Q.If there is no change in concentration, why is the equilibrium state considered dynamic?

Chemical equilibrium can be pictured through an everyday analogy: people continuously moving between the first and second floor of a building. If the number of people moving up in a given interval exactly equals the number moving down in that same interval, the population of each floor stays constant even though people never stop moving between floors -- a genuine, ongoing dynamic balance, not a frozen, static one.

Chemical equilibrium works identically. A reversible reaction does not switch off once equilibrium is reached: the forward and the backward reactions both continue to proceed, at exactly matched rates, so the macroscopic (bulk, measurable) concentrations of every species stay fixed even though, at the molecular level, reactant molecules keep converting to product and product molecules keep converting back to reactant, continuously and simultaneously. This is precisely why chemical equilibrium is called a state of dynamic equilibrium rather than a static one -- illustrated by $H_2(g)+I_2(g)\rightleftharpoons 2HI(g)$, where even once the concentrations of $H_2$, $I_2$ and HI have stopped changing, individual $H_2$ and $I_2$ molecules are still colliding and forming HI just as fast as HI molecules are still decomposing back into $H_2$ and $I_2$.

The 'no change in concentration' observation that defines equilibrium is therefore a net-rate statement, not a no-reaction statement -- exactly the same logic behind Guldberg and Waage's law of mass action, which relates rate (not just concentration) to the active masses of the reacting species, and behind the reaction-quotient framework (Q vs $K_C$) used to track how far a system still has to move before both opposing rates lock into balance.

"Dynamic equilibrium definition with example class 11 chemistry" and "chemical equilibrium important questions" are common search terms for the Equilibrium chapter of the NCERT-aligned CBSE Class 11 Chemistry curriculum, a foundational idea tested in board exams, JEE Main and NEET alike. The building-analogy used here to explain why equilibrium is a net-rate condition, not a no-reaction condition, is a favourite way examiners phrase conceptual questions on this topic.