The Chemical History of a Candle · Lecture III, part 1 · Part 3: Lecture III: Water and hydrogen
7. Water from a candle, and water's changes
Goal: Your child can show that a burning candle makes water, and explain that water is the same substance whether it is ice, liquid or steam.
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
- A small metal bowl with ice and a little water in it
- An oven mitt
- A plastic cup, a marker, and a freezer
- Ice cubes and a glass of water
- Science notebook
What's in the book
Faraday collects a drop of water under a dish of ice and salt held over a candle, tests it with potassium, and shows bottles of water made by burning gas, oil and wax. Then he shows water as ice and steam: iron bottles burst by freezing, a flask of steam rattling its lid, and a tin flask crushed as steam condenses.
The lesson, step by step
Hook question
SayLast time the cold spoon went misty. Can we prove the mist is really water, and where it came from?
Read together
Read pages 68 to 81 (you can read the part about steam and ice more quickly).
we find that that part of the product is just water — nothing but water.
Catch water from a candle
This is Faraday's own set-up, made safe. Wearing the oven mitt, the adult holds the metal bowl of ice water a few inches above the flame for half a minute. Look under the bowl: drops of water collect there, with a little soot. Faraday said:
The head of a poker is a very good thing to try with, and if it remains cold long enough over the candle, you may get water condensed in drops on it
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.
How Faraday tested the drop
Watch only: don't try this at home.
Faraday dropped a bit of potassium, a soft metal, into a basin of water: it lit up and floated about, burning with a violent flame. Then he put potassium on the drop collected from the candle, and it took fire the same way. That proved the drop was water. Potassium is far too dangerous to handle outside a laboratory. If you want to see it, search online for a video of a chemistry teacher doing this demonstration and watch it together.
He also said:
A pint of oil, when burnt fairly and properly, produces rather more than a pint of water.
Ice and steam
Water expands when it freezes. Faraday froze water in thick iron bottles screwed shut until they burst. Try it the safe way: fill a plastic cup halfway with water, mark the level, and freeze it overnight. Check the new level in the morning. (Never freeze water in glass or in anything sealed.)
Drop an ice cube in a glass of water:
Why does the ice float? — think of that, and philosophise.
Faraday's tin flask, crushed when cold water made its steam condense, is watch only: hot steam burns badly.
Say it back and check
Narration prompts: How did Faraday prove the drop was water? What happened to the iron bottles? Why does ice float?
Check questions:
1. Is water made by a candle different from river water?
2. Faraday says a cubic inch of water makes about a cubic foot of steam. How many times bigger is that?
3. Why does ice float?
What we know now
Water does expand by about one-tenth when it freezes, which is why ice floats and frozen pipes burst. Faraday's cubic inch to cubic foot is very close to today's figure: water swells about 1,700 times when it turns to steam at boiling point.
Afterwards
Notice it this week: water appearing where something cold meets warm, damp air: a cold drink "sweating," a misty bathroom mirror, a window on a cold morning.
Tip
Faraday mentions frozen pipes in his own house the week before ("the sad catastrophe"). It was a very cold winter in London.
Answers to the book's problems
1. No. Faraday says it is exactly the same: "Water is one individual thing — it never changes." 2. A cubic foot is 12 × 12 × 12 = 1,728 cubic inches, so about 1,728 times bigger. 3. Ice takes up more room than the water that made it, so it is lighter for its size, and floats.
Worked out for this site from the scan. If a number in your copy differs, trust the book.