The Chemical History of a Candle · Lecture IV, part 3 · Part 4: Lecture IV: Oxygen
12. Burning in oxygen
Goal: Your child can explain that things burn far more fiercely in pure oxygen than in air, and that burning in oxygen still makes the same products.
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
- Three glass jars of clearly different sizes
- A timer
- Science notebook (for a small chart)
What's in the book
A candle, a lamp, iron wire, sulfur and phosphorus burning in oxygen; soap bubbles of hydrogen and oxygen exploded on Faraday's hand; and potassium burning on water and on ice.
The lesson, step by step
Hook question
SayAir is only part oxygen. What would happen if a candle had pure oxygen to burn in?
Read together
Read pages 119 to 126, to the end of Lecture IV.
whatever can burn in air, can burn with a far greater intensity in oxygen, leading you to think that perhaps the atmosphere itself owes all its power of combustion to this gas.
What Faraday did
Watch only: don't try this at home.
In jars of oxygen, a candle blazed like the battery's spark, iron wire caught fire and burned brilliantly ("if it were a bar as thick as my wrist, it would burn the same"), sulfur burned fiercely, and phosphorus gave a glorious light:
I can do it better for you here than you can do it at home.
He even blew soap bubbles of mixed hydrogen and oxygen and exploded them on the palm of his hand. Never copy that. If you want to see it, search online for a video of a chemistry teacher doing this demonstration and watch it together.
The same products
Even in oxygen, the candle made nothing new: the same water and the same products as in air, only faster. Faraday explains the potassium trick too: potassium grabs the oxygen in water and sets the hydrogen free, and the hydrogen burns. It even works on ice, "a sort of volcanic action."
A jar race at home
The more air (and so the more oxygen) a flame has, the longer it can burn. Light a tea light, cover it with the smallest jar and time how long it burns. Air out the jar and repeat with the middle and the largest jar. Make a small chart: jar size and seconds.
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 adult lifts the warm jars.
Say it back and check
Narration prompts: What happened to the iron wire in oxygen? Why does potassium set water on fire?
Check questions:
1. Does a candle burning in oxygen make anything different from a candle burning in air?
2. Which jar kept the candle burning longest, and why?
3. What does Faraday promise at the end of the lecture about candles, gas and our fireplaces?
What we know now
Pure oxygen is used in hospitals and by divers and pilots. Because almost anything burns fiercely in it, there must be no flames, sparks or smoking anywhere near an oxygen tank.
Afterwards
Notice it this week: why do people blow on or fan a campfire or barbecue? (More air, more oxygen.) Watch with an adult.
Tip
Faraday closes by promising that none of these strange effects happen with an ordinary candle or fire "so long as we confine ourselves within the laws that Nature has made for our guidance." A nice line to end on.
Answers to the book's problems
1. No: the same water and the same products, only much faster and brighter. 2. The largest jar, because it holds the most air, so the flame takes longer to use up the fresh air. 3. That none of these strange and harmful actions happen with an ordinary candle, gas lamp or fire, as long as we keep within the laws of Nature.
Worked out for this site from the scan. If a number in your copy differs, trust the book.