Why does a fire smoke?

Burning

We’ve all see smoke rising from a bonfire or the chimneys of factories and houses.

When a log in a fireplace burns, some of the burning wood is changed into invisible gases.  These hot gases, which consist mostly of water vapor and carbon dioxide.  Are very light.  They rise up the chimney and drift away with the air.

If burning were complete we could not see the hot gases, but usually many tiny bits of ash and black specks of unburned fuel (know as soot) are also carried away in the rising clouds of gases.  The soot and ash color the gases gray or almost black and make them visible in the form of smoke.  Some soot sticks to the flue of the chimney, blackening it.

The chimney does more than just carry off the smoke.  As the column of hot gases rises up the chimney it causes a draft that draws air into the fire.  This causes the fire to burn hotter and smoke less.

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