# Why won't water molecules dissociate?

1. Jul 25, 2015

### Lim Y K

Pure water is not an electrolyte. This is because the hydrogen atoms and oxygen atom will not dissociate into individual charged particles. Why won't they dissociate?

2. Jul 26, 2015

### Staff: Mentor

They do dissociate, as described by a water ion product. However, concentrations of H+ and OH- thus produced are quite low, so the specific resistance of a pure water is quite high (18 MΩ).

3. Jul 26, 2015

### tommyxu3

Besides, that's not two atoms'( $\text{H}$ and $\text{O}$ ) dissociation, but $\text{H}^+$ and $\text{OH}^-.$

4. Jul 27, 2015

### PWiz

Isn't the actual ionic equilibrium $2H_2 O (l) ⇔ H_3 O^{+} (aq)+ OH^{-} (aq)$ ? If I remember correctly, H+ ions are just used instead of hydronium ion for simplification of the equation, but the actual equation is the one above.

5. Jul 27, 2015

### Staff: Mentor

Protons can be solvated by several water molecules (so there exists whole series of cations of the general formula H2n+1On+). H+ is about as good as H3O+ IMHO - works OK as a symbol, doesn't reflect the reality. Add to that fact that OH- is not isolated in the solution, but solvated as well (yielding H2n+1On+1- anions), and you should start to see the picture

6. Jul 27, 2015

### PWiz

Ahh, I see. But is it safe to say that n is not large? I mean after all, you'll eventually crowd up the entire area around the H+ ion so that the repulsion between the lone pair of electrons on the O atoms of water molecules solvating the ion and the lone pairs on the O atoms of 'unlinked' water molecules is greater than any attractive force the 'unlinked' water molecules might feel toward the H+ ion, preventing these 'unlinked' molecules from solvating the H+ ion as well.
P.S. I can't find a replacement word for 'unlinked' from my Chemistry verbiage memory bank right now :P

7. Jul 27, 2015

### Staff: Mentor

8. Jul 27, 2015

### PWiz

9. Jul 27, 2015

### DrDu

I think the difference is that in H3O+, the proton is covalently bounded to a water molecule while the additional water molecules in the larger clusters are bound via hydrogen bridges.

10. Jul 27, 2015

### Staff: Mentor

I would expect the change to be gradual - for small n (like 2 or 3) I would guess the cluster to be quite symmetrical, and all water molecules to be equivalent. Once the structure gets larger the outer layers will be definitely bonded differently.

11. Jul 27, 2015

### DrDu

The only symmetric hydrogen bond I know about is the FHF-. So even in small water clusters, you can uniquely define which one is the hydronium ion.