Concepts explained
What Does a Candle Eat?
A birthday candle burns down to nothing. Where did it go? It did not vanish. It joined the air, became invisible — and grew heavier. Chemistry begins with the scale that caught this.
The old answer — something leaves
For a very long time people thought of burning as something leaving. Burn wood and a little ash remains, far lighter than the wood. So it was natural to think something had left the body during the fire.
In the early 1700s a German doctor gave the leaving thing a name: , the stuff of fire. What burns well has lots of it; what will not burn has none. It fitted perfectly, and it stayed in textbooks for nearly a century.
The scale said something odd
But one thing was strange. Burn iron — iron does burn, very slowly, we call it rusting — and the rusted iron weighed more than the iron before.
You can check at home. Turn a glass upside down over a candle. Soon it goes out, with candle to spare. The air inside the glass has run out of what the flame was eating. That something is oxygen.
Where the missing candle went
Now the first question can be answered. The candle did not vanish. What it was made of joined the oxygen in the air and became two invisible gases — the one we breathe out (carbon dioxide), and water vapour. Hold cold glass over a flame and droplets form: that is why.
In the 1780s a chemist in Paris nailed this down with a scale. He burned things inside a tightly sealed vessel and weighed it before and after. The same. Whatever burned and whatever formed inside, the vessel as a whole did not change by a grain.
A little further in
Then why is ash so light?
Burn wood and little ash is left because most of the wood turned into gas and flew away — smoke, water vapour, carbon dioxide. Gather all of that and weigh it and it is heavier than the wood, by exactly the oxygen that joined. The old thinkers simply could not count what had flown off.
Your body does the same thing. Eat, breathe, and the food joins oxygen slowly, giving off heat and sending out carbon dioxide. A candle burns fast; we burn very slowly. That is why we are warm.
Phlogiston has its own piece in the series 'Wrong, and Useful Anyway' — how a wrong idea held on for a hundred years.
One word slipped past in this piece: the candle joins the oxygen. What exactly does joining mean? What is a candle made of, that it can join anything? The next piece keeps cutting things smaller.
The question that remainsIf burning is not vanishing but joining — does anything in the world ever truly disappear?