Why are some salts acidic when others are neutral?

Question

Need some help explaining this.

Answer 1

It has to do with the ability of the ions produced to interact with the water molecule. For example, when cations such as Al³⁺ are introduced into solution, the 3+ charge on the cation causes the electron distribution in the water molecule to shift more towards the oxygen, essentially making it easier for protons (hydrogen ions) to fall off the water molecule. When the concentration of the hydrogen ion increases, the pH decreases (becomes more acidic). Basically (pun intended) the water molecule becomes more acidic.

Certain anions can also affect the pH if they are capable of abstracting (taking) protons from water. When this happens hydroxide is left behind from the water molecule which increases the concentration of OH- and raises the pH.

Many ions are spectators in acid-base chemistry, Alkali metal ions, conjugates of the strong acids / bases etc. These ions would not affect the pH and we would expect solutions that contain exclusively those ions to remain neutral.

Answer 2

Neutral salts are salts formed by reaction of a strong acid and a strong base. The ions of such a salt do not undergo a hydrolysis reaction with water, so they are neutral.

An acidic salt is a salt made by reaction of a strong acid and a weak base. The weak base ion reacts with water to from some of the weak base, removing hydroxyl ions and leaving an excess of hydrogen ions that makes the solution acidic.

For example, NH4Cl dissociates into NH4¹⁺ and Cl^1- ions. The NH4¹⁺ ions react with water in this way

NH4¹⁺ + H2O ↔ NH₄OH + H¹⁺ This results in an excess of H¹⁺ ions. Cl^1- does not react with water as it comes from a strong acid.