The Chemical History of a Candle · Lecture III, part 3 · Part 3: Lecture III: Water and hydrogen
9. Hydrogen, the water-maker
Goal: Your child can describe hydrogen (very light, burns with a hot pale flame, makes only water when it burns) and explain why it is not a home experiment.
You'll need
- A party balloon blown up with breath
- A helium party balloon, if you can get one (optional)
- Science notebook
What's in the book
Faraday names hydrogen, makes it with zinc and acid in a "philosopher's candle," burns it to make water, blows soap bubbles and balloons with it, gives its weight against water, and ends with the flash of a large voltaic battery.
The lesson, step by step
Hook question
SayWhat do you think is the lightest gas in the world?
Read together
Read pages 90 to 98, to the end of Lecture III.
This is hydrogen — a body classed among those things which in Chemistry we call elements, because we can get nothing else out of them.
And this gas has been so named hydrogen, because it is that element which, in association with another, generates water.
Why we watch only
Faraday wanted his audience to make hydrogen at home with zinc and acid, and he gave a careful warning:
I am not afraid to shew you, for I wish you to make experiments, if you will only make them with care and attention, and the assent of those around you.
Watch only: don't try this at home. Hydrogen mixed with air explodes when lit, and the acids are dangerous. Faraday himself fills his little bottle nearly full of liquid for exactly that reason. If you want to see it, search online for a video of a chemistry teacher doing this demonstration and watch it together.
What hydrogen does
Faraday lit a jet of hydrogen:
It is a foolish feeble sort of a flame, you may say; but it is so hot that scarcely any common flame gives out so much heat.
A cold glass held over it was soon wet: burning hydrogen makes water, and nothing else.
It is important to remember that this hydrogen is the only thing in nature which furnishes water as the sole product of combustion.
Light gas, heavy water
Soap bubbles blown with hydrogen rose to the roof of the theater. Compare: let go of a breath-filled balloon (it sinks) and a helium balloon (it rises). Helium is the safe light gas we use today.
Faraday's numbers: a pint of hydrogen weighs three-quarters of a grain; a pint of water weighs 8,750 grains. Ask your child to work out about how many times heavier the water is (8,750 ÷ ¾).
The battery
Watch only: don't try this at home. Faraday ended with a huge battery of forty zinc plates, making a brilliant flash and burning an iron file. He said that, used wrongly on himself, it "would destroy me in an instant." Never experiment with electricity from wall sockets.
Say it back and check
Narration prompts: What is hydrogen's flame like? What does hydrogen make when it burns? How did Faraday show it is light?
Check questions:
1. What does the name hydrogen mean?
2. Why did Faraday fill the bottle nearly full before lighting the gas at the tube?
3. Where is the hydrogen in a candle?
What we know now
Hydrogen is the lightest element of all (symbol H). Balloons and airships were once filled with it, but it burns so easily that they were dangerous; helium, which does not burn, is used instead. Hydrogen and oxygen are still burned together as a rocket fuel.
Afterwards
Notice it this week: keep checking the rusting nail. Also look at the ingredients on a candle or lamp-oil label; these fuels are made mostly of carbon and hydrogen.
Tip
Let your child do the division with paper or a calculator; the answer is not meant to be exact, just enormous.
Answers to the book's problems
1. "Water-maker" (from the Greek words for water and to generate, as the book's footnote shows). 2. Hydrogen mixed with air explodes; the air had to be swept out of the bottle first. 3. It is part of the wax; when the candle burns, its hydrogen joins with something from the air to make water. Arithmetic: 8,750 ÷ 0.75 ≈ 11,667, so a pint of water is roughly 11,700 times heavier than a pint of hydrogen.
Worked out for this site from the scan. If a number in your copy differs, trust the book.