Equilibrium can be established in:

Chemical equilibrium is a state in a reversible reaction where the rate of the forward reaction equals the rate of the backward reaction. At this point, the concentrations of reactants and products remain constant over time. ### Why a Closed System is Required: 1. **Matter Conservation:** In an **open system**, matter can be exchanged with the surroundings. For example, if a gas is produced as a product, it can escape into the atmosphere. If the product is lost, the reverse reaction ($Product \rightarrow Reactant$) cannot occur, preventing the system from reaching equilibrium. 2. **Reversibility:** Equilibrium requires all components (reactants and products) to be present simultaneously. A **closed system** prevents the loss of matter, ensuring that products are available to react back into reactants. 3. **Phase Systems:** While equilibrium can occur in solid, liquid, or gas phases, it is not restricted to 'only' one of them (e.g., $H_2O(l) \rightleftharpoons H_2O(g)$). However, it *must* be closed to prevent the vapor from escaping. Therefore, equilibrium can be established in a **closed system**.