What is the most important clue that a chemical reaction is taking place?

I will give you two clues just incase:

-Change in temperature-usually when a reaction takes place heat will be absorbed or released which result in a temperature change.-eg: put a piece of sodium in water. you will feel or observe that the temperature is increased

-Change in colour-Some times we see a colour change in the media when a reaction takes

How important each one is depends on what you are learning about a chemical reaction. Hope I helped!

Icant see the pictures