The Chemical History of a Candle · Lecture V, part 3 · Part 5: Lecture V: The air and carbonic acid
15. The invisible product: carbonic acid
Goal: Your child can make carbon dioxide safely, show that it is heavier than air and puts out a flame, and connect it to chalk, shells and soda water.
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
- Baking soda and white vinegar
- A tall pitcher or large jug, and a spoon
- Soap-bubble mixture and a wand, and a large bowl or plastic tub
- A bottle of plain sparkling water (optional)
- Science notebook
What's in the book
The gas from a chimney over a candle puts a light out and turns lime-water milky. Faraday shows that chalk, shells and marble hold the same gas (Dr. Black's "fixed air"), makes it with acid and marble, dissolves it in water, pours it like water, and floats a soap bubble on it.
The lesson, step by step
Hook question
SayWe have caught the soot and the water from a candle. Is anything else going up that we can't see?
Read together
Read pages 147 to 158, to the end of Lecture V.
it is not moisture — it is not water — it is not condensible; and yet, after all, it has very singular properties.
The lime-water test
Watch only: don't try this at home.
Faraday held a bottle over a chimney above a candle, then shook clear lime-water in it. The lime-water turned milky white; plain air did nothing to it. The white stuff turned out to be chalk. Making lime-water uses quicklime, which burns skin and eyes. If you want to see it, search online for a video of a chemistry teacher doing this demonstration and watch it together.
Chalk, shells and marble
All limestones contain a great deal of this gas which issues from the candle, and which we call carbonic acid. All chalks, all shells, all corals contain a great quantity of this curious air.
Faraday made the gas by dropping marble into muriatic acid, and from chalk with strong sulfuric acid. Watch only: strong acids are not for home. Our safe way uses baking soda and vinegar.
Pour an invisible gas
Put two spoonfuls of baking soda in the tall pitcher and add a good splash of vinegar. Let it fizz for a minute; the gas collects in the pitcher. Light a tea light on the plate. Tip the pitcher slowly over the flame as if pouring water, without letting the liquid come out. The flame goes out.
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.
This is Faraday's pouring and bucket experiment. He warns:
indeed, we too often have real wells of carbonic acid
Float a bubble
Make more gas in the large bowl or tub (more baking soda and vinegar, with the tub's sides high). Blow soap bubbles gently over it. Some will sink and then hover on the invisible layer:
It is floating, as the balloon floated, by virtue of the greater weight of the carbonic acid than of the air.
Optional: taste a sip of plain sparkling water. Faraday found water with the gas dissolved in it "tastes a little acid to the mouth."
Say it back and check
Narration prompts: How did Faraday find the third product of the candle? Why did the flame go out when we "poured" the gas? Why did the bubbles float?
Check questions:
1. What did Dr. Black call this gas, and why?
2. Name three things that contain it.
3. Is it heavier or lighter than air? How do you know?
What we know now
Faraday's "carbonic acid" is what we call carbon dioxide (CO₂). Today "carbonic acid" means carbon dioxide dissolved in water, as in soda water. Because it is heavy, carbon dioxide can collect in low places like wells, pits and cellars, where people have been overcome by it. Never climb into such places.
Afterwards
Notice it this week: the fizz in soda, bread rising with baking soda or yeast, and the shells or chalk around your house. All of them involve this one gas.
Tip
Go slow when tipping the pitcher. The gas pours out invisibly, and pouring too fast mixes it with air.
Answers to the book's problems
1. "Fixed air," because he found it fixed in solid things like marble and chalk, where it had lost its quality of air and become part of a solid. 2. Any three: limestone, chalk, marble, shells, corals, and the products of a burning candle. 3. Heavier: it can be poured downward into a glass, it sinks onto a flame and puts it out, and a soap bubble floats on it. Faraday says plainly that it is heavier than the atmosphere, about 16 grains to the pint.
Worked out for this site from the scan. If a number in your copy differs, trust the book.