What law is known as the law of action-
reaction?

Law of Action and Reaction mean "To every action there is equal and opposite reaction.

Explanation:

It is Newtons third law which explained few  things such as :-

1. The nature of the forces between the two interacting objects.

2. According to this law , the force that is exerted from Object-1 to Object-2 is said to be equal in magnitude and opposite to the direction to the force exerted to object 2 upon object 1.

3. Eaxmple - As we push against the wall - Action , the wall in return opposes equal force to resist the pressure of the hand - Reaction.

nacl is added to the aqueous of what? ? in order to get a proper answer you have to specify the exact composition of the aqueous solution.