What is fire and how does it burn?

Hannah - So, what is fire and why does it burn? We ignite the issue with professor of chemistry and fire science, Richard Hull from the University of Central Lancashire.

Richard - Imagine gas coming out of the cooker or from a cigarette lighter. The gas mixes with air, but nothing happens. But the small spark sets off a chain reaction, resulting straight away in a hot flame.

What causes, this huge difference? The spark's got enough energy to break a few molecules into pieces: we call them free radicals, which react billions of times faster than the fuel and air gas molecules.

When they react, they give out lots of heat and light, producing a flame. That flame can produce more radicals and spread to other things, starting a fire.

When a solid - like wood - is heated, maybe with a flame, some of it turns to gaseous fuel, which mixes with air and, like the lighter, it too starts to burn. Fire is incredibly important. Being able to control it opened the door to cooking, the Bronze Age, the Iron Age, almost all forms of power generation and most atmospheric pollution.

Yet, still, it can grow out of control and have absolutely devastating consequences...

Hannah - Thanks, Richard and as long as there's oxygen, fuel and heat, a fire, once ignited, can burn indefinitely. The world record for the longest burning fire is a bed of coal 30 metres underground beneath Mt. Wingen in New South Wales, Australia. And it's still smouldering, thought to be ignited by a lightning strike hitting the top of the coal over 5,000 years ago!