Which of the following statement is incorrect:

The correct answer is: "At equilibrium, concentration of reactants must be equal to concentration of products."

This is the incorrect statement among the given options. Let us analyze each statement one by one to understand why.

Statement 1: "At equilibrium, concentration of reactants must be equal to concentration of products."

This statement is INCORRECT. At chemical equilibrium, the rate of the forward reaction equals the rate of the backward reaction — not the concentrations of reactants and products. The concentrations of reactants and products become constant at equilibrium, but they are not necessarily equal to each other.

For example, consider a reaction with an equilibrium constant:

$K_{\mathrm{eq}}=\frac{\left[}{\text{Products}}$

If $K_{\mathrm{eq}}$ is very large (say 10⁵), the concentration of products is far greater than that of reactants at equilibrium. If $K_{\mathrm{eq}}$ is very small (say 10⁻⁵), the concentration of reactants is far greater than that of products. The concentrations are equal only in the special case when $K_{\mathrm{eq}}=1$, which is not a general requirement.

Statement 2: "Equilibrium can be attained in both homogeneous and heterogeneous reactions."

This statement is correct. A homogeneous equilibrium involves all reactants and products in the same phase (e.g., all gases or all in solution), while a heterogeneous equilibrium involves species in different phases (e.g., a solid reacting with a gas). Both types of systems can reach equilibrium.

Statement 3: "Approach to the equilibrium is fast in the initial state but gradually it decreases."

This statement is correct. Initially, only reactants are present, so the forward reaction rate is high while the backward rate is zero or negligible. As products accumulate and reactant concentrations decrease, the forward rate slows down and the backward rate speeds up. The net change in concentrations therefore decreases over time until equilibrium is reached, where the net change is zero.

Statement 4: "Equilibrium is dynamic in nature."

This statement is correct. At equilibrium, although the macroscopic concentrations remain constant, the forward and reverse reactions continue to occur at the molecular level at equal rates. This is why it is called a dynamic equilibrium, as opposed to a static one.

Conclusion: Statement 1 is incorrect because equilibrium does not require equal concentrations of reactants and products — it only requires that their concentrations remain constant over time, with the forward and reverse reaction rates being equal.