What Are Coal and Petroleum, Really?
Coal and petroleum are fossil fuels — and that word “fossil” tells you everything. These aren’t just rocks or liquids; they’re millions of years of compressed, transformed biological material. Dead plants, animals, and marine organisms that lived 300–400 million years ago got buried under layers of earth, and over geological time, heat and pressure converted them into energy-rich fuels.
We burn these fuels today and release energy that was originally captured from sunlight by ancient plants through photosynthesis. So in a very real sense, when you switch on a light powered by a coal plant, you’re using stored sunlight from the age of dinosaurs.
The reason this chapter matters beyond Class 8 — it’s foundational for understanding energy, environmental science, and even economics. Plus, this content has shown up consistently in CBSE board papers and is a guaranteed 3–5 mark question.
Key Terms and Definitions
Fossil Fuels Fuels formed from the remains of living organisms buried millions of years ago. Coal, petroleum, and natural gas are all fossil fuels.
Carbonisation The slow conversion of dead vegetation into coal over millions of years, under heat and pressure. The plant matter progressively loses moisture and other substances, becoming progressively carbon-rich.
Petroleum From Latin petra (rock) + oleum (oil) — literally “rock oil.” A dark, viscous liquid found trapped between layers of rock beneath the earth’s surface.
Refining / Fractional Distillation The industrial process of separating crude petroleum into useful products based on differences in boiling points.
Coke The solid residue left after destructive distillation of coal. About 90–95% pure carbon — not to be confused with the beverage.
Coal Tar A thick, black, oily liquid obtained as a by-product when coal is heated without air. Contains over 200 different chemicals, many of which are used to make dyes, medicines, and synthetic fibres.
Coal Gas A mixture of hydrogen, methane, and carbon monoxide produced during the destructive distillation of coal. Used as a fuel and in lighting (historically called “town gas”).
Petrochemicals Chemical products derived from petroleum — plastics, synthetic rubber, fertilisers, detergents, dyes.
Formation of Coal
The Process — Step by Step
Step 1: Accumulation About 300 million years ago, dense forests grew in swampy regions. When these trees and plants died, they fell into swamps and were covered by water — cutting off oxygen, which slowed down decomposition.
Step 2: Peat Formation Partially decomposed plant matter accumulated as a spongy material called peat. Peat itself is the lowest grade of coal — still used as fuel in some countries.
Step 3: Lignite Over millions of years, more sediment piled on top. Increased pressure squeezed out more moisture. Peat hardened into lignite (brown coal) — still relatively low in carbon content (~25–35%).
Step 4: Bituminous Coal Greater depth, higher pressure and temperature — lignite transformed into bituminous coal, the most commonly used type (~45–85% carbon).
Step 5: Anthracite The highest grade. Under maximum pressure and temperature, bituminous coal becomes anthracite — the hardest, shiniest coal with ~90–95% carbon content and the highest heat output.
Remember the sequence: Dead plants → Peat → Lignite → Bituminous → Anthracite. Carbon content increases at each step. This sequence is asked directly in CBSE exams.
Products of Coal (Destructive Distillation)
When coal is heated strongly in the absence of air, we get:
| Product | Description | Uses |
|---|---|---|
| Coke | Solid, nearly pure carbon | Steel manufacturing, fuel |
| Coal Tar | Black, oily liquid | Dyes, medicines, naphthalene balls, synthetic fibres |
| Coal Gas | Mixture of gases | Fuel (historically for street lighting) |
| Ammoniacal Liquor | Watery liquid | Fertilisers |
CBSE 2023 asked: “Name the product of coal used in manufacturing of steel.” Answer: Coke. Coal gas and coal tar are also common answer targets.
Formation of Petroleum
Petroleum formed from marine organisms — tiny sea plants and animals that died and sank to the ocean floor. Over millions of years, layers of sand and rock covered them. Heat and pressure, combined with bacterial action, converted these organic remains into crude oil and natural gas.
The crude oil collected in porous rocks and got trapped between non-porous rock layers. Natural gas, being lighter, floated above the oil layer. This is why petroleum wells often yield natural gas first.
Fractional Distillation of Petroleum
Crude petroleum (crude oil) is a complex mixture of hundreds of different hydrocarbons. We separate them using fractional distillation — heating the crude oil and collecting different fractions at different temperatures.
| Fraction | Boiling Range | Carbon Atoms | Uses |
|---|---|---|---|
| Petroleum Gas (LPG) | Below 40°C | C₁–C₄ | Cooking fuel |
| Petrol / Gasoline | 40–75°C | C₅–C₁₀ | Car fuel, dry cleaning |
| Naphtha | 75–120°C | C₆–C₁₀ | Making chemicals |
| Kerosene | 120–240°C | C₁₀–C₁₆ | Jet fuel, cooking |
| Diesel | 240–350°C | C₁₄–C₁₉ | Trucks, generators |
| Fuel Oil / Heavy Oil | 350–400°C | C₂₀–C₃₀ | Ship fuel, power plants |
| Lubricating Oil | Above 400°C | C₂₀+ | Machine lubrication |
| Bitumen (Asphalt) | Residue | C₃₀+ | Road making, roofing |
The key principle: lighter hydrocarbons (smaller carbon chains) have lower boiling points and are collected near the top of the fractionating column. Heavier ones are collected lower down or remain as residue.
A simple way to remember the order from top to bottom of the fractionating column: LPG, Petrol, Kerosene, Diesel, Lubricating oil, Bitumen. Lighter and more volatile fractions rise higher.
Natural Gas
Natural gas is primarily methane (CH₄) — up to 85–90% of its composition. It’s found with petroleum deposits and is now one of India’s primary cooking fuels (as CNG and PNG).
CNG (Compressed Natural Gas) is used in vehicles as a cleaner alternative to petrol and diesel. Delhi’s bus fleet switched to CNG in the early 2000s — a move driven by Supreme Court orders on air pollution.
Uses of Natural Gas:
- Cooking fuel (domestic and industrial)
- CNG in vehicles
- Making fertilisers (urea, ammonia)
- Generating electricity
- Raw material for chemicals
Solved Examples
Example 1 — CBSE Level (Easy)
Q: Why are coal and petroleum called exhaustible natural resources?
A: Coal and petroleum took millions of years to form from dead organisms. Once we use them up, they cannot be replaced in any human timescale. At current rates of consumption, coal reserves may last 200–300 more years, and petroleum reserves could run out within 50–60 years. Since their formation rate is negligible compared to our usage rate, they are exhaustible (non-renewable).
Example 2 — CBSE Level (Medium)
Q: What is coke? How is it different from coal? State two uses.
A: Coke is obtained by heating coal strongly in the absence of air (destructive distillation). Unlike coal, coke is nearly pure carbon (90–95%) and contains very little sulphur, making it ideal for specific industrial uses.
Differences:
- Coal is a natural rock; coke is an industrially processed product
- Coke has higher carbon content and burns at higher temperatures
- Coal contains impurities like sulphur; coke has very few
Uses of Coke:
- In blast furnaces to manufacture iron and steel
- As a fuel in industrial furnaces requiring very high temperatures
Example 3 — CBSE Level (Medium-Hard)
Q: Rohit’s grandmother uses wood for cooking. His mother uses LPG. Which fuel would you recommend and why? Give three scientific reasons.
A: LPG is strongly recommended over wood for cooking.
Reasons:
- Higher calorific value — LPG releases more heat per kilogram than wood, so less fuel achieves the same cooking result
- Cleaner combustion — LPG burns completely, producing mainly CO₂ and H₂O. Wood burning produces smoke, soot, and CO (carbon monoxide), which is toxic
- No deforestation — Using wood accelerates cutting of trees, destroying forests and habitats. LPG use preserves forests
- Better for health — Wood smoke causes respiratory diseases. LPG combustion is much cleaner, especially important for enclosed kitchens
- Easier temperature control — LPG flames can be precisely controlled, wood fires cannot
Exam-Specific Tips
CBSE Class 8 Board Pattern
This chapter typically carries 5–8 marks in Class 8 annual exams. Questions appear as:
- 1-mark: Name/identify questions (“Name the highest grade of coal”)
- 2-mark: Difference-based (“Difference between coal and coke”)
- 3-mark: Process explanation (“How is petroleum refined?”)
- 5-mark: Long answer (“Explain the formation of petroleum with a diagram”)
The table of petroleum fractions with their uses is a guaranteed scoring table — memorise at least 5 fractions with their boiling ranges and primary uses. Draw the table in your answer; examiners give full marks for neat, complete tables.
Common Exam Questions (PYQs Pattern)
- Why should we use fossil fuels judiciously?
- What is the difference between coke, coal tar, and coal gas?
- Arrange in order of increasing carbon content: Lignite, Anthracite, Peat, Bituminous
- Which petroleum fraction is used in jet aircraft?
- What is petrochemical? Give two examples.
Common Mistakes to Avoid
Mistake 1: Confusing Coke with Coal Students write “coal” when asked about the reducing agent in steel manufacturing. The answer is coke — it’s the processed form, not raw coal, used in blast furnaces.
Mistake 2: Wrong Order of Coal Formation Many students write Lignite → Peat instead of Peat → Lignite. Remember: Peat is first (less compressed), Lignite comes next as pressure increases.
Mistake 3: “Petroleum is formed from plants” Petroleum is formed from marine organisms (sea plants and animals). Coal is formed from land plants (forests). Don’t mix these up — the question often specifically asks which type of organisms.
Mistake 4: Saying CNG = LPG CNG (Compressed Natural Gas) is mainly methane, used in vehicles. LPG (Liquefied Petroleum Gas) is mainly propane and butane, used for cooking. Different compositions, different uses, different storage methods.
Mistake 5: Incomplete Answers on “Uses of Coal Tar” Students write just “making dyes” and lose marks. Coal tar has many applications — dyes, medicines (antiseptics), perfumes, synthetic fibres, explosives, and naphthalene balls. List at least 3 in exam answers.
Practice Questions
Q1. Name the four grades of coal in order of increasing carbon content.
Peat → Lignite (Brown Coal) → Bituminous Coal → Anthracite. Carbon content increases from roughly 10% in peat to over 90% in anthracite.
Q2. What is destructive distillation of coal? Name the products obtained.
Destructive distillation is the process of strongly heating coal in the absence of air. Products obtained are: (1) Coke — solid residue, nearly pure carbon; (2) Coal Tar — thick, black, oily liquid containing many organic chemicals; (3) Coal Gas — mixture of hydrogen, methane, and carbon monoxide; (4) Ammoniacal Liquor — used in making fertilisers.
Q3. Which fraction of petroleum is used as: (a) aircraft fuel, (b) road surfacing, (c) cooking gas at home?
(a) Aircraft fuel — Kerosene (specifically aviation kerosene / ATF — Aviation Turbine Fuel); (b) Road surfacing — Bitumen (also called asphalt); (c) Cooking gas at home — LPG (Liquefied Petroleum Gas), a fraction with mainly propane and butane.
Q4. Why is natural gas considered a cleaner fuel than coal or petroleum?
Natural gas (mainly methane, CH₄) burns more completely than coal or petroleum. It produces mainly carbon dioxide and water, with very little sulphur dioxide (which causes acid rain) or particulate matter. Coal and petroleum produce sulphur compounds, nitrogen oxides, and soot when burned. Natural gas also has higher calorific value per unit of pollution produced.
Q5. Distinguish between petrol and diesel based on: (a) composition, (b) boiling range, (c) use.
(a) Composition: Petrol contains shorter hydrocarbon chains (C₅–C₁₀); Diesel contains longer chains (C₁₄–C₁₉). (b) Boiling range: Petrol — 40–75°C; Diesel — 240–350°C. (c) Use: Petrol powers cars, motorcycles, and is used as a solvent; Diesel powers trucks, buses, generators, and agricultural equipment. Diesel engines are more fuel-efficient but heavier.
Q6. The statement says: “Coal should be used as a ‘last resort’.” Do you agree? Give two reasons.
Yes, I agree. (1) Coal is a non-renewable resource formed over millions of years — once used, it cannot be replaced. Using it as a last resort ensures our reserves last longer. (2) Coal combustion produces the most air pollution among fossil fuels, releasing CO₂ (greenhouse gas), SO₂ (acid rain), and particulate matter. Cleaner alternatives like solar, wind, and natural gas should be preferred wherever possible.
Q7. What are petrochemicals? Give four examples of everyday products made from petrochemicals.
Petrochemicals are chemicals derived from petroleum or natural gas. They form the raw materials for a huge range of products. Examples: (1) Polythene and PET plastic — packaging, bottles; (2) Nylon — clothes, ropes, toothbrushes; (3) Synthetic rubber — tyres, footwear; (4) Detergents and soaps; (5) Fertilisers (urea); (6) Medicines (aspirin and many others).
Q8. Why do we need to use fossil fuels judiciously? Suggest three alternative energy sources.
We must use fossil fuels carefully because: they are non-renewable and took millions of years to form; at current consumption rates, reserves will be exhausted within decades; their burning causes air pollution, acid rain, and climate change. Alternative energy sources: (1) Solar energy — from sunlight using photovoltaic cells; (2) Wind energy — using wind turbines; (3) Hydroelectric power — using flowing water; (4) Geothermal energy — using Earth’s internal heat; (5) Biogas — from organic waste decomposition.
Frequently Asked Questions
Q: Why is coal called a “fossil” fuel?
The word fossil comes from Latin fossilis — meaning “dug up.” Fossil fuels got their name because they’re formed from ancient organisms that have been buried and preserved, similar to how fossils of bones and shells form. The biological material was “fossilised” over geological time into carbon-rich fuel.
Q: What is the difference between CNG and LPG?
CNG is Compressed Natural Gas — mainly methane, stored under high pressure. It’s used in cars and buses (Delhi’s public transport, for example). LPG is Liquefied Petroleum Gas — mainly propane and butane, stored as a liquid under moderate pressure. Used for domestic cooking. Both are cleaner than petrol and diesel, but they’re different compounds with different storage requirements.
Q: How long will coal and petroleum last?
Current estimates suggest: petroleum reserves will last approximately 50–60 years at current consumption rates; coal reserves may last 200–300 years; natural gas around 50–60 years. These are rough estimates — new discoveries and changing consumption patterns affect them. The real answer is: not long enough, which is why renewable energy is critical.
Q: Which is better — coal or petroleum?
For most purposes, petroleum-derived fuels (petrol, diesel, LPG) are more convenient — higher energy density, cleaner burning, easier to transport. Coal is bulkier, dirtier, but is still essential for steel manufacturing (as coke) and many power plants. They serve different purposes. The real question is: how do we replace both with renewables?
Q: Why is natural gas found above petroleum in underground reservoirs?
Natural gas (mainly methane) is less dense than liquid petroleum. In underground rock formations, the lighter natural gas rises above the denser crude oil. So the typical layering from top to bottom is: cap rock (non-porous, traps everything) → natural gas layer → crude oil → water. This is why drilling a petroleum well often encounters natural gas first.
Q: What household items come from coal tar?
Many more than you’d expect: naphthalene balls (used in cupboards to repel insects), synthetic dyes (the colours in your clothes), antiseptics (like carbolic acid — found in some soaps), perfumes and flavouring compounds, aspirin and other medicines, and even saccharin (an artificial sweetener). Coal tar is one of the most chemically diverse industrial by-products we have.
Q: Is petroleum formed from dinosaur remains?
This is a popular misconception — petroleum did not form from dinosaurs. Petroleum formed mainly from microscopic marine organisms (phytoplankton, zooplankton, algae) that died and sank to the seafloor hundreds of millions of years ago. Dinosaurs appeared much later (around 230 million years ago), and petroleum formation was already well underway before they existed.