Where Does a Candle Go When It Burns? Conservation of Mass

Light a candle and leave it. A few hours later, it’s a puddle of wax around a burnt wick, or nothing at all. The wax has gone. But where?

The natural answer is that it was used up: burned away, turned into light and heat. It’s the same intuition that says a log becomes a pile of ash and a few sparks. And it’s wrong in a way that turned out to be one of the most important discoveries in science.

The law

In a chemical reaction, matter is neither created nor destroyed. The total mass stays the same.

The French chemist Antoine Lavoisier established this in the 1770s and 1780s by doing something that sounds obvious and was, at the time, unusual: he weighed everything. He burned and heated things in sealed glass vessels, so that no gas could escape or get in, and weighed them before and after. Whatever happened inside, the total weight didn’t change.

It mattered because chemists at the time believed in phlogiston, an invisible substance that things supposedly gave off when they burned. The theory had trouble with one awkward fact: some metals got heavier when burned, which seemed impossible if they were losing something. Lavoisier’s scales showed why. Burning wasn’t losing something. It was gaining something from the air, the gas he named oxygen.

Lavoisier was executed by guillotine in 1794, during the French Revolution, for his role as a tax collector. His law outlived the Revolution. The Russian scientist Mikhail Lomonosov had reached a similar idea in the 1750s, and the principle that matter is never lost goes back, as philosophy, to ancient Greece. Lavoisier was the one who proved it with a balance.

What happens to the candle

Candle wax is mostly paraffin, made of long chains of carbon and hydrogen atoms. When it burns, it reacts with oxygen from the air:

• The carbon joins with oxygen to make carbon dioxide.
• The hydrogen joins with oxygen to make water vapour.

Both are invisible gases that drift off into the room. The light and heat come from energy released as the new chemical bonds form, not from wax “turning into” energy.

If you burned the candle in a sealed jar on a scale, the scale wouldn’t move. The wax and the oxygen would slowly become carbon dioxide and water, and the total would stay exactly the same, until the flame ran out of oxygen and went out. The toy at the top shows it: burn the candle down and watch the mass move from wax to gases without the total changing.

Here’s the surprise: the gases weigh more than the wax. Each carbon atom picks up two oxygen atoms, which are heavier than it is. Burn 10 grams of wax and you make about 31 grams of carbon dioxide and 13 grams of water vapour. The extra came from the air.

In the winter of 1860, the scientist Michael Faraday gave a famous series of Christmas lectures for young people at the Royal Institution in London, later published as The Chemical History of a Candle. He collected the water from a burning candle in a cold tube, showed the children it was ordinary water, and asked them where it could have come from. It’s still one of the best introductions to chemistry ever written.

Rust gets heavier

The same law explains a surprise about rust: when iron rusts, it gets heavier. Rust is iron combined with oxygen and water from the air, so the rusty object has gained atoms. It crumbles and looks as if it’s wasting away, but if you caught every flake, the rust would weigh more than the iron did. A gram of pure iron that rusts completely to iron oxide becomes about 1.4 grams.

This was exactly the puzzle that broke phlogiston theory. A metal heated in air turns to a powder that weighs more than the metal. Phlogiston believers had to suggest phlogiston might have negative weight. Lavoisier simply weighed the air.

The classroom demonstration that surprises everyone

There’s a classic demonstration in school chemistry labs. Balance a fluffy pad of fine steel wool on a scale, then touch it with a flame, or the terminals of a battery. It glows and burns with a crackle, turning dark and brittle. Ask the class what the scale will show, and most predict it’ll drop, because burning things get lighter: look at a log fire.

The scale goes up. The steel wool has combined with oxygen from the air, and the iron oxide it became is heavier than the iron was. A log fire only seems to lose weight because its products, carbon dioxide and water vapour, escape up the chimney. Steel wool’s product is a solid that stays on the scale. Same law, different outcome, depending only on where the atoms end up.

Where does a tree get its weight?

Here’s a question that catches out a lot of science graduates. An oak tree can weigh many tonnes. It started as an acorn. Where did all that mass come from?

Most people say the soil. In the 1640s, the Flemish chemist Jan Baptist van Helmont tested it.

A tree’s dry mass is mostly carbon, and the carbon comes from the carbon dioxide in the air, taken in through its leaves. Using sunlight’s energy, the tree joins it with water to make sugar, and builds wood from the sugar. Trees are, in a real sense, made of air. When you burn a log, the reaction runs the other way: the carbon goes back into the air as carbon dioxide, and the small pile of ash is the minerals that did come from the soil.

Where does fat go?

The same question applies to your own body. When you lose weight, where does the fat go? Many people guess it’s burned off as energy, or leaves as sweat. In fact, most of it leaves through your lungs.

Fat is made mostly of carbon, hydrogen and oxygen. When your body breaks it down for energy, the carbon leaves as carbon dioxide, which you breathe out, and the hydrogen leaves as water. A 2014 study by Ruben Meerman and Andrew Brown worked through the chemistry and found that about 84% of the mass of lost fat is breathed out. There’s more in our article on where fat goes.

Dissolving doesn’t destroy either

Stir sugar into tea and it disappears. But it hasn’t gone anywhere: the sugar molecules have spread out among the water molecules. The tea weighs as much more as the sugar did, and it tastes sweet. You can get the sugar back by evaporating the water.

Dissolving isn’t even a chemical reaction in the usual sense: the sugar molecules stay sugar molecules. They’re just too small and too spread out to see. When things seem to vanish, conservation of mass says to ask a better question: where did the atoms go?

Does E = mc² break the law?

A careful reader might object: Einstein showed that mass and energy are interchangeable, and a candle gives off energy, so shouldn’t it lose a little mass?

Strictly, yes. But in chemical reactions the amount is absurdly small. The energy released by burning a kilogram of wax corresponds to a loss of about half a millionth of a gram. No balance could detect it. In nuclear reactions, inside the Sun or a power station, the mass changes are large enough to matter, and the right law there is conservation of mass and energy together. For anything you’ll do in a kitchen or a chemistry lab, conservation of mass holds.

How to use the idea

Conservation of mass is one of the most useful habits of thought in science, because it lets you follow matter through any change. When something disappears, ask where its atoms went. When something grows, ask where its atoms came from. Most of the time the answer involves the air, the part of the world we forget we’re swimming in, because we can’t see it.

The candle and the rusting nail are in the free chemistry test, and the tree question is in the biology test.