Answer: D) The amount of air in a room never changes.
Explanation: Equilibrium is defined as the state in which the concentration of reactants and products is constant or the rate of forward direction is equal to the rate of backward direction.
a) The percentage of nitrogen gas in a room decreases steadily: the amount decreases that means the reaction is taking place in one direction and hence the reaction is not in equilibrium.
b) A blade of grass grows taller : the plant gets taller that means the reaction is taking place in one direction and hence the reaction is not in equilibrium.
c) The amount of water in a cup decreases as it evaporates: The amount of water is decreasing that means the reaction is taking place in one direction and hence the reaction is not in equilibrium.
d) The amount of air in a room never changes: the amount of air does not change as the amount of air leaving the room is same as that of the air entering the room and hence the reaction is in equilibrium.
