When a reversible reaction happens in a closed container, it can achieve a dynamic equilibriumA dynamic equilibrium is one in which the rate of the forward reaction equals the rate of the reverse reaction and the concentrations of the reactants and products remain constant.
At equilibrium:
the forward and backward reactions are still happening
the rates of the forward and backward reactions are the same
the concentrations of the reactants and products remain constant (they do not change)
equilibrium can only be achieved if none of the reactants or products can escape. For example if calcium carbonate is heated in a sealed container, breakdown occurs and the following equilibrium is made: CaCO3(s) ⇌ CaO(s) + CO2(g)
if this reaction is carried out in an open test tube the breakdown goes to completion: CaCO3(s) → CaO(s) + CO2(g)
this occurs as the carbon dioxide gas escapes and the backwards reaction cannot occur.