How does a fire happen?

Answer 1

Fire needs fuel, oxygen and heat. Unless the temperature of the fuel rises because of a hot environment, fires usually also need a source of ignition (flame, spark etc)

(More detailed answer)

Technically oxidation occurs.

For instance, when wood burns, the carbon found in the cellulose of plant material will readily combine with the oxygen in the air and form carbon dioxide when given enough energy.

This is because if there is sufficient thermal energy in the system, the electrostatic attraction between the carbon, hydrogen, oxygen and nitrogen within the cellulose of the wood with begin to weaken. The carbon found in the wood will be highly attracted to the oxygen gas found in the air, so the oxygen will then break the double bond with itself to allow the carbon to combine with it, this in turn will form carbon dioxide.

Since the oxygen is so highly attracted to the carbon, there will be much kinetic energy involved in the transfer, in other words, the atom of oxygen in the air will rush towards the carbon atom in a rather violent manner. As this reaction occurs energy is often distributed to other neighboring atoms, if the energy is sufficient enough, it will induce the same reaction over again with more carbon and oxygen atoms, this is known as a chain reaction.

The bright flame that forms with a fire is due to the nitrogen and carbon dioxide gas being liberated from the cellulose with locally high energy/temperature. These gases have such a high temperature that the electrons bound to the atom are rapidly accelerating and decelerating and jumping up and down electron shells, and in turn continually emitting photons that we perceive as 'fire'.

The smoke is mainly composed of Di methoxy-phenol, phenol and other carbonyl compounds. This is essentially due to the sugars in the wood being 'caramelized' with the heat, hence the slightly sweet smell of burnt wood.

Answer 2

fire spreads by air. Air gives more oxygen to the fire to make it expand. It also makes the fire move by wind

Fire spreads by transfer of heat by conduction, convection and radiation. The rate of spread depends upon the fuel, heat and amount of oxygen.