The Chemical History of a Candle · Lecture VI, part 2 · Part 6: Lecture VI: Carbon, breathing and the conclusion
17. We burn like candles: breathing
Goal: Your child can explain how breathing is like a burning candle: both use oxygen and give out water and carbon dioxide, and food is our fuel.
You'll need
- A small mirror (put it in the fridge for ten minutes first)
- A timer
- Science notebook
What's in the book
Faraday's board with a groove and glass tubes: his breath puts a candle out without blowing; one breath spoils a jar of air; breathed-out air turns lime-water milky; how food is "burned" in the body; and sugar turned to black carbon.
The lesson, step by step
Hook question
SayIs there anything burning inside you right now?
Read together
Read pages 173 to 182, stopping at "And if I make arrangements so as to oxidize the carbon of sugar."
In every one of us there is a living process of combustion going on very similar to that of a candle
What Faraday did
Watch only: don't try this at home.
He let a candle burn in a tube fed by a channel of air. Then he put his mouth over the opening and let only his breathed-out air go in, without blowing:
I did not blow the candle out.
The flame went out for want of oxygen. He breathed one breath in and out of a jar, and a taper went out in it. He breathed out through lime-water, and it turned milky.
Even one inspiration, you see, has completely spoiled this air, so that it is no use my trying to breathe it a second time.
If you want to see it, search online for a video of a chemistry teacher doing this demonstration and watch it together.
Your breath is like candle smoke
Breathe on the cold mirror: water mist, just like the cold spoon over the candle. Candle: wax + oxygen → water + carbonic acid + heat. You: food + oxygen → water + carbonic acid + warmth.
we may thus look upon the food as fuel.
More fuel, more breathing
Count your child's breaths for one minute while sitting still. Then do jumping jacks for a minute and count again. Write both numbers down. Say: Working muscles burn more fuel, so they need more oxygen, and you breathe faster.
Sugar is full of carbon
Faraday's sugar table: 72 parts carbon, 11 hydrogen, 88 oxygen. He points out the hydrogen and oxygen are in the proportion for water, so:
sugar may be said to be compounded of 72 parts of carbon and 99 parts of water
He poured oil of vitriol (strong sulfuric acid) on syrup and it turned into a solid lump of black carbon. Watch only: don't try this at home. If you want to see it, search online for a video of a chemistry teacher doing this demonstration and watch it together.
Say it back and check
Narration prompts: How did Faraday put out a candle with his breath without blowing? How is breathing like burning?
Check questions:
1. Why did the candle go out in Faraday's breath?
2. What did breathed-out air do to lime-water, and what did that prove?
3. What is our "fuel"?
What we know now
Faraday thought the food's carbon met the air in the lungs. We now know the lungs pass oxygen into the blood, and the "burning" happens slowly in the cells all through the body; the blood then carries the carbon dioxide back to the lungs to be breathed out. Breathed-out air still has some oxygen in it, just not enough to keep a candle going.
Afterwards
Notice it this week: warm, misty breath on a cold day, and how much faster you breathe climbing stairs or running.
Tip
Faraday remarks on stuffy, badly aired homes of his time. It is a good moment to open a window and talk about why fresh air matters.
Answers to the book's problems
1. His lungs had taken oxygen out of the air, so there was not enough left to feed the flame. 2. It turned it milky, showing that breathed-out air carries carbonic acid (carbon dioxide), as a candle's gases do. 3. Food, such as sugar, which contains carbon. Arithmetic check: 72 + 11 + 88 = 171, and 11 + 88 = 99.
Worked out for this site from the scan. If a number in your copy differs, trust the book.