Hydrogen chloride

When hydrogen reacts with chlorine, hydrogen chloride is formed. Hydrogen chloride is a gas, and has the formula HCl(g).

When hydrogen chloride dissolves in water, hydrochloric acid is formed. This has the same formula, but you can tell the difference because of the state symbol (aq), which stands for ‘aqueous’. The formula is written as HCl(aq).

Hydrogen chloride is made from molecules. The hydrogen atom and the chlorine atom are joined by a covalent bond. When hydrogen chloride forms hydrochloric acid, the molecules split into ions.

HCl(aq) → H+(aq) + Cl-(aq)

The H+ ions make this aqueous solution acidic. The solution also conducts electricity because it contains ions that are free to move.

However, when hydrogen chloride gas dissolves in a solvent called methylbenzene, the molecules do not split up. A solution of HCl in methylbenzene does not contain hydrogen ions, so it is not acidic. The solution also has a low electrical conductivity.

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