Final answer:
Yes, the equality of the normal force and the weight of the chair is an illustration of Newton's third law of motion, which states that for every action there is an equal and opposite reaction.
Step-by-step explanation:
The equality of the normal upward force exerted by the floor on a chair being equal in size but opposite in direction to the weight of the chair does illustrate Newton's third law of motion.
This law states that for every action, there is an equal and opposite reaction. In the context of the chair, the weight of the chair acts as the action force upon the floor, and in response, according to Newton's third law, the floor exerts an equal and opposite reaction force upward, which is the normal force. Similar examples include a bathroom scale showing the normal force during an elevator ride, or a professor pushing backward on the floor and the floor pushing forward on the professor.