Observation :
Initially, the colourless gas (N2O4) turns to reddish brown (NO2) gas.
After sometime,
The colour becomes lighter indicating the formation of N2O4 from NO2.
Inference :
This indicates that the reaction is reversible. In such reaction, the reactants combine to form the products and the products combine to give the reactants.
As soon as the forward reaction produces any NO2, the reverse reaction begins and NO2, starts combining back to N2O4.
At equilibrium,
The concentrations of N2O4 and NO2 remain unchanged and do not vary with time, because the rate of formation of NO2 is equal to the rate of formation of N2O4.
[Note : For any reversible reaction in a closed system whenever the opposing reactions (forward and reaction) are occurring at different rates, the forward reaction will gradually become slower and the reverse reaction will become faster. Finally, the rates become equal and equilibrium is established.]
