What Is Fire? The Chemistry of Flames, Explained

Fire has been part of human life for hundreds of thousands of years. It cooked our food, kept us warm, scared away predators and lit up the night. We light candles on birthday cakes, gather around campfires and use flames in engines and power stations. Yet most of us would struggle to say what fire actually is. Is it a thing? A substance? A form of energy?

Fire is a chemical reaction

Fire is not a substance but a process: a fast chemical reaction called combustion. During combustion, a fuel reacts with oxygen and releases energy as heat and light. For wood, paper, gas or wax, the main products are carbon dioxide and water vapour, along with smoke, soot and ash if burning is incomplete.

The reaction releases more heat than it needs to keep going. That extra heat warms nearby fuel, which starts reacting too. This is why a fire can grow and spread on its own.

The fire triangle

A fire needs three ingredients, often drawn as the fire triangle:

• Fuel: anything that can burn, such as wood, paper, fabric, oil, petrol or gas.
• Oxygen: about 21% of air is oxygen, which is plenty for most fires.
• Heat: enough to raise the fuel to its ignition temperature and keep the reaction going.

Firefighters often add a fourth element, the chemical chain reaction itself, forming a fire “tetrahedron”. Some extinguishers work by interrupting that chain.

What burns is the gas

Here is a surprise: solid wood does not really burn. When wood is heated, it breaks down and releases flammable gases, a process called pyrolysis. It is these gases, mixing with oxygen, that burn in the flames. That is why flames dance above a log rather than sitting on its surface. Glowing embers are different: there, the carbon left behind, charcoal, reacts directly with oxygen without a flame.

Inside a candle flame

A candle is a perfect laboratory. In the 1840s, scientist Michael Faraday gave a famous series of lectures for young people, The Chemical History of a Candle, using it to explain combustion.

1. The heat of the flame melts the wax near the wick.
2. The liquid wax is drawn up the wick by capillary action, like water soaking up a paper towel.
3. Near the top of the wick, the wax vaporises into a gas.
4. The wax vapour mixes with oxygen from the air and burns, producing heat that melts more wax.

The flame has distinct zones. The dark core near the wick is unburnt wax vapour. The bright yellow region glows because tiny particles of soot, unburnt carbon, are heated to over 1,000 °C and shine like the filament of an old light bulb. At the blue base, where oxygen is plentiful, burning is more complete, and the blue light comes directly from excited molecules created by the chemical reaction.

Why flames have different colours

• Yellow and orange flames, from candles, wood and campfires, contain glowing soot. They are usually cooler and produce more smoke.
• Blue flames, like those on a gas cooker, burn with plenty of oxygen and produce little soot. They are usually hotter.
• Other colours come from chemical elements. Copper gives green or blue flames, sodium bright yellow, strontium red and potassium lilac. Fireworks use these to create their colours.

Why flames point upwards

On Earth, the hot gases of a flame are less dense than the surrounding air, so they rise. That draws fresh, oxygen-rich air in at the bottom and gives flames their teardrop shape. In microgravity, there is no “up”. Experiments on the Space Shuttle and the International Space Station, described by researchers such as Daniel Dietrich, showed that candle flames become spherical and dim blue, because oxygen only reaches them slowly by diffusion.

How to put a fire out

Remove one side of the fire triangle, and the fire dies:

Never throw water on a burning pan of oil: the water instantly turns to steam and hurls burning oil into the air. Turn off the heat and cover the pan instead.

Staying safe

Most deaths in house fires are caused not by flames but by smoke, which contains toxic gases such as carbon monoxide. Fire safety agencies recommend having working smoke alarms on every floor, testing them regularly, never leaving cooking unattended and planning how to get out of your home quickly if a fire starts.

In short

• Fire is combustion: a fast reaction between fuel and oxygen that releases heat and light.
• It needs the fire triangle: fuel, oxygen and heat.
• Solid fuels release flammable gases when heated; the gases burn in the flame.
• Yellow flames glow from hot soot; blue flames burn more completely and are usually hotter.
• In space, flames are round and blue because hot gas does not rise.
• Remove fuel, oxygen or heat to put a fire out; never use water on burning oil.