Which of the following is not a general characteristic of equilibria involving physical processes? (i) Equilib

NCERT Exemplar · Q3

Q. Which of the following is not a general characteristic of equilibria involving physical processes?

(i) Equilibrium is possible only in a closed system at a given temperature.

(ii) All measurable properties of the system remain constant.

(iii) All the physical processes stop at equilibrium.

(iv) The opposing processes occur at the same rate and there is dynamic but stable condition.

Punjab PsebMCQ· 1mImportance★★★★★est

67% · 104/155 Questions

✓ Free question

Concept understanding — Phase Equilibrium

Phase Equilibrium: From Intuition to Precision

Imagine a glass of ice water on a warm day. The ice cubes are melting, but the water around them stays cold. If you wait long enough, the ice stops melting — not because the room got colder, but because the system has reached a balance. The ice and water coexist without any net change. That balance is phase equilibrium.

The Intuition: A Tug-of-War Between Two Tendencies

Every substance wants to exist in the phase (solid, liquid, or gas) that has the lowest energy under the given conditions. But there's a catch: nature also loves disorder (entropy). A solid is low-energy but highly ordered; a gas is high-energy but very disordered. At a specific temperature and pressure, these two opposing drives — minimising energy and maximising disorder — exactly cancel each other out. The result? Two phases can coexist peacefully, with molecules constantly jumping between them but no net change in the amount of each phase.

Think of it as a tug-of-war. On one side, molecules in the solid phase are tightly bound and low in energy. On the other side, molecules in the liquid phase have more freedom and higher entropy. At the melting point, the rope doesn't move — the rates of melting and freezing are equal.

The Precise Statement

Phase equilibrium is the condition in which two or more phases of a substance coexist in thermodynamic equilibrium, with no net transfer of mass between phases. For a pure substance, this occurs at a unique temperature for a given pressure (or vice versa), defined by the equality of the chemical potential (or Gibbs free energy per mole) in all phases.

In simpler terms: at equilibrium, the driving force for a molecule to move from phase A to phase B is exactly the same as the driving force to move from B to A. The system is dynamic — molecules are still crossing the boundary — but macroscopically static.

The Key Condition: Equal Chemical Potential

The rigorous condition for phase equilibrium in a pure substance is:

\mu_{\text{solid}} = \mu_{\text{liquid}} = \mu_{\text{gas}}

where $\mu$ (mu) is the chemical potential — the Gibbs free energy per mole. When chemical potentials are equal, no phase has a "preference" to grow at the expense of another.

Important

For a pure substance, phase equilibrium occurs only along specific lines on a phase diagram (the melting curve, boiling curve, and sublimation curve). At the triple point, all three phases coexist.

A Concrete Example: Water at 0°C

At 1 atm pressure and 0°C, ice and liquid water are in equilibrium. Here's what's happening at the molecular level:

Water molecules at the surface of the ice are vibrating. Some gain enough energy to break free and enter the liquid.
Simultaneously, water molecules in the liquid near the ice surface lose energy and attach to the ice crystal.
The rates of these two processes are equal. So the amount of ice and the amount of liquid water remain constant — even though individual molecules are constantly switching sides.

If you raise the temperature slightly above 0°C, the melting rate exceeds the freezing rate — the ice melts completely. Below 0°C, freezing dominates. Only at exactly 0°C (at 1 atm) do you get stable coexistence.

Why This Matters

Phase equilibrium is the foundation for understanding:

Phase diagrams — the maps that show which phase is stable at any temperature and pressure.
Boiling and condensation — the liquid-vapour equilibrium curve.
Sublimation — solid-vapour equilibrium (like dry ice "evaporating" without melting).
Colligative properties — how solutes shift equilibrium temperatures (freezing point depression, boiling point elevation).

The One Thing to Remember

Note

Phase equilibrium is not a static state where nothing happens. It is a dynamic balance where opposing processes occur at equal rates. The system appears unchanged because the forward and backward rates are identical.

When you see a phase diagram, every line on it represents a condition of phase equilibrium — a specific temperature and pressure where two phases can coexist indefinitely. That's the power of this concept: it tells you exactly where nature's tug-of-war ends in a draw.

A quick search for "Phase Equilibrium class 11 chemistry" or "NCERT chemistry syllabus phase equilibrium" will confirm what's true here: this concept is a standard, curriculum-aligned part of Class 11 Chemistry. Given how often it's tested in JEE Main, NEET and state CET Chemistry papers, it's worth revisiting this explanation until the reasoning feels automatic, not just the final formula.

Equilibrium in physical processes is dynamic, meaning the opposing processes continue to occur but at equal rates, leading to constant macroscopic properties. The statement that all physical processes stop at equilibrium is incorrect.

When a system reaches equilibrium, it appears static at a macroscopic level, but at a microscopic level, it is a state of constant activity. This is the core idea of dynamic equilibrium. Let's understand this concept and then evaluate the given options.

Consider a physical process like the evaporation of water in a closed container. Initially, water evaporates. As the concentration of water vapor increases, the rate of condensation also increases. Eventually, the rate of evaporation becomes equal to the rate of condensation. At this point, the amount of liquid water and water vapor remains constant, and the pressure exerted by the vapor also remains constant. This is a state of equilibrium.

Here's why this approach works: We need to identify the statement that does not align with the fundamental characteristics of equilibrium, particularly its dynamic nature.

Analyze Option (i): Equilibrium is possible only in a closed system at a given temperature.
- Closed System: For equilibrium to be established and maintained, there must be no exchange of matter with the surroundings. If matter can enter or leave, the system's composition or the amount of substance in a particular phase will change, preventing a stable equilibrium from forming.
- Given Temperature: Temperature is a critical factor that influences the rates of physical processes and the position of equilibrium. For a stable equilibrium state, the temperature of the system must be constant. Any change in temperature would shift the equilibrium.
- This statement accurately describes a necessary condition for physical equilibrium.
Analyze Option (ii): All measurable properties of the system remain constant.
- At equilibrium, the macroscopic properties of the system, such as pressure, temperature, volume, density, concentration of substances in different phases, and color (if applicable), do not change with time. This constancy of macroscopic properties is how we experimentally identify that a system has reached equilibrium.
- This statement is a defining characteristic of equilibrium.
Analyze Option (iii): All the physical processes stop at equilibrium.
- This statement is incorrect. Equilibrium is dynamic. It means that the forward and reverse (opposing) physical processes continue to occur, but they do so at exactly the same rate. For example, in a liquid-vapor equilibrium, water molecules are continuously evaporating from the liquid phase and condensing from the vapor phase, but the rates of these two opposing processes are equal. This results in no net change in the amounts of liquid and vapor, but the individual processes have not ceased.
- This is a common misconception about equilibrium.
Analyze Option (iv): The opposing processes occur at the same rate and there is dynamic but stable condition.
- This statement perfectly describes dynamic equilibrium. The "opposing processes occur at the same rate" explains the microscopic activity, while "dynamic but stable condition" refers to the macroscopic observation of constant properties despite continuous microscopic changes.
- This statement is a correct and fundamental characteristic of equilibrium.

Based on the analysis, the statement that is not a general characteristic of equilibria involving physical processes is that all physical processes stop at equilibrium.

✓Final answer

The statement that is not a general characteristic of equilibria involving physical processes is (iii) All the physical processes stop at equilibrium.

←Previous questionQ2 — For the reaction H2(g) + I2(g) ⇌ 2HI (g), the standard free energy is …→Next questionQ4 — PCl5, PCl3 and Cl2 are at equilibrium at 500K in a closed container an…

Q.Which of the following is not a general characteristic of equilibria involving physical processes?

🎓Unlock everything free for 14 days

Unlock everything free for 14 days