Coal, Petroleum and Combustion
Coal, Petroleum and Combustion
This topic gives 1 to 2 questions and is the one place where chemistry meets resource conservation, which makes it attractive to a teaching paper. Fuel products, the conditions for combustion and fire safety are the three examinable areas.
Fossil Fuels
Coal, petroleum and natural gas are fossil fuels, formed from the remains of organisms buried millions of years ago. They are exhaustible natural resources, meaning they cannot be replaced within a human timescale.
| Fuel | Formed from | Key products |
|---|---|---|
| Coal | Dead vegetation of ancient forests | Coke, coal tar, coal gas |
| Petroleum | Dead marine organisms | Petrol, diesel, kerosene, LPG, lubricating oil, paraffin wax |
| Natural gas | Found above petroleum | CNG, used as fuel and raw material |
The slow conversion of dead vegetation into coal is called carbonisation, and it is asked by name. Petroleum is separated into its products by fractional distillation in a refinery, which links back to the separation techniques topic.
Petroleum is called black gold because of its value. CNG is a cleaner fuel because it burns with less pollution, and it is used in vehicles and as a raw material in the chemical industry.
Combustion and Its Conditions
Combustion is a chemical process in which a substance burns in the presence of oxygen, releasing heat and usually light. Three conditions must be met together.
- Fuel — the substance that burns.
- Oxygen or air — the supporter of combustion.
- Ignition temperature — the lowest temperature at which the substance catches fire.
Ignition temperature is the concept most often tested. A substance below its ignition temperature will not burn however much oxygen is present, which is why blowing on a small flame with a strong draught can put it out by cooling.
- Rapid combustion is the burning of LPG; spontaneous combustion is phosphorus catching fire on its own; an explosion produces gas and heat suddenly.
- Inflammable substances have very low ignition temperatures — petrol, alcohol, LPG.
✗ A substance burns as long as enough oxygen is present | ✓ It must also reach its ignition temperature; oxygen alone is not enough
Fire Control and Safety
A fire is put out by removing any one of the three requirements. Water works by cooling below the ignition temperature and is the commonest extinguisher.
| Fire type | Correct extinguisher | Why |
|---|---|---|
| Ordinary solids | Water | Cools below ignition temperature |
| Oil or petrol fire | Foam or CO₂, never water | Oil floats on water and spreads the fire |
| Electrical fire | CO₂, never water | Water conducts and causes shock |
The never use water on an oil or electrical fire rule is the most important practical point in the topic, and a science teacher is expected to know it. Carbon dioxide works because it is heavier than air and cuts off the oxygen supply while also cooling.
Flame and Fuel Efficiency
A candle flame has three zones: the innermost dark zone of unburnt vapour, the middle yellow zone of partial combustion, and the outermost blue zone, which is the hottest because combustion there is complete. Goldsmiths use the outermost zone for exactly that reason.
A good fuel has a high calorific value, a moderate ignition temperature, leaves little residue and is easy to store and transport. Incomplete combustion produces carbon monoxide, a poisonous gas, which is why coal should never be burnt in a closed room.
तेल या बिजली की आग पर पानी कभी नहीं डालना चाहिए।
TRE pointer: Three items dominate: the three conditions of combustion with ignition temperature as the concept, the fire-extinguisher table with its safety rule, and the fact that the outermost blue zone of a flame is hottest. The last one is counterintuitive and therefore a good discriminator. Carbon monoxide from incomplete combustion is a public-safety point a teacher must convey, and it is asked in exactly that framing.
60-Second Recap
- Coal, petroleum and natural gas are exhaustible fossil fuels.
- Carbonisation forms coal; fractional distillation separates petroleum products.
- Combustion needs fuel, oxygen and ignition temperature together.
- Never use water on an oil or electrical fire; use foam or carbon dioxide.
- The outermost blue zone of a flame is the hottest, with complete combustion.
- Incomplete combustion produces poisonous carbon monoxide.
Frequently Asked Questions
What are the three conditions necessary for combustion?
Fuel, a supporter of combustion such as oxygen, and attainment of the ignition temperature. All three must be present together. A substance below its ignition temperature will not burn no matter how much oxygen surrounds it, which is the point most often tested.
Why should water not be used on an oil fire?
Because oil is lighter than water and floats on top, so the burning oil spreads across the water surface and the fire grows instead of dying. Foam or carbon dioxide is used instead. Water is also dangerous on electrical fires, because it conducts electricity.
Which zone of a candle flame is the hottest?
The outermost blue zone, where combustion is complete because oxygen is freely available. The innermost dark zone contains unburnt vapour and is the coolest. Goldsmiths direct the outermost zone onto metal for melting, which is the example used in textbooks.
Why is carbon monoxide dangerous?
Because it is produced by incomplete combustion, is poisonous, and has no smell to warn you. Burning coal in a closed room can release enough of it to be fatal. This is why adequate ventilation is essential wherever fuel is burnt indoors.