Types of reaction · Process
Combustion
A collar of metal at the bottom of a burner decides whether the flame is clean and hot or dirty, cool and poisonous. Why should a hole in the side matter that much?
Start here
Two flames on the same burner. One is clean and roaring. One is lazy, yellow and quietly making a poison.
Same gas, same tap, same room. The only thing that changed was a metal collar at the bottom of the burner. Hold a beaker over the yellow one and it comes away black; hold it over the blue one and it stays clean.
What is the collar changing?
How much air — and therefore how much oxygen — reaches the gas. With plenty of oxygen every carbon atom in the fuel ends up in carbon dioxide. With too little, some of them end up as soot and some as carbon monoxide, which is colourless, has no smell, and is the reason gas boilers are serviced by law.
Combustion is a fuel reacting with oxygen and giving out energy. Nothing burns without oxygen, which is why a jar over a candle puts it out — and why a rocket leaving the atmosphere has to carry its own oxygen with it.
When there is enough oxygen for the reaction to finish, it is complete combustion. When there is not, it is incomplete combustion, and the products are different — worse in every way.
Your turn · the burner bench
Pick a fuel, set the air supply, and see what comes out
The fuel
The air supply
Before you light it: with the air hole shut, what will be in the products that was not there before?
The flame
Blue, roaring, around 1500 °C
Nothing solid in it to glow.
Energy released
All of it
The reaction finished.
Beaker held over the flame
None — the beaker stays clean
Carbon that never made it to carbon dioxide.
In the air around it
Nothing harmful from this reaction
No carbon monoxide. Every carbon atom reached carbon dioxide.
The word equation for this run
methane + oxygenmakescarbon dioxide + water
Complete combustion. Enough oxygen arrived for every atom in the fuel to finish reacting, so all of the available energy came out and nothing was left over.
The flame
Yellow, lazy, around 1000 °C
The brightness is glowing soot.
Energy released
Less than the fuel could give
Unfinished reactions release less.
Beaker held over the flame
Black deposit on the beaker within seconds
Carbon that never made it to carbon dioxide.
In the air around it
Carbon monoxide
Carbon monoxide: colourless, odourless, and it stops your blood carrying oxygen.
The word equation for this run
methane + oxygenmakescarbon monoxide + carbon + water
Incomplete combustion. The fuel still reacted and it did not finish: some carbon stopped at carbon monoxide, some never reacted at all and came out as soot, and the energy you wanted stayed locked in both of them.
The flame
Blue, roaring, around 1500 °C
Nothing solid in it to glow.
Energy released
All of it
The reaction finished.
Beaker held over the flame
None — the beaker stays clean
Carbon that never made it to carbon dioxide.
In the air around it
Nothing harmful from this reaction
No carbon monoxide. Every carbon atom reached carbon dioxide.
The word equation for this run
candle wax + oxygenmakescarbon dioxide + water
Complete combustion. Enough oxygen arrived for every atom in the fuel to finish reacting, so all of the available energy came out and nothing was left over.
The flame
Yellow, lazy, around 1000 °C
The brightness is glowing soot.
Energy released
Less than the fuel could give
Unfinished reactions release less.
Beaker held over the flame
Black deposit on the beaker within seconds
Carbon that never made it to carbon dioxide.
In the air around it
Carbon monoxide
Carbon monoxide: colourless, odourless, and it stops your blood carrying oxygen.
The word equation for this run
candle wax + oxygenmakescarbon monoxide + carbon + water
Incomplete combustion. The fuel still reacted and it did not finish: some carbon stopped at carbon monoxide, some never reacted at all and came out as soot, and the energy you wanted stayed locked in both of them.
The flame
Orange glow, hardly any flame
The solid itself is glowing, not a gas above it.
Energy released
All of it
The reaction finished.
Beaker held over the flame
None — the beaker stays clean
Carbon that never made it to carbon dioxide.
In the air around it
Nothing harmful from this reaction
No carbon monoxide. Every carbon atom reached carbon dioxide.
The word equation for this run
carbon + oxygenmakescarbon dioxide
Charcoal is mostly carbon, with a little ash and other substances in it, so the equation is written for the carbon.
Complete combustion. Enough oxygen arrived for every atom in the fuel to finish reacting, so all of the available energy came out and nothing was left over.
The flame
A dull, dark glow
Less oxygen reaching it means less of it burning.
Energy released
Less than the fuel could give
Unfinished reactions release less.
Beaker held over the flame
Black deposit on the beaker within seconds
Carbon that never made it to carbon dioxide.
In the air around it
Carbon monoxide
Carbon monoxide: colourless, odourless, and it stops your blood carrying oxygen.
The word equation for this run
carbon + oxygenmakescarbon monoxide + carbon
Charcoal is mostly carbon, with a little ash and other substances in it, so the equation is written for the carbon.
Incomplete combustion. The fuel still reacted and it did not finish: some carbon stopped at carbon monoxide, some never reacted at all and came out as soot, and the energy you wanted stayed locked in both of them.
The flame
Almost invisible, very hot
Nothing solid in it to glow.
Energy released
All of it
The reaction finished.
Beaker held over the flame
None — there is no carbon in the fuel
There is no carbon in this fuel to leave behind.
In the air around it
Nothing harmful from this reaction
No carbon monoxide is possible, whatever the air supply.
The word equation for this run
hydrogen + oxygenmakeswater
Complete combustion. Enough oxygen arrived for every atom in the fuel to finish reacting, so all of the available energy came out and nothing was left over.
The flame
Pale and quiet
Nothing solid in it to glow.
Energy released
Nearly all of it
Unfinished reactions release less.
Beaker held over the flame
None — there is no carbon in the fuel
There is no carbon in this fuel to leave behind.
In the air around it
Nothing harmful from this reaction
No carbon monoxide is possible, whatever the air supply.
The word equation for this run
hydrogen + oxygenmakeswater
Incomplete combustion, and without carbon in the fuel it looks quite different. Less oxygen still means less of the hydrogen finishes reacting, so less energy comes out — but there is no carbon to stop half way, so there is no soot and no carbon monoxide. The only product is still water.
Same fuel, both air settings. Shutting the air off did not stop the reaction and did not make it hotter — it made it unfinished. Less energy out, soot on the glass, and carbon monoxide in the room. A yellow flame is never the flame you heat something with.
Key fact
Combustion is a fuel reacting with oxygen and giving out energy. With enough oxygen the carbon in the fuel ends up as carbon dioxide and the hydrogen ends up as water. With too little, some carbon comes out as soot and some as carbon monoxide, and less energy is released.
Three fuels · predict the products
Complete combustion, plenty of oxygen. What comes out?
One rule does all three: every carbon atom in the fuel ends up in carbon dioxide, and every hydrogen atom ends up in water. Read the fuel, commit, then check.
Propane, in a camping stove
Made of carbon and hydrogen only. Burned with plenty of air.
propane + oxygenmakescarbon dioxide + water
Carbon and hydrogen in the fuel, so carbon dioxide and water out. Every hydrocarbon burned completely gives these two products and no others.
Charcoal, on a barbecue
Almost pure carbon. No hydrogen in it at all.
carbon + oxygenmakescarbon dioxide
No hydrogen means no water. One product only — which is why a charcoal barbecue in an enclosed space is so dangerous when the air runs short and that product becomes carbon monoxide instead.
Hydrogen, in a hydrogen-powered bus
No carbon anywhere in the fuel.
hydrogen + oxygenmakeswater
No carbon means no carbon dioxide and no soot. The exhaust is water. This is the reason hydrogen is interesting, and the reason the argument about it happens elsewhere — in how the hydrogen was made.
Think again
“The big yellow flame is hotter — you can see more fire.”
It is taller, brighter and looks far more like a fire. Commit before you read on.
The blue flame is the hot one — around 1500 °C at its tip, against roughly 1000 °C for the yellow. What you can see in the yellow flame is glowing soot: unburnt carbon, hot enough to give off light but not burning. The brightness is the fuel being wasted, and the soot lands on whatever you were trying to heat.
Which is why the blue flame is almost invisible. There is nothing solid left in it to glow — every carbon atom has already made it to carbon dioxide. In a flame, visible is not the same as hot. The safety flame is yellow so you can see the burner is lit; the working flame is blue because it is finishing the job.
Reference · three things a fire needs
Take away any one of them and it stops
Fuel
A firebreak clears the fuel out of the way so a wildfire has nothing to reach.
Oxygen
A fire blanket, a pan lid or a carbon dioxide extinguisher cuts the air off.
Heat
Water cools the fuel below the temperature at which it can keep going.
Never water on a chip-pan fire or on an electrical fire: hot oil throws burning droplets everywhere, and water conducts. Cutting off the oxygen is the move in both cases.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What are the products of the complete combustion of a fuel containing carbon and hydrogen?
Rung 2 · The one that catches people
A Bunsen burner is giving a tall yellow flame. What does that tell you?
Rung 3 · Explain
A gas fire is burning with a yellow flame and there is black marking on the wall above it. Explain what is going wrong chemically, why it is dangerous, and why the danger cannot be detected by smell.
Rung 4 · Take it somewhere new
A rocket has to burn its fuel hundreds of kilometres above the Earth, where there is effectively no air. A candle in a sealed jar goes out within seconds. Use the same idea to explain both, and say what a rocket must therefore carry.
Key note
Combustion is a fuel reacting with oxygen, giving out energy. Complete combustion — plenty of oxygen — turns the carbon in the fuel into carbon dioxide and the hydrogen into water, and releases all the available energy. Incomplete combustion, with too little oxygen, leaves soot and produces carbon monoxide, and releases less. A yellow flame is the visible warning; the poison is not visible at all.
Going further
Carbon monoxide is dangerous for a reason that belongs to biology, and you meet it properly in The gas exchange system. It sticks to the haemoglobin in your red blood cells far more strongly than oxygen does, and it does not let go — so the blood keeps circulating while carrying less and less oxygen. There is no smell and no taste, and the early symptoms are a headache and tiredness, which is exactly what you would ignore. A boiler burning with a yellow flame instead of a blue one is producing it, which is why the servicing is a legal requirement rather than a suggestion.
Hydrogen is the fuel that breaks the pattern: it contains no carbon at all, so complete combustion gives water and nothing else — no carbon dioxide, no soot, no carbon monoxide. That is the whole appeal of a hydrogen car. The catch is not in the burning; it is that hydrogen has to be made first, and most of it today is made from natural gas, which releases carbon dioxide somewhere else. A clean flame is not the same as a clean fuel, and knowing where to look is the difference.
Before this lesson
Next in this unit
- Symbol equations and balancing
- The gas exchange system
Where carbon monoxide does its harm.
At GCSE this becomes
- Fuels, the atmosphere and pollutants, and combustion as an exothermic reaction with a reaction profile.
Where to next
- Next: Thermal decomposition
- Previous: Symbol equations and balancing
Chemical reactions
Ask Mr Badmus AI
Still not sure why the blue flame is the hot one?
Anything held in a flame stays hot long after it stops looking it, and glass shows nothing at all — the sooted beaker is moved with tongs and left to cool. Open the collar only while you are actually heating something, and turn the burner back to the yellow flame as soon as you stop, so that everyone can see it is lit.
Lesson content © MrBadmusAI.