Science · 3 min read
Where Does a Candle Go When It Burns? Conservation of Mass
A burning candle seems to vanish. It doesn't: its atoms leave as gases. Conservation of mass explained, with the candle, rust, and where lost body fat goes.
By the Cognosc team ·
Light a candle and leave it for a few hours. It gets smaller and smaller until there’s almost nothing left. Where did the wax go?
It seems to have been destroyed, turned into light and heat. But matter can’t be destroyed in a chemical reaction. The wax is still out there, as invisible gases in the room.
Try it
Where does a candle go?
A candle burns in a sealed jar on a scale. Burn it down and watch what’s inside.
4.0 g of wax has gone. In its place are 12.5 g of carbon dioxide and 5.3 g of water vapour, made with 13.8 g of oxygen from the air. The scale still reads 50.0 g: nothing was destroyed, only rearranged.
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 by weighing reactions in sealed containers. Burning, rusting, dissolving: when he caught everything that went in and everything that came out, the mass never changed. Atoms were just rearranged into new substances.
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 bonds form, not from wax turning into energy.
If you burned the candle in a sealed jar on a scale, the scale wouldn’t change. The wax and oxygen would slowly turn into carbon dioxide and water, and the total would stay exactly the same (until the oxygen ran out and the flame died).
Rust gets heavier
The same law explains a surprise: 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, but if you caught every flake, it would weigh more than the iron did.
Where does fat go?
When you lose weight, where does the fat go? Many people guess it’s burned into 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 out that about 84% of the mass of lost fat is breathed out as carbon dioxide, and the rest leaves as water.
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.
The fine print
Nuclear reactions, like those in the Sun or a power station, are different: they really do turn a tiny amount of mass into energy, as Einstein’s E = mc² describes. In ordinary chemistry, the effect is far too small to measure, and for all practical purposes, mass is conserved.
Try it
Burn the candle above down in its sealed jar and watch the scale.
The free chemistry test has the candle question and nine other everyday chemistry myths, and the biology test asks where lost fat goes.