When can chemical equilibrium be reached according to the constant density?

The density ρ=m/v, and the general reaction is carried out in a fixed closed container, so V remains unchanged as long as the total mass m of the gas changes.

Reversible reaction: 2NO2(g) → 2NO(g)+O2(g). According to the conservation of mass, the total mass of gas is constant, so the density of mixed gas is constant at any time of the reaction.

Reversible reaction: 2Ag(s)+Cl2(g) → 2AgCl(s), because there are solid substances, so as long as the reaction does not reach equilibrium, the total mass of the gas will definitely change and the density will also change. Similarly, if the density is constant, the reversible reaction will reach an equilibrium state.