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salt in water

Posted: Fri Jun 08, 2018 2:40 pm
by Briana Lopez 4K
Dr. Lavelle said in the lecture today that salts do not affect the pH of water. Why is this? And how is related to not affecting the boiling point as well?

Re: salt in water

Posted: Fri Jun 08, 2018 4:43 pm
by Andrew Evans - 1G
Okay hi Briana,

So Dr. Lavelle DIDN'T say salts do not change pH of water. In fact the entire lecture was giving examples of how salts can change pH. Where you're mistaken is that at the beginning of lecture he said how TABLE SALT (NaCl) doesn't affect pH. That's a specific case and is similar to most other salts involving group 1 and group 2 metal cations.
Salts with these types of ions, characterized as elements of strong acids and strong bases do not change the pH of water alone. This is because they dissociate easily and remain as individual ions floating around in the solution without interactions with H2O that actually break its bonds.

Meanwhile salts with ions that are themselves weak acids (they have some extra proton somewhere that doesn't need to be there), that proton will dissociate from that ion and form hydronium. This would make the solution acidic.
Small, highly charged metal ions can also act as acids. We learned that coordination complexes consist of transition metal cations that adopt ligands which provide lone pairs. When water acts as a ligand for some transition metal cations, that water molecule sometimes loosens its bond with one of its H's, allowing it to break off and enter the solution as a free H+ proton and form hydronium.
Salts with ions that are themselves the conjugate bases of weak acids (they have some lone pair that could pick up a proton) they will adopt a proton from water making a hydroxide (OH-). This would make the solution alkaline (basic)

So some salts do indeed change the pH of the solution, but only if they tend to form interactions with water that alter H2O's structure. If it doesn't change the structure of H2O it won't change the pH (Cl-, Na+...)

Reading back over this it seems like a big jumble of ideas, but I hope it helps a little.

-Andrew Evans
Section 1G

Re: salt in water

Posted: Fri Jun 08, 2018 5:54 pm
by Eli Esagoff_1I
He was explaining that both Na+ and Cl- in table salt does not change the pH of water. This does not apply to all types of salts.