2.7.1. Aqueous Solutions of Various Salts Can Be Neutral, Basic, or Acidic. How Cations and Anions Produced on Dissociation of Salts Interact with Water.
On addition of an acid such as HCl or H2
to water, the original pH value of 7 goes down because the concentration of the H+
goes up. On addition of a base such as NaOH or Ca(OH)2
to water the pH goes up because the concentration of the H+
decreases due to an increase in the concentration of the OH-
. Will the neutral pH of 7 of pure water change on addition of a soluble salt? It might and it might not.
Some salts such as NaCl, Ba(NO3
, and K2
have no effect on the pH of water. However, if we dissolve baking soda (NaHCO3
) in water, the resultant solution will be slightly basic (pH ≈ 8). On dissolution of Na2
in water, the pH goes to 11 — no kidding!
— thereby indicating that a solution of Na2
is quite alkaline. On the contrary, aqueous solutions of ZnCl2
are acidic, the pH being as low as 1 at a high concentration of zinc chloride. Why? Because some ions produced on dissociation of a salt can and do interact with water molecules in an interesting way.
Let us consider an aqueous solution of ZnCl2
(Figure 2-87). Being a salt, ZnCl2
readily dissociates in water into the ions Zn2+
. Water dissociates too, although to a much smaller degree, to give rise to H+
. The blue and red ellipses in Figure 2-87 show interactions of the oppositely charged ions derived from ZnCl2