Did you know that scientists didn’t used to believe in oxygen? Oxygen in the air helps things to burn. But chemists used to think that anything that could be burned contained a mysterious element called phlogiston.
The element that weighed less than zero
Scientists thought that the red hot glow of a burning metal was evidence of phlogiston escaping. They even decided that, because metal weighs more after burning, phlogiston must weigh less than zero! (We now know that the extra weight comes from oxide that forms on metal when it’s heated.)
More phlogiston nonsense
Joseph Priestly (1733-1804) was the first scientist to trap oxygen – but he didn’t realise what he’d done. The phlogisticians thought that when they placed a burning candle under a glass, it gave off phlogiston until the air in the glass was completely saturated with phlogiston.
So when a candle burned even more brightly in the “air” Priestly collected, he reasoned that the air must not contain any phlogiston at all. He called his oxygen sample “dephlogisticated air”!
Of course, this was exactly backwards. From The Mystery of the Periodic Table:
“The air Priestly thought was full of phlogiston was actually emptied of oxygen. The air he thought was entirely emptied of phlogiston, was actually full of oxygen.”
Goodbye phlogiston, hello oxygen
It was French scientist Antoine Lavoisier (1743-1794) who finally sorted things out and put phlogiston in its rightful place (the history books). How did he do it?
Lavoisier wanted to find out what really happened when a metal was heated. Was something removed from the metal and released into the air (as the phlogisticians believed), or was the reverse true – was something removed from the air and drawn into the metal?
He had the genius idea of measuring the volume of gas in his apparatus before and after the metal was heated. The result? Lavoisier found that when he heated metal, the volume of air around it decreased. Some of the air had combined with the metal!
Next, Lavoisier heated the specks that had formed on the metal and measured how much gas they gave off. Of course, it was the exact same amount as had left the air and gone into the metal previously.
Lavoisier had proved that neither phlogiston nor dephlogisticated air were real. He renamed dephlogisticated air, “oxygen”.
(Water + phlogiston) + (Water − phlogiston) = Water?
Even before Lavoisier’s breakthrough, scientists had begun to figure out that water was a combination of two separate things.
Henry Cavendish (1731-1810) had found that two parts of what he called”inflammable air” [hydrogen] combined with one part of “dephlogisticated air” [oxygen] made water.
But Cavendish’s inability to see beyond phlogiston got him in a bit of a pickle.
“He thought that inflammable air (hydrogen) was actually water plus phlogiston, and that dephlogisticated air (oxygen) was actually water minus phlogiston. What happens when you add water-plus-phlogiston to water-minus-phlogiston? The plus and minus phlogistons ‘cancel’ each other out, and you are left with only water!”
The Mystery of the Periodic Table
Thankfully Lavoisier – debunker of phlogiston – was able to put things in order. He made water by sparking oxygen with some of Cavendish’s “inflammable air”.
Now that he had proved that phlogiston didn’t exist, Lavoisier realised that inflammable air must also be an element itself. He named this gas, “hydrogen” (Greek, for water-generator).
Atomic Pancakes
The French rewarded Lavoisier for his services to science by chopping off his head. (They were a bit guillotine-crazy back then.) We decided to honour the great scientist by making atomic pancakes.
You need
- Pancake batter
- White chocolate chip “protons”
- Dark chocolate chips “neutrons”
- Small sweets e.g. M&Ms (all the same colour) – “electrons”
- Chocolate sauce (and a toothpick for spreading it into “orbits”)
(We actually used red and green grapes as protons and neutrons, but we struggled to fit them all into the nucleus of our oxygen atom.)
Proton and neutron grapes, and white chocolate chip electrons
{See full instructions here.}
First, we made two small hydrogen pancakes.
Hydrogen atom
Each hydrogen atom has one proton at its centre, and one electron orbiting the nucleus.
Then we made one big pancake for our oxygen atom.
Oxygen atom
Oxygen has 8 protons and 8 neutrons in its nucleus.
Oxygen also has 8 electrons – one pair in its first orbit, and 2 more pairs in its second orbit. The second orbit also contains 2 single electrons.
To make our water molecule, we put the 2 small pancakes beside the large pancake, lining up the 2 sets of unpaired electrons.
Water molecule
In reality, electrons are really far away from the protons and neutrons. If a proton were as big as a grape, you would need to walk an hour before you set down your electron!
Putting electrons into orbit
After eating up our atomic pancakes, we moved onto making the real thing. Come back soon to find out how we made oxygen and hydrogen out of water!
Resources
The Mystery of the Periodic Table - A wonderful living book about the history of chemistry – a great read aloud for all ages.
CSIRO - Australia’s national science agency’s website. I came across atomic pancakes via their (free) Science by Email program.
Chemistry, a Volatile History – Fascinating BBC documentary series with a whole episode on the phlogiston blind-alley. We saw it a few years ago and would love to see it again. YouTube has clips. Please let me know if you find the whole thing available somewhere!
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I’m appreciatively linking up here:
Science Sunday at Adventures in Mommydom
After School Link Up at Planet Smartypants
Weekly Wrap-Up at Weird, Unsocialized Homeschoolers
The HomeEd Linkup Week 7 at Adventures in Home Schooling
Science Saturday at Suzy Homeschooler
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