What is the difference between dilute #HCl# and liquid #HCl#?
One is a solution of a strong acid, the other is just a liquid (no acidic properties).
Liquified HCl has been cooled until the gas turns to a liquid, in the same way most other gases change on cooling. It is a pure substance in the liquid phase.
"Dilute" HCl implies you have dissolved a small amount of HCl gas into water. In this case, an ionization reaction occurs as follows:
This is a complete reaction (hence HCl is a strong acid), and the resulting hydronium ion,
Liquid HCl does not display these properties, as no reaction with