Question

what is the best evidence that a gas has been produced in a chemical reaction? change in color change in temperature formation of bubbles

Explanation:

When a gas is produced in a chemical reaction, it is often seen as bubbles forming. A change in color indicates a different chemical change (like a new compound with different color - related properties). A change in temperature indicates an exothermic or endothermic reaction. But specifically for gas production, bubble formation is the most direct evidence.

Answer:

formation of bubbles