Fun to Imagine: Fire
A log can sit next to the oxygen in the air for years and never burn. Strike a match and the two rush together in a blaze of light and heat. Richard Feynman explains why, then follows the wood back to where it came from and finds something surprising: a burning log is giving back the sunshine a tree stored up as it grew.
Why Doesn't Wood Just Burst into Flame on Its Own?
There's a hill in the way. Oxygen would love to snap onto the carbon in wood, but up close the two push each other apart first. Richard Feynman pictures a ball rolling towards a deep hole with a hill in front of it. Too slow and it rolls a little way up and back again; fast enough and it goes over the top and drops in.
Mostly from the air. It's tempting to think trees grow out of the ground, but the carbon in wood comes from carbon dioxide in the air, and the water comes down as rain. Only a little comes from the soil, as minerals. Richard Feynman says it plainly: trees come out of the air.
Put Your Instincts to the Test
Think about atoms snapping together, then pick an answer for each question and see if your instincts were right.
The rub gets the first atoms over the hill. Friction heats the chemicals in the match head, so a few of their atoms jiggle fast enough to react. Each reaction gives out heat that sets off the next. Within a moment the head is alight and lighting the wood of the stick.
It pulls the oxygen off the carbon. Carbon and oxygen hold on to each other very tightly in carbon dioxide. Richard Feynman asks how a tree manages to undo that and dismisses any mysterious force of life: the sunshine does the work. The tree keeps the carbon to build wood and breathes the oxygen back out.
From stored sunlight. It took the sun's energy to pull carbon and oxygen apart. When they snap back together in a fire, that energy comes out again as light and heat. Richard Feynman calls a burning log stored sun coming out.
Understanding the Science
Tap each card to see how a fire works, from atoms snapping together to a tree in the sunshine.
Key Concepts
Atoms That Like Each Other
Tap to learn moreAtoms attract each other by different amounts. Oxygen in the air would very much like to be next to carbon. When the two join, they hold on tightly and give out a lot of energy. That joining is a chemical reaction; the hold between them is a chemical bond. Burning is the reaction of fuel with oxygen.
The Hill Before the Hole
Tap to learn moreBefore they can snap together, atoms have to get very close, and close up they push each other apart. So there's a hill to climb before the hole. Chemists call the height of that hill the activation energy. Wood and oxygen don't burn on their own because, at room temperature, almost no oxygen arrives with enough speed to get over it.
Only the Fast Ones Get Over
Tap to learn moreIn any gas, some molecules move slowly and a few move very fast. Heating something doesn't make every molecule fast; it makes many more of them fast enough to get over the hill. That's why many reactions run about twice as fast for every 10 °C rise and why a fire needs a match, a spark or kindling to get it going.
The Catastrophe
Tap to learn moreEach time an oxygen snaps onto carbon, the energy it gives out sets the neighbouring atoms jiggling faster, so more of them get over the hill, and they set off more again. Richard Feynman calls this a horrible catastrophe. Once it starts, it keeps itself going. Take away the fuel, the oxygen or the heat and the chain breaks, which is how every fire extinguisher works.
What a Fire Makes
Tap to learn moreWhen carbon burns completely, each carbon atom joins two oxygens to make carbon dioxide, which goes up the chimney. Wood also holds hydrogen, which burns to make water vapour. Smoke is tiny bits of wood that didn't burn completely. The grey ash left behind is mostly minerals, the small part of the tree that really did come from the ground.
Trees Come Out of the Air
Tap to learn moreIn the 1640s Jan van Helmont grew a willow in a pot of weighed soil for five years. The tree gained about 74 kg (164 pounds), but the soil lost only about 57 g. He thought the water had made the wood. We now know most of a tree's mass is carbon taken from carbon dioxide in the air, built together with water from the ground.
Sunlight Does the Hard Work
Tap to learn morePulling oxygen off carbon takes energy, because they hold on so tightly. In a leaf, green chlorophyll catches sunlight and uses its energy to split carbon dioxide and water, keeping the carbon and hydrogen to build sugar and wood. The oxygen is a by-product; the tree breathes it out. This is photosynthesis, and nearly all the oxygen we breathe comes from it.
Stored Sunlight
Tap to learn moreEnergy is never made or destroyed, only moved. The sunshine that split carbon dioxide in the leaves is held in the wood, ready to come out when carbon and oxygen join again. So the light and heat of a log fire really is old sunlight. Coal, oil and gas are the same idea on a grand scale: sunlight stored by plants that lived millions of years ago.
Try It: Light the Log
Light the log with a single match, put it out without water, then bring it back without a match.
Wood and the oxygen in the air sit side by side for years without burning, because an oxygen molecule has to arrive fast enough to get over a hill before it can snap onto the carbon. Heat makes them faster. Each snap gives out heat and light, which speeds up the next ones. Wood catches at about 300 °C and its flames reach 600 to 1,100 °C. The hill in the magnifier is drawn lower than the real one, so that you can see the snaps. Even in a roaring fire only a few in every hundred oxygen molecules get over; it burns because, in real wood, trillions arrive every second.
Richard Feynman's Pictures
Richard Feynman explained fire with pictures you can run in your head. Tap each one to see what it shows.
The Ball and the Volcano Hole
Tap to learn moreA ball rolls along towards a deep hole, like a volcano's crater, but there's a hill around it. Roll it gently and it climbs part way, then rolls back; it never finds the hole. Roll it fast enough and it goes over the top and falls in. Oxygen and carbon are the same: without enough speed they bounce apart, never knowing they could have snapped together.
Getting It Started
Tap to learn moreThe air is always bumping into the wood, and nothing happens. Heat a spot somehow, with a match or a spark, and a few oxygen molecules come fast enough to get over the top and snap in. That's all a fire needs to begin: a handful of atoms given a push over the hill.
The Horrible Catastrophe
Tap to learn moreEach snap makes a lot of jiggly motion, which hits other atoms and speeds them up, so they can climb the hill and snap in too, and so on. One after another, faster and faster. Richard Feynman calls it a horrible catastrophe; we call it a fire. Once it's started, the heat keeps making the conditions for more heat.
A Cup of Coffee Over the Fire
Tap to learn moreHold a cup of coffee over all that activity and the cup gets shaken by the jiggling atoms rising from the fire, so its own atoms jiggle more and the coffee heats up. Richard Feynman links fire straight back to his picture of heat: the warmth of a fire is jiggling, passed upwards to whatever is above it.
Trees Out of the Air
Tap to learn morePeople look at trees and think they grow out of the ground. Ask where the substance comes from and the answer is the air: carbon dioxide goes into the tree, the oxygen is pushed away and the carbon stays. Even the water came down from the sky. Most of a tree, Richard Feynman says, is out of the air.
The Sun Knocks the Oxygen Off
Tap to learn moreCarbon and oxygen hold each other so tightly; how is the tree so smart that it can pull them apart? Richard Feynman pretends to reach for a mysterious force of life, then drops it: the sun is shining, and the sunlight knocks the oxygen away from the carbon. It takes sunlight to make a plant work.
Stored Sun Coming Out
Tap to learn moreBurn the wood and the carbon and oxygen rush back together with all that activity, giving out light and heat. That's the light and heat of the sun that went in. Richard Feynman calls the log fire stored sun coming out: a tree spends years catching sunlight and a fireplace gives it back in an evening.
How Is the Sun So Hot?
Tap to learn moreRichard Feynman's last question is how the sun got so jiggly and hot, and he leaves it for us to imagine. The answer is the same hill-and-hole idea at a much bigger scale. In the sun's core, at about 15 million °C, hydrogen nuclei jiggle so fast that they get over their own enormous hill and fuse into helium, giving out the energy that reaches us as sunlight.
Apply Your Knowledge
Put each part of a log fire in its place in Richard Feynman's story.
Match each part of a fire to its job
Click an item to select it, then click the matching description to place it.
Real-World Challenge
Firefighters talk about the fire triangle: fuel, heat and oxygen. Use Richard Feynman's hill and hole to explain why taking away any one side puts a fire out, then name one safe way firefighters take away each side. Which one does a fire blanket remove?
Slow Fires and Stored Sunlight
No flames needed. These three try-it-at-home ideas show the same chemistry, slowly and safely. Ask an adult before you start.
Sprinkle cress seeds on two pads of damp kitchen roll. Put one on a sunny windowsill and the other in a dark cupboard, keep both damp and look after five days. What to look for: The sunny cress grows green and strong. The cress in the dark grows pale, thin and floppy, using up the food stored in its seed, because without light it can't pull carbon out of the air. Stay safe: Wash your hands after handling the seeds and don't eat cress grown on used kitchen roll.
Soak a pad of steel wool in vinegar for a minute, squeeze it out and push it into a small jar with a thermometer. Put the lid on loosely and read the thermometer every minute. What to look for: The temperature creeps up by a few degrees. The iron is joining with oxygen to make rust, which is a very slow kind of burning, and it gives out heat just like a fire. Stay safe: Wear rubber gloves (steel wool splinters) and wash your hands afterwards.
Open the outer packet of a disposable hand warmer and feel it over the next few minutes. What to look for: It warms up as soon as air gets in. Inside is iron powder that rusts quickly, so it's the same slow fire as the steel wool, made faster. Stay safe: Never open the inner pouch and don't hold it against bare skin for a long time.
What Has Changed Since This Programme
Fun to Imagine was filmed for BBC television in 1983. Here's what science has found since then.
Updated: In a way, yes. In the late 1980s Ahmed Zewail used laser flashes a few femtoseconds long, a few millionths of a billionth of a second, to film chemical bonds breaking and forming. He won the 1999 Nobel Prize in Chemistry for it.
Updated: Yes. Fusion in the sun's core gives off ghostly particles called neutrinos. For years detectors caught too few of them, until in 2001 and 2002 the Sudbury Neutrino Observatory in Canada showed they change type on the way here and that the total matches what fusion predicts. That work shared the 2015 Nobel Prize in Physics.
Updated: Better than ever. Since late 2018 a laser called GEDI, on the International Space Station, has measured the height and shape of forests across much of the Earth. Scientists use it to estimate how much carbon the world's trees are holding.
Test Your Understanding
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Reflection
Richard Feynman stops with a question and says he'll leave us something to imagine. Pick something you used today that needed energy: a meal, a bus ride, a warm room. Can you follow its energy back, step by step, to the sun?
Episode Discussion
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