Sulfur Dioxide (SO₂):
* Effect on litmus: SO₂ dissolves in water to form a weak acid (sulfurous acid, H₂SO₃). Acids turn blue litmus paper red.
* Observation: When exposed to SO₂, blue litmus paper will turn red.
Chlorine (Cl₂):
* Effect on litmus: Chlorine is a strong oxidizing agent. It reacts with water to form hydrochloric acid (HCl) and hypochlorous acid (HOCl), both of which are acidic.
* Observation: Blue litmus paper exposed to chlorine will also turn red.
The Problem:
While both gases turn blue litmus red, there's a key difference:
* SO₂: The red color change caused by SO₂ is reversible. If you remove the SO₂ and expose the litmus paper to air, it will usually return to its original blue color. This is because the sulfurous acid formed is relatively weak and can easily decompose back into SO₂ and water.
* Cl₂: The red color change caused by Cl₂ is irreversible. The strong acids produced by Cl₂ will permanently bleach the litmus paper, making it turn white instead of returning to blue.
Conclusion:
* If the litmus paper turns red and then returns to blue when exposed to air, it was likely SO₂.
* If the litmus paper turns red and then stays red or turns white, it was likely Cl₂.
Important Note: While this method can be helpful, it's best to use it in conjunction with other tests to confirm the identity of the gas. Always handle these gases with caution, as they are both toxic.
