Science Puzzle
The Charcoal Lifecycle
A 5 kg log of wood burns completely on a campfire. When the fire is out, the ash weighs about 100 grams. The wood has lost almost all of its mass.
Where did the other 4.9 kg go?
The Answer
Into the air, as invisible gases. Wood is mostly water, cellulose, and lignin, compounds built from carbon, hydrogen and oxygen atoms. When wood burns, these compounds react with oxygen from the air and are converted into carbon dioxide gas (CO₂) and water vapour (H₂O), both of which are colourless gases that disperse invisibly into the atmosphere.
The ash is what remains: the minerals in the wood that were not combustible, calcium, potassium, phosphorus and similar elements, which cannot be oxidised further.
Antoine Lavoisier demonstrated in the eighteenth century that if you could collect all the gases produced by combustion and weigh them alongside the ash, they would together equal the original mass of the wood plus the oxygen consumed. Mass is conserved; it simply changes form.
This is the law of conservation of mass: in a chemical reaction, mass is neither created nor destroyed. The 4.9 kg is now floating invisibly overhead as part of the atmosphere.
The principle: Conservation of mass in combustion. Wood is mostly carbon, hydrogen and oxygen. Burning converts these into CO₂ and H₂O gases that disperse into the atmosphere. The ash is just the non-combustible mineral residue.