2.11      understand the difference between hydrogen chloride gas and hydrochloric acid

Hydrogen chloride is a gas at room temperature and is made of molecules, H–Cl  with a covalent bond between the H and Cl atoms (see section 1.40). Its formula is HCl(g).

When we dissolve hydrogen chloride gas in water, it reacts. The H–Cl covalent bond breaks and it makes a solution which contains H+ ions and Cl- ions. This solution is called hydrochloric acid. (It is acidic because of the H+ ions). Its formula now is HCl(aq).