Why does burning wood produce ash?

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Why not just a block of coal instead of powder?

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The ash is made of the particles that did not turn into a gas after they burned. The basic structure of wood is mostly cellulose, which turns into carbon dioxide and water vapor when burned completely. The particles that become ash are largely not directly connected to each other when they are in wood.

Wood is made mostly of hydrocarbons. Long chains of hydrogen and carbon, with some nitrogen and oxygen on there too. Then there’s water. These all become a gas during the burning of the wood.

Then there are the minerals. Not much of the wood’s weight is minerals, but they’re important to the tree. As the tree burns, the minerals generally do not become a gas. For instance, the potassium forms potassium oxide. The phosphorous, phosphorous pentoxide. These form your ash.

So the order goes:

Wood burns by releasing the non-carbon part of hydrocarbons. This makes a billowing flame.

The leftover carbon, the coals, burn at their surface. They do not create a billowing flame but instead glow.

Finally, as the coals slowly vanish, you’re left with ash.