Curious Kids: why does wood crackle in a fire?

To know why this happens, we need to understand what happens when you put a wooden log on the fire. First, the wood starts to get warmer. Inside the wood are pockets of trapped water and tree sap, which is the goo you sometimes see on trees.

Likewise the water in a kettle heats up and turns into steam, as does the water trapped inside the log. Then, when the fire gets hotter, the water and lymph inside begin to boil and turn into gas. As the fire gets even hotter, these gases begin to take up more space and expand (enlarge).

Gary Chartrand

How do gases explode?
As the water and sap are transformed into steam, something also happens to the wood. Wood contains something called cellulose, which is the material that plants are mainly made of.

When cellulose is heated, it begins to break down or “decompose”. If you’ve ever left an apple in your lunch box over the weekend and it turns brown and gross, it means it’s rotted. When something in nature (like a fruit) decomposes, it changes.

Read more: Curious children: When I slide a match, how does it make a fire?

When the wood in the fire gets hot enough, the cellulose inside begins to turn into gas. This is when we see smoke coming out of the wood, sometimes even before that piece of wood has caught fire.

Flames occur when the gas escaping from the wood begins to mix with the oxygen in the air. Oxygen is like food for fires: it makes them burn really intensely.

As the wood burns, the mix of expanding gas and disintegrating cellulose causes the pockets of vapor trapped by the wood to explode, one by one. This is why you hear crackling and crackling noises.

So the more water and sap there is in the wood, the louder the fire will be. If you’ve ever put wet wood on a fire, you may have noticed that it makes a lot more noise than really dry wood.

How does wood get water into it?
But in the first place, how do water and sap get into the wood?

Well, wood isn’t as solid as it looks. It has many small holes, too small for our eyes to see, and these holes contain water and sap.

We know that wood comes from trees. And when the trees are alive, they keep themselves healthy by carrying water along the trunk through these tiny holes, called xylem vessels. When the tree is felled to produce firewood, there is still water trapped inside these xylem vessels.

There are other ways that water can enter wood. If the wood is left out in the rain, it can absorb water in this way. Or sometimes insects make small holes in the wood, which let water in.
Sitting in front of a fire and watching the flames and listening to the crackle and pop of wood can be fun. Most of the time the mini bursts of escaping steam are small.

But sometimes they can be large and may even cause small pieces of burning wood to fly off the fire! This is why it is important to always keep a safe distance from the fire or use a fire extinguishing device.

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