The Chemical History of a Candle · Lecture VI, part 1 · Part 6: Lecture VI: Carbon, breathing and the conclusion
16. Carbon burns with a spark
Goal: Your child can explain that carbonic acid is made of carbon and oxygen, that carbon burns as a glowing solid, and why it matters that its product is a gas.
You'll need
- A used, burnt match or a piece of barbecue charcoal
- A slice of very dark toast (optional)
- A pencil
- The clay water models from earlier, plus clay in a third color
- Science notebook
What's in the book
Japanese candles with hollow wicks; turpentine smoke burned up in oxygen; charcoal glowing in oxygen; carbonic acid made of 6 parts carbon to 16 of oxygen; potassium taking the oxygen out of carbonic acid; carbon from wood and coal-gas; and lead powder that catches fire by itself.
The lesson, step by step
Hook question
SayCharcoal is black and solid. When it burns, where does it go? Why is there hardly any ash?
Read together
Read pages 159 to 173, stopping at "Now, I must take you to a very interesting part of our subject."
I do not know a finer experiment than this, to shew that carbon burns with a spark.
What Faraday did
Watch only: don't try this at home.
He burned smoky turpentine in a flask of oxygen and the smoke vanished. He dropped red-hot powdered charcoal into oxygen: each grain burned as a spark, not a flame. A lump of charcoal glowed and slowly "dissolved" away. Then he burned potassium in carbonic acid and found black carbon left behind, proof that the gas is carbon plus oxygen. He also broke a tube of finely powdered lead that burst into fire the moment air touched it. If you want to see it, search online for a video of a chemistry teacher doing this demonstration and watch it together.
Find the carbon
If you take a piece of wood, and partly burn it, and then blow it out, you have carbon left.
Look at the burnt match or charcoal. Rub a little on paper. Compare it with burnt toast and with pencil marks: all black carbon. (Pencil "lead" is graphite, another form of carbon.)
whenever carbon burns under common circumstances, it produces carbonic acid.
Why carbon is a perfect fuel
Suppose all fuel had been like iron, which, when it burns, burns into a solid substance.
Iron and lead burn into solid crusts, so the air cannot reach the fuel underneath, and you are left with "more ash than fuel." Carbon burns into a gas that floats away, leaving fresh carbon open to the air. That is why coal, charcoal and wood make good fires.
Numbers and a model
Faraday says 6 parts of carbon and 16 parts of oxygen by weight make 22 parts of carbonic acid; with 28 parts of lime, that makes 50 parts of chalk, as in an oyster-shell.
Check: 6 + 16 = 22, and 22 + 28 = 50.
Clay model: one black ball (carbon) with two oxygen balls, one on each side. That is one piece of carbon dioxide.
Say it back and check
Narration prompts: How does charcoal burn in oxygen? How did Faraday get carbon back out of carbonic acid?
Check questions:
1. What two things is carbonic acid made of?
2. Why does pure charcoal leave no ash?
3. Why would iron make a bad fuel for a fireplace?
What we know now
Carbon dioxide is CO₂: one carbon atom and two oxygen atoms. Using today's atom weights, carbon to oxygen is 12 to 32, exactly the same proportion as Faraday's 6 to 16.
Afterwards
Notice it this week: the glowing embers of a fireplace or barbecue (from a safe distance, with an adult): carbon burning without flame.
Tip
Faraday points out that the Japanese candles have hollow wicks, like Argand's lamp from Lecture II. Ask your child why that would help (more air reaching the middle of the flame).
Answers to the book's problems
1. Carbon and oxygen. 2. It burns into a gas, carbonic acid, which passes away into the air, so nothing solid is left. 3. Iron burns into a solid crust that blocks the air from the iron underneath, and it would leave a heap of heavy ash.
Worked out for this site from the scan. If a number in your copy differs, trust the book.