I went to St. John’s College in Annapolis for a visit for a day and missed a lot of the lecture prior to the days of my visit, and they specialize in doing old school science experiments following original lab journals.
It was fascinating.
They had a lab assignment in which they replicated the original experiment used to demonstrate how water was H2O, by turning it into steam, running it through charcoal (carbon), and igniting it out the other side in a flask, to which we “realized” you can determine some gasses based on the sound they produce when they ignite. Hydrogen “chirps”.
Since I was only there for one day I didn’t get to learn the bulk of the material. Instead, I just learned how to replicate the building and initiation of the experiment.
I wonder to this day: how did the first scientist use what he had to determine that it was 2 parts hydrogen, instead of (let’s say) three parts hydrogen (for example)?
Thanks!
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Stoichiometry and a truly staggering mountain of logical deductions and algebraic calculations. The origin of all this was a man named Jeremias Benjamin Richter realizing that when you split water into hydrogen and oxygen you always got a ratio of 8 to 1 by weight. He did other reactions such as iron and oxygen and weighed the result. By making vast tables of chemical reactions the world of chemistry was eventually able to deduce how much every element weighed and realized that the mass of hydrogen produced by splitting water couldn’t be 1 part but 2.
[This discussion of how atomic weights were derived ](https://hsm.stackexchange.com/questions/3398/how-did-chemists-figure-out-atomic-weights-when-hydrogen-etc-were-first-disc) might be interesting to you. The short version is that until the 1800s there wasn’t a clear concept of atoms, molecules and compounds, so the determination of atomic mass, molecular mass and molecular formulas was fairly hit or miss.
Avogadro developed the theory that an equal volume of any gas contains the same number of molecules – in other words, a balloon at sea level with a volume of 1 liter contains the same number of molecules regardless of whether it’s full of Hydrogen, Helium, Oxygen, Nitrogen or any other gas.
Assuming Hydrogen has a value of 1, if you compare the mass of the balloon when filled with different gases you can determine relative molecular masses. Avogadro also theorized that even pure elements like elemental Hydrogen could exist as H2, O2, etc, rather than as single-atom molecules.
Others built on Avogadro’s theory to determine the molecular formulas for successively more complex chemicals using molecular weights and other methods. It’s really fairly brilliant how theoretical most of the history of chemistry was until we reached the point where methods like gas chromatography came along that allowed us to more directly analyze the composition of the stuff sitting in the sample tube.
A fascinating example of the “art” behind chemical discoveries is August Kekule’s discovery of the ring structure of benzene in the 1800s. The chemical formula for benzene was known, but it didn’t follow the same rules as other organic compounds because it was unsaturated (the ratio of hydrogen atoms to carbon atoms was lower than expected). Kekule theorized the benzene ring after having a daydream of a snake eating its own tail. He went on to publish papers backing up his theory with papers on the isomers of benzene, but the initial theory was a a logical exercise combined with an inspirational leap. That’s true of a lot of the advances in the understanding of chemistry as it evolved from the alchemical understanding of the universe.
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