Salts

When an acid and a base react, we call it a neutralisation reaction. The products are water and a salt. Until now, we have looked at strong acids (like HCl) reacting with strong bases (like NaOH). The salt is neutral (pH = 7), so the final (salt) solution is neutral.

SOURCE: https://byjus.com/chemistry/neutralization-reaction/

However, this is not the case when we use a weak acid or a weak base. The salts themselves react with water, so affect the pH. For example, when we react hydrochloric acid (strong acid) with ammonia (weak base), the product is acidic ammonium chloride.

SOURCE: http://acids-are-pretty-basic.weebly.com/conjugate-acids-and-bases.html

We know it is acidic because one of the ions in the salt, ammonium, can act as a weak acid.

We see that the ammonium ion increases the concentration of hydronium ions, so making the solution more acidic (lowering the pH).

NOTE; The chloride ion does not react with water, so has no effect upon pH.

Learning Outcomes

• Identify the ion in the salt that affects the pH
• Use an equilibrium equation to show how this ion reacts with water
• Use the equation to explain the nature of the salt (acidic or alkaline)