What is fire?
We often hear about destructive fires in the media. We know that fire is dangerous and can cause severe damage and destruction and, at times, death. Since our earliest days, humans have sought to find out what fire is, how it starts and what keeps it going.
Sometimes we might think that fire is a living thing! It moves, ‘eats’ things and seems to breathe. The ancient Greeks thought it was one of four major elements, along with water, earth and air. They could feel, see and smell fire just like they could the earth, water and air, but fire is something completely different. Earth, water and air are all matter – they are made up of billions of atoms. Fire is not matter at all, and it is not a living thing. It’s the visible effect of a chemical reaction between oxygen in the atmosphere and some sort of fuel.
It is important to understand this chemical reaction and how it happens. This helps us to know how to prevent and manage fire disasters.
Throughout New Zealand’s history, there have been many kinds of destructive fires. The Ballantynes department store fire in Christchurch in 1947 was the worst in New Zealand’s history – 41 people died. Other fires have included the destruction of houses, factories, storehouses, shops, land and bush (wildfires).
Fire is the visible effect of the process of combustion – a special type of chemical reaction. It occurs between oxygen in the air and some sort of fuel. The products from the chemical reaction are completely different from the starting material.
The fuel must be heated to its ignition temperature for combustion to occur. The reaction will keep going as long as there is enough heat, fuel and oxygen. This is known as the fire triangle.
Combustion is when fuel reacts with oxygen to release heat energy. Combustion can be slow or fast depending on the amount of oxygen available. Combustion that results in a flame is very fast and is called burning. Combustion can only occur between gases.
Chemical reaction in the combustion process
Fuels can be solids, liquids or gases. During the chemical reaction that produces fire, fuel is heated to such an extent that (if not already a gas) it releases gases from its surface.
Only gases can react in combustion. Gases are made up of molecules (groups of atoms). When these gases are hot enough, the molecules in the gases break apart and fragments of molecules rejoin with oxygen from the air to make new product molecules – water molecules (H2O) and carbon dioxide molecules (CO2) – and other products if burning is not complete.
The heat generated by the reaction is what sustains the fire. The heat of the flame will keep remaining fuel at ignition temperature. The flame ignites gases being emitted, and the fire spreads. As long as there is enough fuel and oxygen, the fire keeps burning.
Fuel + oxygen (from the air) = combustion products (mainly CO2 + H2O) + heat energy.
In complete combustion, the burning fuel will produce only water and carbon dioxide (no smoke or other products). The flame is typically blue.
For this to happen, there needs to be enough oxygen to combine completely with the fuel gas.
Incomplete combustion produces products such as carbon (C) and carbon monoxide (CO) as well as water and carbon dioxide. The burning flame is typically yellow or orange and there is smoke.
Many of us use methane gas (CH4), commonly known as natural gas, at home for cooking. When the gas is heated (by a flame or spark) and if there is enough oxygen in the atmosphere, the molecules will break apart and reform totally as water and carbon dioxide. Less heat energy is released during incomplete combustion than complete combustion.