Question

Water and kerosene oil both are liquid but kerosene catch fire easily ,why?

Answer 1

Water can also catch fire! All we need is a more powerful oxidant like fluorine. Only thing is fluorine is not readily available in its elemental form. 

Kerosene and Petroleum catch fire when they come in contact with oxygen, where oxygen acts as an oxidizing agent. The product of this combustion(kerosene + oxygen) can be further oxidized in the presence of more powerful agent like fluorine. 

The technical name for catching fire or burning is combustion. Combustion is a process where a fuel is oxidized. And combustion process means: 

fuel + oxidant => product 
the above product + more powerful oxidant => second product 
the second product + yet more powerful oxidant => third product, and so on. 

So, if we can provide a more powerful oxidant water can also catch 'fire'...

Answer 2

Kerosene has low ignition level whereas water has slightly higher level than it.