The Chemical History of a Candle · Lecture V, part 1 · Part 5: Lecture V: The air and carbonic acid
13. What air is made of: oxygen and nitrogen
Goal: Your child can explain that air is mostly nitrogen with about one part in five oxygen, and why the nitrogen matters.
You'll need
- 100 dried beans or buttons in two colors (or graph paper and colored pencils)
- Science notebook
What's in the book
Faraday asks why a candle burns more gently in air than in oxygen. He tests for oxygen with a gas that turns red, finds a gas left over that will not burn or support burning, and gives a table of the air: 20 parts oxygen to 80 of nitrogen by bulk.
The lesson, step by step
Hook question
But then you have a right to ask me, "How is it that the air and the oxygen do not equally well burn the candle?"
Let your child guess before reading on.
Read together
Read pages 127 to 134, stopping at the table of the atmosphere.
oxygen, which burns our candles, our phosphorus, or anything else; and this other substance — nitrogen — which will not burn them.
What Faraday did
Watch only: don't try this at home.
Faraday used a "test-gas" that turns red when it meets oxygen. He kept adding it to a jar of air and shaking it with water until no more red appeared. Some gas was left over that the test could not touch. That leftover gas was nitrogen. The test-gas is poisonous. If you want to see it, search online for a video of a chemistry teacher doing this demonstration and watch it together.
A "boring" gas that does a great job
It has no smell; it is not sour; it does not dissolve in water; it is neither an acid nor an alkali
But Faraday says imagine an atmosphere of pure oxygen:
When you see a fire in an iron grate, imagine where the grate would go to if the whole of the atmosphere were oxygen.
The nitrogen lowers it down and makes it moderate and useful for us
Model the air
we find that 5 pints of the atmosphere contain only 1 pint of oxygen, and 4 pints, or 4 parts, of nitrogen by bulk.
Count out 100 beans: 20 of one color for oxygen and 80 of another for nitrogen (Faraday's figures). Mix them in a bowl. That is a handful of air.
Say it back and check
Narration prompts: How did Faraday find the nitrogen? What would happen to a fire grate in pure oxygen?
Check questions:
1. Why does a candle burn more gently in air than in pure oxygen?
2. Name three things nitrogen does not do.
3. What else does nitrogen do, according to Faraday, besides calming the oxygen?
What we know now
Measured carefully, dry air is about 78 parts nitrogen, 21 parts oxygen and about 1 part argon, with a tiny amount of carbon dioxide. Argon was discovered in 1894, after Faraday's time; he counted it with the nitrogen. To show today's numbers, change two nitrogen beans: one into oxygen (21) and one into a third color for argon (1), leaving 78 nitrogen.
The book's note calls Faraday's test-gas "nitrous oxide." Today it is called nitric oxide, and the red gas it makes is nitrogen dioxide.
Afterwards
Notice it this week: every breath you take is about four-fifths nitrogen, which goes in and comes back out unchanged.
Tip
If your child likes numbers, have them turn 20 out of 100 into "one in five" and check it against Faraday's 5 pints.
Answers to the book's problems
1. Air is only about one-fifth oxygen; the nitrogen dilutes it and moderates the burning. 2. Any three: it will not burn, will not let a taper burn, has no smell, is not sour, does not dissolve in water, is neither acid nor alkali. 3. It carries away the fumes from burning and spreads them through the air to places where they help plants grow.
Worked out for this site from the scan. If a number in your copy differs, trust the book.