Suppose a log's mass is 5 kg. after burning, the mass of the ash is 1 kg. explain what may have happened to the other 4 kg

Diffrent things could have happened to the other 4 kg
1 burnt up in the fire
2 turned to ash and blew away

When the components of would are combusted (burnt in presence of oxygen (O), which is mostly carbohydrate (C-H-O), the C-C and C-H bond are broken, combining to make many gases. Much of this gas is carbon monoxide (CO) and carbon dioxide (CO2). These gases expand and are very light so they travel off in the smoke. So over the time that the wooden log is burnt, much of the mass in the carbons and oxygens dissipate into the air as these gases.

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