On a forest campfire you observe that the Wood taken from a tree nearby initially burns with a flame...........

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On a forest campfire, you observe that the Wood taken from a tree nearby initially burns with a flame but later only glows red and gives out heat without a flame. What could be a possible reason behind this?

Volatile gases in the wood fuel burn out gradually.

The moisture content of wood reduces on burning.

It is a gradual chemical change.

The latent heat of fusion of the wood fuel reduces afterwards.

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Explanation:

It is possible that a fuel has some moisture content even before burning. Moisture content as such does not increase or reduce flame generation. So, B is wrong.
Many burning processes may be called as chemical change. Moreover, even after the flame dies out, the process of burning remains as a chemical change. So, C is incorrect.
When a substance changes phase, that is it goes from either a solid to a liquid or liquid to gas, the energy, it requires energy to do so. This energy is called as latent heat. It is fixed for a certain kind of fuel (in KJ). So, D is also wrong.
A candle or the LPG in the gas stove burns with a flame. However, you will observe the coal or charcoal in an ‘angithi’ sometimes just glows red and gives out heat without a flame.
This is because a flame is only produced when gaseous substances burn. When wood or charcoal is ignited, the volatile substances present vapourise and burn with a flame in the beginning.

? Later when the volatile substances burn out, and only the soild fuel remains, you will see only heat and red glow without flames.
? A luminous flame is seen when the atoms of a gaseous substance are heated and start to glow. The colour produced by each element is a characteristic property of that element.