Question

Why is it more difficult to burn some combustible substances than other??

Answer 1

Combustible substances are difficult to burn because they are highly reactive. they can easily catch fire and in severe cases they can even cause explosion. Moreover, very strict precautionary measures are required to tackle and burn combustible substances and; The conditions in which they are to be burnt must be kept in mind. Like the person must be aware of the ignition temperature or the temperature of the environment. 

I hope it helps.

Answer 2

Some substances have a lower combustion point and therefore they catch fire much more easily and faster as compared to other substances. The temperature at which a substance catches fire is the most important parameter in determining the combustibility of different substances.