Chlorine gas turns damp starch-iodide paper blue-black. 

• The chlorine oxidizes the iodide ions on the paper into iodine, which then reacts with the starch to produce a blue-black compound.
• Chemical reaction: The chemical reaction is:

 Cl\(_2\)(aq) + 2I\(^−\)(aq) → I\(_2\)(aq) + 2Cl\(^−\)(aq)