Question

Write any two observations in an activity which may suggest that a chemical reaction has taken place. Give an example in support of your answer.

Answer 1

when a chemical reaction takes place , any of these changes will be observed:-
change in color
change in state
evolution of gas
change in temperature
formation of precipitate
ex:- when lead nitrate and potassium iodide reacts , a yellow precipitate ie lead iodide is formed 

Answer 2

• Evolution of gas.

For example: When sodium carbonate reacts with dilute hydrochloric acid, carbon dioxide gas is evolved.

• Formation of a precipitate.

For example: When potassium iodide solution is added to a solution of lead nitrate, yellow precipitate of lead iodide is formed.