Clean and sooty flames

The idea

Saturated hydrocarbons generally burn with a clean flame and unsaturated compounds with a yellow flame and black smoke; a limited air supply causes incomplete combustion, giving a sooty flame even with saturated hydrocarbons.

Blackened vessels

A cooking vessel is put on a gas stove. After some time the bottom of the vessel is black with soot.

What does this tell us about the air holes of the stove?

• They are blocked
• They are wide open
• The air holes do not matter

Fuel and air

Choose the type of fuel and switch the air supply. A teacher does such burning experiments in the lab.

Two causes of soot

A sooty flame has two causes: the fuel is unsaturated, or the air supply is limited so the burning is incomplete.

Saturated hydrocarbons generally burn with a clean flame and unsaturated compounds with a yellow flame and black smoke. A limited air supply causes incomplete combustion, giving a sooty flame even with saturated hydrocarbons.

Notes

Saturated hydrocarbons generally burn with a clean flame; unsaturated compounds with a yellow flame and black smoke. A limited air supply causes incomplete combustion and a sooty flame even with saturated hydrocarbons.

Check yourself

How do saturated hydrocarbons generally burn, with enough air?

How do unsaturated carbon compounds burn?

A saturated hydrocarbon burns with a limited supply of air. What happens?

The bottoms of cooking vessels are getting blackened. What does it mean?

• With a yellow flame and lots of black smoke. That is how unsaturated compounds burn.
• With a clean flame — correct. Yes!
• They do not burn. Saturated hydrocarbons burn, and generally give a clean flame.
• With a clean flame. Saturated hydrocarbons generally give a clean flame. Unsaturated compounds give a yellow flame with black smoke.
• Without any flame. Unsaturated compounds give a yellow flame with lots of black smoke.
• With a yellow flame and lots of black smoke — correct. Yes!
• Incomplete combustion gives a sooty flame — correct. Yes!
• It burns with a clean flame. A limited supply of air results in incomplete combustion, even for saturated hydrocarbons.
• It stops being a saturated hydrocarbon. The compound is still saturated. The sooty flame is caused by incomplete combustion.
• There is too much air. Blackening means the air supply is limited, so the burning is incomplete.
• The air holes are blocked and fuel is getting wasted — correct. Yes!
• The fuel is burning completely. Complete burning gives a clean blue flame. A black deposit shows incomplete burning.