The concentration of a pure solid or liquid phase is not included in the expression of equilibrium constant because

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Prashant Kotian · Accepted Answer

Correct answer: A. Step 1 - Define Molar Concentration for Any Substance The molar concentration ($C$) of a substance is defined as the number of moles ($n$) of the substance present per unit volume ($V$): $$C = \frac{n}{V}$$ We know that the number of moles is the ratio of the mass of the substance ($m$) to its molar mass ($M$): $$n = \frac{m}{M}$$ Substituting this expression for $n$ into the concentration formula gives: $$C = \frac{m}{M \cdot V} = \left(\frac{m}{V}\right) \frac{1}{M}$$ Step 2 - Express Concentration in Terms of Density Since density ($d$) is defined as mass per unit volume ($d = \frac{m}{V}$), we can substitute $d$ into our concentration equation to get: $$C = \frac{d}{M}$$ Step 3 - Analyze the Properties of Pure Solids and Liquids For any pure solid or pure liquid, both its density ($d$) and its molar mass ($M$) are intensive physical properties. At a constant temperature, these properties remain strictly constant: * If we increase or decrease the total amount of a pure solid or pure liquid, its mass ($m$) and volume ($V$) change in the exact same proportion. * As a result, the ratio $\frac{m}{V}$ (density, $d$) remains completely unchanged. Since both density ($d$) and molar mass ($M$) are constant, the molar concentration ($C = \frac{d}{M}$) of a pure solid or liquid is always constant and completely independent of the quantity or amount of the substance present in the reaction mixture. Conventionally, because their concentrations do not change during the reaction, they are omitted from the expression of the equilibrium constant (or their constant value is mathematically incorporated into the equilibrium constant $K$, taking their active mass/activity as $1$). Step 4 - Evaluate the Given Options * **(A) solid and liquid concentrations are independent of their quantities:** Correct. As shown mathematically, the concentration of a pure solid or liquid depends only on its density and molar mass, making it constant and independent of the total quantity present. * **(B) solid and liquids react slowly:** Incorrect. The rate of a chemical reaction is a kinetic property and does not determine whether a concentration term is included in the thermodynamic equilibrium constant expression. * **(C) solid and liquids at equilibrium do not interact with gaseous phase:** Incorrect. In heterogeneous equilibria, such as the decomposition of calcium carbonate ($\ce{CaCO3(s) <=> CaO(s) + CO2(g)}$), the solid phases must be in direct contact and interact with the gaseous phase for equilibrium to be established. * **(D) the molecules of solids and liquids cannot migrate to the gaseous phase:** Incorrect. Molecules of solids and liquids can migrate to the gaseous phase through processes like sublimation and evaporation. Therefore, the correct option is A. $$\boxed{\text{A}}$$

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