wood charcoal — destructive distillation of wood; adsorbs gases, so gas masks and water filters.
- Animal charcoal — heating bones without air; adsorbs colouring matter, so decolourises sugar solution.
- Sugar charcoal — heating sugar, or concentrated sulphuric acid on sugar; the purest amorphous carbon, used as a reducing agent.
- Heating used charcoal restores its adsorbing power, showing nothing chemical occurred.
Test yourself by naming the four products of coal with a use for each, then matching the three charcoals to their properties — those two lists carry most of the marks in this chapter.