When we put water on fire, it evaporates very fast! and that evaporation stops the fire. say we put water at 10 degree C on fire and it takes 3 sec to evaporate so the fire will be cooled in 3 seconds or so. but when we put water at 50 degree C it will evaporate in 1.5 sec or so as IT IS MORE HOTTER and the fire will get blown out in 2 seconds aprox. So hot water is more effective!