LearnChem · Part I — What is stuff made of?
One candle, opened all the way down — the way Faraday did it for children.
“What is a flame actually made of, and where does the candle go?”
The alchemist's furnace burned for a thousand years and never made gold — and the course's first page says why: chemistry rearranges electrons and leaves every nucleus alone. A flame can cook, smelt and glaze, and it will never transmute. But that page never asks what the fire itself is. What is actually happening in the commonest chemical event in the house? This module takes one candle and opens it, stage by stage, all the way down.
Think of the last candle you watched — a birthday cake, a power cut, a table set for a festival. A stick of solid wax, a thread down the middle, a steady teardrop of light on top. Now the awkward question: what exactly is burning? A match held to the side of the candle barely melts the wax, and a lump of wax with no wick cannot be lit with one at all. The wick alone — a bare cotton thread — flares and dies in a second. Neither part burns for long on its own, yet together they burn for hours.
Michael Faraday — the bookbinder's apprentice who became the finest experimenter of his century — put this exact puzzle at the centre of his lectures for children at the Royal Institution in London, and said of it: “There is no better, there is no more open door by which you can enter into the study of natural philosophy, than by considering the physical phenomena of a candle.” By natural philosophy he meant all of science. This module holds him to it.
Watch the candle the way he did. The flame's own heat melts the wax below it into a little pool, and the rim of solid wax holds that pool like a cup. The thread stands in the pool, and liquid wax climbs it — the same trick by which a towel's dipped corner slowly drinks a puddle. Near the top of the wick it gets so hot that the liquid stops being liquid: it becomes an invisible vapour. And there the trail goes cold — whatever happens next happens inside the flame. Look closely at one: it has a dark middle, a small hollow-seeming region wrapped round the wick. Faraday reached into it.
He did it on stage, with a thin glass tube — a demonstration to watch, not to try: hot glass, an open flame and a lit match are no combination to improvise.
Faraday dips one end of a thin glass tube into the dark middle of a candle flame. A pale, heavy smoke drifts out of the far end, away from the flame. What is that smoke?
Commit before the reveal. First answers are counted anonymously, never named.
Faraday's next move: a lit match at the tube's far mouth, and the pale smoke catches — a small flame burning at the far end of the tube, fed through the glass. Spent gas has already burned and cannot burn a second time; hot air has nothing in it to burn. What the tube stole from the middle of the flame was fuel: wax, as a vapour, that had not yet burned. That vapour is what a candle actually burns. The candle is a machine in three stages — solid in the stick, liquid in the cup, vapour at the top — and only the last stage burns.
Sit with how strange that is. The middle of a flame — the place that looks like the heart of the fire — is the one place where nothing is burning. There is no air in there; the vapour streams up through it unburned. Burning happens only in a thin skin near the flame's outer edge, where the fuel from inside finally meets the air from outside. A flame is hollow.
You have seen the vapour without knowing it. When a candle is blown out, a ribbon of white smoke climbs from the wick for a few seconds. That ribbon is the fuel line still running — wax vapour with no flame left to burn it, condensing back into a fog of tiny wax droplets as it cools.
Quick check — where in a candle flame is nothing burning?
So a flame has geography. A dark core of unburned vapour. Around it, the bright yellow zone — the light you read by. Wrapped over everything, almost invisible, the thin skin where fuel finally meets air: that skin is where the burning runs. At the base, a small skirt of blue, which §3 will explain. And rising off the top, invisible, the finished gases leaving. None of it is solid, none of it stays put, and yet the shape holds steady for hours. What kind of thing holds its shape while its substance streams through?
Geography can be surveyed. In 2005, fire-safety researchers at NIST — the United States' national measurement laboratory — put ordinary paraffin candles on instruments and measured what a candle actually does. Not guesses, not textbook lore: a balance under the candle, a camera on the flame, arithmetic on the heat. Their numbers are the bench below.
Two of the numbers were measured outright. The flame stands 42 millimetres tall, give or take one. The candle feeds it a tenth of a gram of wax every minute, steadily — the paper starts its clock at minute 15, once cup and wick have settled into rhythm. The other two are flagged for what they are: the 77 watts of heat output is worked out from that burning rate, and the roughly 1400 °C at the hottest point is a figure the paper quotes from the combustion literature — hot enough to melt copper, and not something its own instruments touched. Move the slider to read off how much wax is gone after any number of minutes.
And the coloured maps of temperature at every point inside a flame, the kind the internet is full of? They usually come with no source at all. This page could not find a measured map it could cite, so it shows none.
Why does the flame shine? The finished gases leaving a candle are invisible, and the vapour inside the dark core is invisible too. Something in between must be making the light.
Faraday pulled the answer out of the flame on the end of a wire. When he lowered a metal gauze into the bright zone, the glow above the gauze went out at once, and a ribbon of smoke streamed up through it instead. The cold metal bleeds away the heat, so the burning stops halfway — and what escapes when burning stops halfway is black.
He caught the black directly, too. A cold plate held in the bright zone for a moment came away coated with soot — carbon, the same stuff as charcoal and pencil-black.
So the bright zone is full of solid specks of carbon, cooked out of overheated fuel that has not yet found its share of air. While they ride up through the flame they are white-hot, and white-hot solids glow — a blacksmith's iron, the coil of a room heater, same rule. The candle's yellow light is millions of microscopic embers, each alight for a fraction of a second before it finishes burning in the thin skin at the flame's edge.
A well-behaved candle sends up almost no smoke, because almost every speck burns away before it escapes the shell. A draught that tips the flame lets some specks out early — that is the sooty ribbon a guttering candle leaves, and the black stain above a lamp that sits too close to a wall.
The stove answers the same question the other way round. A kitchen gas burner mixes its fuel with air before the flame — the air rushes in at the base of the burner and rides in with the gas. With air already alongside every parcel of fuel, the burning runs to completion at once: no starving middle, no overheated fuel, no carbon specks. Nothing solid to glow means no yellow. The faint blue is the light of the reacting gases themselves, and it is faint — a stove burns thousands of watts and barely lights the kitchen, while one candle's 77 lights the room. A candle is yellow because it mixes late; a stove is blue because it mixes early. Turn the logic around and it becomes a diagnostic: yellow tips on a gas burner report soot in the flame — fuel meeting air too late — and mean the burner needs servicing. And the candle's own blue skirt, promised in §2, is the same effect in miniature: at the base, the first air reaches the vapour before any soot has had time to form, so the only light there is the gas's own faint blue.
Quick check — the yellow light of a candle comes from…
Everything so far has watched the wax. But the flame's shell — the only place where burning happens — is where fuel meets air. Is the air just the space the fire lives in, or is it an ingredient?
A short candle burns on a table. A big glass jar — a few litres — is turned over it, rim to the table. What happens?
Predict first.
The flame dims, gutters and dies — with the candle barely shorter than when the jar came down. Fuel to spare, heat to spare, and still it dies. The jar changed exactly one thing: no fresh air. So the flame was consuming the air. Air is an ingredient — a reactant, chemists say — not a backdrop.
One refinement, and it matters. The flame does not use air whole. Air is a mixture, and only about a fifth of it — 21 parts in 100 — is the working part, a gas called oxygen. The rest, mostly a gas called nitrogen that takes almost no part in burning, passes through the flame and comes out the far side unchanged. That is why the jarred candle dies with most of the jar's air still in there. It does not even need to finish the oxygen: once the share around the flame falls from about 21 towards roughly 16 parts in 100, the flame can no longer keep itself hot enough, and it dies with most of the jar's oxygen still unspent.
Once you know air is an ingredient, small fire facts click into place. A flame organises its own supply line — its rising heat pulls fresh air in at the base; the waterfall's trick again. A wood fire fanned with a plate brightens, and the blacksmith's bellows are the same trick with intent: more oxygen delivered, faster burning. And a whole family of fire-safety rules — smother the flame, wrap the person in a fire blanket — comes down to one idea: cut the oxygen delivery, and the fire is an ingredient short.
Water attacks a fire's other supports. It soaks up an enormous amount of heat on its way to becoming steam, and §1 showed what that steals: no heat, no fresh vapour, no fuel line. The steam then crowds fresh air away from the fuel. Cooling plus smothering — for wood, cloth and paper, water is the right tool.
For burning oil — a kadai of it on the stove, say — water is the worst tool there is. Water is denser than oil, so it slips under the burning layer, straight into contact with metal and oil far above 100 °C. There it flashes to steam in an instant, and expanding steam under burning liquid throws that liquid upwards and outwards: a fireball of flying burning oil. UK fire-safety guidance, linked at the end of this page, says it in one line: “Never use water on chip pan fires, as this will cause a fireball.” The rest of the advice holds anywhere: never move the pan; turn the heat off only if that is safe; get out; call the fire brigade — 101 in India, or 112 from any phone. The rule learned young — water puts out fire — meets its exception exactly where the stakes are highest.
The wax vanishes — centimetres of solid candle, gone by morning. The air loses its oxygen. And a scale under the burning candle falls, slowly and steadily, at that measured tenth of a gram a minute. Where does it all go?
Faraday caught the first product on stage. A cold, dry surface held over the flame mists over at once — and the mist is ordinary water. Not water that was hiding in the candle: wax repels water, and the mist keeps coming for as long as the flame burns. The burning is making water, out of a fuel and a gas that are neither of them wet. The second product never shows itself: an invisible gas that chemists trap by shaking it with lime-water, a clear liquid that turns milky when this gas is bubbled through it — the standard test for it. Its name is carbon dioxide — and the carbon in its name is an old acquaintance: the same carbon whose glowing specks lit §3, now fully burned and locked away in a gas.
No mystery about where the ingredients for those two came from, either. Wax is built almost entirely of two elements — carbon and hydrogen. The oxygen from the air takes the carbon to make carbon dioxide, and takes the hydrogen to make water. So the candle does not go away. It goes elsewhere — off the top of the flame, into the room, as gas.
And the heat? The heat and the light are not substances — nothing you could weigh on any balance; the mass they carry away is billions of times too small to see. They are the difference: the new arrangement holds less energy than the old one did, and the difference is paid out to the room. Why the new arrangement is cheaper is a question a later module opens properly. For now the books just have to balance — and that suggests an audit.
Every gas the flame gives off is gathered and weighed — all the carbon dioxide, all the water. Against the wax that vanished, the gathering weighs…
Commit before the books are opened.
More — and by a lot. Wax and oxygen go in; carbon dioxide and water come out; and every atom that went in is out there somewhere. The falling scale was never matter being destroyed — only matter leaving as gas, unweighed because it no longer sits on the pan. The same nuclei, in new company: a flame rearranges the atoms' outer parts and touches nothing deeper, which is the whole reason fire can cook dinner and never make gold.
Chemists' bookkeeping for candle wax runs close to this: every 1 gram of wax takes about 3.45 grams of oxygen with it, and returns about 3.12 grams of carbon dioxide and 1.33 grams of water. In: about 4.45 grams. Out: the same 4.45. Those ratios come from the recipe for a wax particle — and how anyone could know the recipe for a particle nobody has seen is exactly the next module's job. Watch the ledger hold:
A caution about what this bench proves: nothing. The ratios come from a balanced recipe, so in = out is built into the arithmetic — the bench illustrates the bookkeeping. The evidence for it is weighing, and weighing is the next module's whole method.
Notice what the ledger says quietly, though. Most of what a candle sends into the room is not from the candle at all; it is borrowed air, returned in new form. Substances in, different substances out — that is what chemists call a chemical reaction, and a flame is the one you can watch: a hollow shell of reaction, standing in the air it feeds on. A tenth of a gram of it drifts into the room every minute. How many particles is that? Counting them sounds impossible. The next module does it with a kitchen scale.
Quick check — a cold, dry steel spoon, held for a moment in the gases above a candle flame, comes away misted. The mist is…
These check themselves, and “New numbers” deals a fresh set. Within a few per cent counts as right — these are a fire-watcher's estimates, not examination arithmetic.
1. A candle burns steadily at the NIST bench's measured 0.105 grams of wax per minute. Over , how many grams of wax does it consume?
2. By the §5 ledger, each gram of wax takes about 3.45 grams of oxygen from the air. A candle that consumes of wax takes how many grams of oxygen?
3. Nothing is lost: whatever goes into the flame comes out of it as gas. When of wax burns (each gram taking its 3.45 grams of oxygen), what total mass of new-made gas — carbon dioxide and water together — leaves the flame?
4. About 21 parts in 100 of air is oxygen. A sealed jar traps of air. How many litres of oxygen are in the jar?
A 25-gram candle is lit at nine in the evening and burns at the NIST candle's measured 0.105 grams per minute until the wax is gone. Work out how long it lasts, how much oxygen it draws from the room, and how much carbon dioxide it leaves behind. Type the last of these — the grams of carbon dioxide.
(Use the §5 ledger: each gram of wax takes about 3.45 g of oxygen and returns about 3.12 g of carbon dioxide.)
Worked through. Time first: 25 grams at 0.105 grams a minute is 25 ÷ 0.105 ≈ 238 minutes — very nearly four hours, so the flame dies around one in the morning. Oxygen: 25 × 3.45 ≈ 86 grams drawn from the room's air. Carbon dioxide: 25 × 3.12 ≈ 78 grams left drifting in the room. Worth putting beside it: a closed bedroom of a dozen square metres, under an ordinary 2.5-metre ceiling, holds about 30 cubic metres of air — roughly 36 kilograms, about 8 kilograms of it oxygen by weight. The candle's 86 grams is around one per cent of that. The room's oxygen was never in danger. The reason nobody should sleep with an open flame is not suffocation; it is that a flame is a self-feeding pattern, and given fuel it does not stay 42 millimetres tall.
Which adds carbon dioxide to the room's air faster — the burning candle, or you, breathing? What you know: the candle's ledger runs at 0.105 grams of wax a minute, giving about 0.33 grams of carbon dioxide a minute. A resting person takes very roughly fifteen half-litre breaths a minute, and about 4 parts in 100 of each breath out is carbon dioxide; carbon-dioxide gas weighs about 1.8 grams per litre at room temperature. There is no marking here — write your reasoning down before looking.
One way through it. Your side: fifteen breaths of half a litre is about 7.5 litres of air out per minute; 4 parts in 100 of that is 0.3 litres of carbon dioxide; at 1.8 grams a litre, that is about 0.54 grams a minute. The candle manages 0.33. So you out-produce the candle — by half as much again on these rough numbers. (Measured resting figures run nearer 0.4 grams a minute: still ahead of the candle.) Every estimate here is rough and the answer survives anyway. And it lands somewhere worth standing: you run on the same chemistry as the flame. Fuel plus oxygen, carbon dioxide and water out, energy as the difference — slower, cooler, and with no glowing carbon, but the same ledger. What breathing actually does with the oxygen is a later module's question.
A flame is a reaction you can see, and it eats the air.