A silver ring reacts with compounds containing sulfur in the air to form silver sulfide,
a black substance that makes up the tarnish on the surface of silver objects. To
remove the tarnish from the ring, students placed it in a pan lined with aluminum foil
and added hot water. Baking soda was added to the hot water and stirred. Students
made observations about the process.

Which observation of this process provides evidence of a chemical reaction?