Complete combustion gives carbon dioxide and water

1. Burning a hydrocarbon in a plentiful supply of oxygen is complete combustion.
2. Every carbon atom ends up in carbon dioxide and every hydrogen atom in water.
3. The general pattern is: $$hydrocarbon+oxygen\to carbon\; dioxide+water$$
4. Methane burning completely gives: $${CH}_4+2O_2\to {CO}_2+2H_{2}O$$
5. Energy is given out, which is the reason hydrocarbons are burned at all.

Incomplete combustion happens in a limited supply of oxygen

1. With too little oxygen there is not enough to convert every carbon atom fully.
2. Some carbon is only partly oxidised, giving carbon monoxide, $CO$.
3. Some carbon is not oxidised at all, and appears as soot.
4. Water is still formed, because hydrogen is oxidised first.
5. Less energy is released than from complete combustion of the same fuel.

Carbon monoxide is toxic

1. Carbon monoxide is colourless and odourless, so it gives no warning of its presence.
2. Breathed in, it binds to the haemoglobin in red blood cells.
3. Haemoglobin that has bound carbon monoxide can no longer carry oxygen.
4. The blood therefore delivers less oxygen to the body's cells.
5. The effects range from headache and drowsiness to unconsciousness and death.

Problems caused in appliances that burn fuels

1. A boiler or heater starved of air produces carbon monoxide indoors.
2. Soot builds up inside the appliance and in the flue.
3. A soot deposit blocks the airway, which makes the oxygen supply worse still.
4. Soot that glows in the flame makes it yellow, which is a visible warning sign.
5. Regular servicing keeps the air supply clear and the combustion complete.