The concentration of a pure solid or liquid phase is not included in the expression of equilibrium constant because : (a) solid and liquid concentrations are independent of their quantities. (b) solids and liquids react slowly. (c) solids and liquids at equilibrium do not interact with gaseous phase. (d) the molecules of solids and liquids cannot migrate to the gaseous phase.

Physical chemistry is a branch of chemistry that deals with the study of how matter behaves on a molecular and atomic level, and how chemical reactions occur. It combines principles of physics and chemistry to understand the physical properties of molecules, the forces that act upon them, and the energy changes that accompany chemical reactions. One of the crucial aspects of physical chemistry is the study of chemical equilibrium, which looks at how reversible reactions evolve over time to reach a state where the rates of the forward and backward reactions are equal. This deep dive into the molecular world allows us to predict reaction behavior and understand the concept of equilibrium constants, a fundamental piece in the puzzle of physical chemistry.

Concentration plays a pivotal role in the realm of physical chemistry, especially when discussing the various phases of matter – gases, liquids, and solids. A phase denotes a physically distinct section of a system with a uniform set of properties. In context to chemical reactions involving multiple phases, the concentration or the amount of substance in each phase can impact the dynamics of the reaction. For a pure solid or liquid phase, the concentration is assumed to be constant because it's proportional to the substance's density – a value that remains unchanged regardless of the sample size. Understanding this concept is important for correctly formulating the equilibrium constant expression, which is pivotal for predicting the extent of a chemical reaction.

Chemical equilibrium is a state in a reversible chemical reaction where the rate of the forward reaction equals the rate of the backward reaction, leading to constant concentrations of reactants and products. It's important to note that reaching equilibrium doesn't mean reactants and products are in equal amounts; it simply means their rates of formation are balanced.

Equilibrium constants (K) quantitatively describe the equilibrium condition for chemical reactions. These constants are expressions that relate the concentrations of the products over the reactants raised to the power of their stoichiometric coefficients. However, the concentrations of pure liquids and solids are omitted because they're constant and do not influence the position of equilibrium. It is critical to correctly include only the reactive concentrations in the equilibrium constant expression to accurately predict the behavior of the reaction.

Students preparing for competitive exams such as the Joint Entrance Examination (JEE) in India must have a strong understanding of physical chemistry concepts, including equilibrium and concentration in various phases. Mastery of these topics is required for excelling in the chemistry section of the JEE. Candidates should focus on conceptual understanding and practice with numerical problems involving equilibrium constants and phase concentrations. Furthermore, a grasp on the rationale behind including or omitting certain concentration terms in equilibrium expressions can be crucial in solving JEE chemistry problems efficiently. Preparing through repeated exercises, understanding the underlying principles, and practical application of these concepts can significantly boost a student's performance in the examination.