Question

all fuels are combustible substances but all combustible sustances are not fuels..justify with an eg.

Answer 1

There are a lot of substances that are combustible, but not used as fuels. For use as fuel, combustion must be easily controllable. Certain elements combust explosively, making their usage as fuels dangerous. Others, produce highly reactive oxides(or other by-products), which react with the surroundings, corroding them. Such substances are not used as fuels.

Nuclear fuels do not produce energy from combustion, but are fuels nonetheless

Answer 2

a chemical process in which a substance reacts with oxygen to give off heat is called combustion. A substance that undergoes combustion is called a combustible substance It is also called a fuel. A paper is not fuel but it's combustible.