What is necessary for a chemical reaction to reach dynamic equilibrium?
The reaction must be (a) reversible, and (b) there must be a closed system.
A chemical reaction doesn't stop when equilibrium has reached.
Instead, the rate of forward reaction equals the rate of reverse reaction.
That's why the equilibrium is dynamic. Things are still happening.
A dynamic equilibrium exists once the concentrations of reactants and products becomes constant.
Although there is no net change, substances are still moving between reactants and products at the same rate.
The system must be closed to prevent any of the components from escaping.
Consider the equilibrium
CaCO₃(s) ⇌ CaO(s) + CO₂(g)
If the system were open, the CO₂ would escape into the atmosphere, and the equilibrium could not be established.