When the stopper of a bottle full of hydrogen chloride gas is opened, fumes can be seen coming out. Why does this happen?
There is an immediate reaction between hydrogen chloride gas and water vapour in the air.
HCl is a gas at room temperature. It is present as a liquid inside the bottle. When the stopper of the bottle is opened, HCl gas escapes out and comes in contact with water vapours in the atmosphere. Thus, white fumes are formed.