The Chemical History of a Candle · Lecture II, part 1 · Part 2: Lecture II: Inside the flame
4. Inside the flame: wax vapor and the ring of heat
Goal: Your child can describe the dark middle of the flame as unburned wax vapor, and explain that the heat is in a ring where the vapor meets the air.
You'll need
- A plain candle (a short, thick one is best) and a few tea lights
- A large plate or metal baking tray to stand them on
- Matches or a long lighter (for the adult only)
- A glass of water nearby
- Science notebook and colored pencils
What's in the book
Faraday draws vapor out of the dark middle of the flame through a bent glass tube, heats wax in a flask, lights the vapor at the far end of a tube, and shows with a strip of paper that the heat of the flame is in a ring.
The lesson, step by step
Hook question
SayFaraday says a candle disappears, if burned properly, without leaving any dirt in the candlestick. Where does it all go?
That question carries us through the rest of the book.
Read together
Read pages 43 to 49, stopping at "Air is absolutely necessary for combustion."
(You must learn the difference between a gas and a vapour: a gas remains permanent, a vapour is something that will condense.)
What Faraday did
Watch only: don't try this at home.
Faraday put one end of a bent glass tube into the dark middle of the flame. A heavy white vapor flowed out of the other end and sank to the bottom of a flask. When he brought a light to it, it burned. Then he lit the vapor right at the far end of the tube:
Talk about laying on gas — why, we can actually lay on a candle!
He also heated wax in a glass flask until it gave off vapor, poured the vapor out and set it on fire. Hot glass and burning wax vapor are not home experiments. If you want to see it, search online for a video of a chemistry teacher doing this demonstration and watch it together.
The idea in plain words
There are two jobs going on in a flame: first the heat turns the wax into vapor, then the vapor burns where it meets the air. The dark middle is vapor that has not burned yet. The burning happens on the outside, where vapor and air come together.
Faraday showed this with a strip of paper pushed quickly across the flame: it was scorched in a ring, not in the middle.
It is in a ring, exactly in the place where I told you the chemical action was
A note on the paper ring
Faraday called the paper ring "a good experiment for you to make at home." We are skipping it: paper held in a flame catches fire very easily. Many science-demonstration videos show the scorched ring, if you want to see it.
Look and draw
Light the candle in still air and look at the flame from the side. Find the dark zone right around the wick. Draw a large flame in the notebook and label: wick, dark middle (wax vapor), bright part, blue at the base, and "the ring where vapor meets air."
Safety: an adult lights the flame and stays right there; tie back long hair and roll up sleeves; set the candle on a plate on a fireproof surface, away from paper and curtains; keep a glass of water nearby.
Say it back and check
Your child explains how Faraday "laid on a candle" through a tube.
Check questions:
1. What is the difference between a gas and a vapor, in Faraday's words?
2. What is in the dark middle of the flame?
3. Where is the flame hottest, and why there?
Afterwards
Notice it this week: the smell when a candle is blown out. Faraday says that bad smell comes from the wax vapor condensing.
Tip
Keep the flame drawing. You will add to it in the next two lessons.
Answers to the book's problems
1. A gas stays a gas; a vapor will condense back into a liquid or solid when it cools. 2. Wax vapor that has not burned yet. 3. In a ring on the outside, where the wax vapor and the air meet and the chemical action happens.
Worked out for this site from the scan. If a number in your copy differs, trust the book.