The Chemical History of a Candle · Lecture IV, part 2 · Part 4: Lecture IV: Oxygen
11. Two gases from water: oxygen gets its name
Goal: Your child can explain that water splits into two volumes of hydrogen and one of oxygen, and describe the test for each gas.
You'll need
- Modeling clay or play dough in two colors (or marshmallows and toothpicks)
- Science notebook
What's in the book
Faraday collects the two gases from water in separate jars, one filling twice as fast; tests one as hydrogen and finds the other makes a splinter burn brightly; names it oxygen; gives the weights of water's parts; and makes oxygen from black oxide of manganese and chlorate of potash.
The lesson, step by step
Hook question
SayIf we split water and keep the two parts in separate jars, which jar do you think will fill faster?
Read together
Read pages 110 to 119, stopping at "Now, as regards this very property of oxygen supporting combustion."
The gas itself will not burn, but it will make the splinter of wood burn.
Naming oxygen
What shall we call it, A, B, or C? Let us call it O — call it "Oxygen:" it is a very good distinct-sounding name.
Faraday is teasing: the name oxygen was already well known by 1860. But he wants the children to feel they found it themselves.
Two to one
we get two volumes of hydrogen, and one of the body that burns it.
Faraday's table shows that by weight, water is 88.9 parts oxygen and 11.1 parts hydrogen out of 100. So oxygen is the heavy part. He gives the weights: a pint of hydrogen weighs three-quarters of a grain, and a pint of oxygen nearly twelve grains.
Ask: How many times heavier is oxygen than hydrogen, pint for pint? (12 ÷ ¾)
What Faraday did
Watch only: don't try this at home.
Faraday made oxygen by heating black oxide of manganese red-hot in an iron bottle, and by heating chlorate of potash with it in a glass retort. A glowing or burning splinter blazes up brightly in oxygen. These chemicals and hot glass are not for home. If you want to see it, search online for a video of a chemistry teacher doing this demonstration and watch it together.
Make a model of water
Use clay to make one large ball (oxygen) and two small balls (hydrogen), and stick them together. That is one tiny piece of water. Make several, and count: twice as many hydrogens as oxygens, just as Faraday's jar filled twice as fast.
Say it back and check
Narration prompts: How did Faraday test each jar? Why did one jar fill twice as fast?
Check questions:
1. What is the test for hydrogen? For oxygen?
2. Twice as much hydrogen by volume: does that mean twice as much by weight?
3. Where does the candle get its oxygen?
What we know now
We write water as H₂O: two hydrogen atoms joined to one oxygen atom. An oxygen atom is about sixteen times as heavy as a hydrogen atom, which matches Faraday's pint weights.
Afterwards
Notice it this week: the formula H₂O on bottled-water ads or science posters. Now you know what the 2 means.
Tip
Keep the clay models. You will add carbon to them later in the course.
Answers to the book's problems
1. Hydrogen burns with a pale flame at the mouth of the upside-down jar; oxygen does not burn itself but makes a burning splinter burn much more brightly. 2. No. Oxygen is much heavier, so by weight water is mostly oxygen (88.9 parts in 100). 12 ÷ 0.75 = 16 times heavier, pint for pint. 3. From the air.
Worked out for this site from the scan. If a number in your copy differs, trust the book.