Here's why:

* pH is a measure of acidity or alkalinity in a solution. Solutions are mixtures of a solute dissolved in a solvent (usually water).

* Chlorine gas is a pure compound and does not form a solution with water. It reacts with water to form hypochlorous acid (HOCl) and hydrochloric acid (HCl), which do have pH values.

However:

* When chlorine gas is dissolved in water, it forms a solution that is acidic. This is because the hydrochloric acid produced lowers the pH of the solution.

* The pH of a chlorine solution will vary depending on the concentration of chlorine gas dissolved.

Important Note: Chlorine gas is a toxic and hazardous substance. It should only be handled by trained professionals using appropriate safety equipment.