The Chemical History of a Candle · Lecture II, part 2 · Part 2: Lecture II: Inside the flame
5. Fresh air, smoke, and two kinds of burning
Goal: Your child can show that a flame needs fresh air, explain why a starved flame smokes, and tell the difference between burning with and without flame.
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
- Two or three clean glass jars of different sizes (tall enough to cover the candle with room to spare)
- A watch or phone with a timer
- Science notebook
What's in the book
A jar over a candle; the Argand lamp with air let into the middle of the flame; a large smoky flame of turpentine; gunpowder burning with iron filings; and a puff of lycopodium powder.
The lesson, step by step
Hook question
SayIf we put a jar over a candle, the jar is still full of air. So why should the candle go out?
Read together
Read pages 49 to 55.
Air is absolutely necessary for combustion; and, what is more, I must have you understand that fresh air is necessary
The jar over the candle
Stand a tea light on the plate and light it. Lower a glass jar over it, mouth down, and start the timer. Watch the flame stretch, fade and go out. Lift the jar away, let it air out, then try a bigger jar. Record the times in the notebook.
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. The jar gets warm, so the adult lifts it off.
Faraday's explanation:
Not because it wants air merely, for the jar is as full now as it was before; but it wants pure, fresh air.
Starved flames smoke
Faraday showed a lamp burning poorly when air was kept from the middle of the flame, and brightly when Argand's clever design let air in. Then he burned turpentine on a big ball of cotton, which sent a stream of black soot into the air, because a big flame cannot get enough air.
Watch only: don't try this at home. Burning turpentine is dangerous.
SayLess air means imperfect burning, and imperfect burning means soot.
Burning with and without flame
Watch only: don't try this at home.
Faraday mixed gunpowder with iron filings and lit it. The gunpowder burned with a flame; the iron filings were thrown up and burned as sparks, with no flame. Then he puffed lycopodium powder (a fine plant powder) through a flame to make a sudden cloud of fire:
This is the lightning of the pantomimes, and a very good imitation.
He warned his audience himself:
Before I go into these experiments, let me hope that none of you, by trying to repeat them, for fun's sake, will do any harm.
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: What happened to the candle under the jar, and why? What did the iron filings do that the gunpowder did not?
Check questions:
1. Why did the candle go out in a jar that was still full of air?
2. Why does a very large wick make smoke?
3. What two kinds of burning did the gunpowder and iron show?
What we know now
The candle under the jar goes out long before all the oxygen is used up. The air in the jar becomes "partly changed," as Faraday says: it has less oxygen and more of the gases the flame gives off, until there is too little oxygen to keep the flame going.
Afterwards
Notice it this week: a fire or a camping stove that smokes when it is starved of air and burns cleanly when air can reach it.
Tip
Faraday says the starved flame, the smoky flame and the well-fed flame are all worth understanding. Let your child predict each time before you show or read the result.
Answers to the book's problems
1. It needs fresh air; the air in the jar has been partly changed by the burning, and there is not enough fresh air left to keep it burning. 2. A larger flame needs a larger supply of air; without enough air it burns imperfectly and throws off soot. 3. Burning with a flame (the gunpowder) and burning without a flame, as glowing sparks (the iron filings).
Worked out for this site from the scan. If a number in your copy differs, trust the book.