One of the five reactions in Sorting Five Reactions was a fuel burning, and almost every group put it in a category of its own without being able to say why. It deserves the separate treatment. Combustion is the reaction that heats most buildings in this province, moves most vehicles in it, and produces most of the carbon dioxide added to the atmosphere — and the difference between doing it well and doing it badly is a difference you can see in a flame.
Complete combustion
Burn a hydrocarbon with plenty of oxygen and every carbon atom ends up in carbon dioxide and every hydrogen atom ends up in water. Nothing else is produced, because there is nothing else in the fuel.
The flame is blue, or nearly invisible, and it is hot. It leaves no deposit on anything held in it.
Something in that equation is worth pausing on, because it surprises people every year. Burn 10.0 g of propane completely and you produce just under 30 g of carbon dioxide — three times the mass of the fuel you started with. No mass has been created. Two-thirds of the product’s mass walked in from the air as oxygen. You can work that out exactly with the methods in Stoichiometry, and doing so is the clearest possible demonstration that a balanced equation is a quantitative statement rather than a diagram.
Incomplete combustion
Restrict the oxygen and the carbon cannot all reach carbon dioxide. Hydrogen still gets its oxygen — it is more reactive towards oxygen than carbon is — so the water is produced either way. The carbon comes out part-oxidised, or not oxidised at all.
In practice a starved flame does both at once and a real one is a mixture of all three outcomes. Three things change, and all three are observable without instruments:
- The flame turns yellow and luminous. That colour is glowing solid carbon — soot particles heated until they emit light. A blue flame is gas emitting light; a yellow flame is dust emitting light.
- It deposits soot. Hold a cool white surface in it briefly and it comes out black. Hold one in a blue flame and it does not.
- It releases less energy per gram of fuel. Carbon monoxide is a fuel in its own right; producing it means you stopped part-way and left energy in the exhaust. Carbon left as soot is worse still.
On a laboratory burner, a yellow flame is what you get with the air hole closed, and it is a setting. On a furnace, a water heater, or a gas stove in a kitchen, a yellow flame is a fault, and the reason is the next section.
Carbon monoxide is why this matters
Carbon monoxide gives you no warning at all
is colourless, odourless, and tasteless. Your senses cannot detect it at any concentration, including a fatal one.
It does its damage by binding to the haemoglobin in your blood far more tightly than oxygen does. The haemoglobin is then unavailable to carry oxygen, so the body is starved of oxygen while the lungs go on working normally and the person goes on breathing comfortably. Early symptoms — headache, dizziness, nausea, confusion — are easy to mistake for influenza, and confusion is a poor state in which to decide to leave a building.
What follows from that, practically: fuel-burning appliances must be vented outdoors and serviced; never run an engine, a generator, or a fuel-burning heater in a garage or any enclosed space, even with a door open; a working carbon monoxide alarm is the only detection method that exists for a human being. In Ontario, an alarm near the sleeping areas is required by law in homes with a fuel-burning appliance or an attached garage.
Carbon monoxide is also a reason that incomplete combustion matters outdoors, where nobody is being poisoned in a room: it is one of the pollutants tracked in air quality reporting, along with the soot particles, which are small enough to reach deep into the lungs. That thread is picked up in Gases and the Atmosphere and argued about in The Air We Share.
Designing a fair comparison
The natural investigation here is to demonstrate the difference between complete and incomplete combustion. It is a good one to design yourself, because the temptation is to change everything at once and then have no grounds for saying which change caused what.
The design decisions are yours; the reasoning belongs in your journal before you light anything.
- What is your independent variable? The obvious answer is the position of the air hole on the burner. Say exactly what “open” and “closed” mean so somebody else could reproduce them.
- What will you measure or observe? Flame colour is a start but it is subjective. A soot deposit on a cool porcelain dish is physical evidence, and if you weigh the dish before and after you have a number rather than an impression.
- What must stay the same? The same burner, the same gas tap setting, the same distance from the top of the flame to the dish, the same length of time, the same kind of dish. Any one of those changing will move your result on its own.
- How will you know the difference is real? One dish against one dish proves very little. Repeat each condition and say what you would accept as a difference rather than as scatter — see Measuring Well.
- What would falsify your prediction? Write it down first. A yellow flame that leaves no deposit would be genuinely interesting.
Specific hazards for this one
Tie hair back and keep loose sleeves out of the way — a yellow flame is larger and less predictable than a blue one. Use tongs for the dish: hot porcelain looks exactly like cold porcelain and stays hot for a long time. Never lean over an open flame to look down into it. Never attempt to detect carbon monoxide by smell, and do not extend this investigation into any enclosed container. Know where the fire blanket and extinguisher are before the burner is lit, and light it only when your teacher has checked the setup.
Write the comparison up using the structure in Writing a Lab Report. The quantitative half of this — exactly how much carbon dioxide a given mass of fuel must produce — waits until Stoichiometry.
Curriculum connection
C3.2
explain the difference between a complete combustion reaction and an incomplete combustion reaction (e.g., complete and incomplete combustion of hydrocarbon fuels)
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C2.7
design an inquiry to demonstrate the difference between a complete and an incomplete combustion reaction [IP, C]
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