Question

Explain why dilute hydrochloric acid cannot be concentrated by boiling beyond 22.2%.

Answer 1

Distillation slowly increases the strength of a weak hydrochloric acid solution until it has 22.2% HCl by weight and boils at 110°C. Once this level is reached, boiling cannot be used to make the acid stronger anymore. When this concentration is attained, boiling [constant boiling mixture or azeotrope] cannot be used to enhance the acid concentration any further.

Water and HCl are the main components of vapours that form before 110°C, although HCl molecules make up the majority of vapours that form above this temperature.