ANS:A - chemical systems in equilibrium.

Chemical systems in equilibrium refer to situations where a chemical reaction occurs, but the forward and backward reactions proceed at equal rates, resulting in no net change in the concentrations of the reactants and products over time. In other words, the system appears to be static on a macroscopic level, even though at the microscopic level, reactions are continuously occurring. Here's a more detailed explanation:

1. Dynamic Equilibrium: Chemical systems in equilibrium are in a state of dynamic equilibrium. Dynamic equilibrium means that while individual reactions are still occurring (reactants are converting into products and products are converting back into reactants), the rates of the forward and backward reactions are equal. As a result, the concentrations of reactants and products remain constant over time.
2. Reversible Reactions: Equilibrium is achieved in reversible reactions, where reactants can form products, and products can revert to reactants. This occurs when the forward reaction rate equals the backward reaction rate. Reversible reactions are represented using a double arrow (⇌) to indicate that the reaction can proceed in both directions.
3. Equilibrium Constant: For a given reversible reaction, the equilibrium constant (K) describes the ratio of the concentrations of products to reactants at equilibrium. It is constant at a given temperature and provides information about the extent to which the reaction proceeds toward products or reactants.
4. Le Chatelier's Principle: This principle states that if a system at equilibrium is subjected to a change in temperature, pressure, or concentration, the system will adjust itself to counteract the effect of the change and restore equilibrium. For example, if more reactants are added, the system will shift towards the formation of more products to counteract the increase in reactant concentration.
5. Physical Properties: While the concentrations of reactants and products remain constant in equilibrium, other properties such as temperature, pressure, and volume may not necessarily remain constant. However, they will stabilize to specific values characteristic of the equilibrium state.