John Carlos BaezJun 9, 2024

If you could watch an individual water molecule, about once in 10 hours you'd see it do this!

As it bounces around, every so often it hits another water molecule hard enough enough for one to steal a hydrogen nucleus - that is, a proton - from the other!

The water molecule with the missing proton is called a hydroxide ion, OH⁻. The one with an extra proton is called a hydronium ion, H₃O⁺.

This process is called the 'autoionization' of water. Thanks to this, roughly one in ten million molecules in a glass of water are actually OH⁻ or H₃O⁺, not the H₂O you expect.

And this gives a cool way for protons to move through water. Let's watch it!

(1/n)

Show threadDaniel DuqueJun 9, 2024
@johncarlosbaez Nature is fine-tuned so that, in standard conditions, one in 10^7 molecules is doing this. 😆
Show threadxrvsJun 9, 2024

@dduque @johncarlosbaez it’s a rounded number though (but yes, it’s still neat that it’s close to 7 and not 6.84 for example)

(also it’s room condition, not standard condition, which would be at 0°C where it would have fewer ions and higher pH)

Show threadxrvsJun 9, 2024
@dduque @johncarlosbaez now that i’m thinking more of this, pH is defined as -log([H+]) where [H+] is concentration in mol/L, so pH=7 should mean that there’s 10^-7 mol of H+ per litre of H2O, and using rough numbers (1L water = 1000g, molecular mass = 18u), that’s 56 mol, so there’d be one in ~5.6×10^8 molecules that does this?
Show threadDaniel Duque
@xarvos @johncarlosbaez Yes, thank you. I was joking about nature being tuned, of course. And I was sloppy in defining the conditions, and (most importantly) in the thing about being 10^-7 per LITRE, not per H2O molecule. Nevertheless, I am collecting a list of physical figures that have no reason to be "nice numbers", and turn out to be. Like the water viscosity coefficient
Jun 9, 2024 at 5:19pmWeb