Question

It is difficult to burn a heap of green leaves, but dry leaves catch fire easily. Explain.

Answer 1

A substance which has a presence of moisture content in it does not catch fire easily. This means that the moisture present in it would not allow the substance to catch fire easily and will take a lot of time to burn.

Complete answer:
A heap of green leaves have chlorophyll and moisture content in it which will not allow the leaf to catch fire. This moisture also tries to prevent the leaf to attain its ignition temperature. Ignition temperature is the temperature at which a substance starts to burn. Although in the atmosphere, there is a presence of a huge amount of oxygen which is a supporter of combustion, but the substance will burn only when all the moisture is lost from it by the strong heating. The cause of burning after the loss of moisture is because of the breaking of hydrogen bonds present due to the water molecules in the green leaves.
In the case of dry leaves, no moisture content is present and thus, it catches fire easily and rapidly. The green leaves become dry when all the moisture content is lost from it. So, the dry leftover part of the leaf has stored fuel energy in it and which is released in the form of heat when it catches fire.

Note:
The common mistake that a student can commit in this question is mentioning the opposite statement, i.e. the green leaves catch fire easily and the dry leaves don’t. Along with that, one must remember that it is the moisture and not the other pigments in the leaves that prevent it from catching fire.