Fire - combustion or burning, in which substances combine chemically with oxygen from the air and typically give out bright light, heat, and smoke.
Fire is the visible result of the chemical reaction called combustion. For combustion to occur, the following reagents must be present: diverse types of fuels and oxygen, but there will be no fire if heat is not present. Therefore, the heat will bring the fuel to it's combustion temperature and the fire will go on if the amounts of heat, fuel and oxygen are adequately.
Since combustion happens only between gases, the solid or liquid fuel is heated till gases will be released from it's surface.
The visible feature that prove that the combustion is complete is the color of the flame, which has to be blue, while, in case of incomplete combustion, the flame is yellowish.