Q.Define vapour pressure. Name factors affecting vapour pressure.
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🔒 Start your 14-day free trial to unlock the full solution →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:
μsolid=μliquid=μgas
where μ (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.
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. …
Vapour pressure is the pressure exerted by a liquid's vapour when it is in equilibrium with the liquid at a given temperature, and it depends mainly on the nature of the liquid and the temperature. …
Vapour pressure is the equilibrium pressure exerted by a liquid's own vapour above its surface in a closed container; it depends on the liquid's intermolecular forces and rises with temperature.
Definition: When a liquid is kept in a closed container, molecules at its surface continuously escape into the vapour phase (evaporation), while some vapour molecules return to the liquid (condensation). Eventually, the rate of evaporation equals the rate of condensation, and a dynamic equilibrium is established between the liquid and its vapour. The pressure exerted by this vapour at equilibrium, at a given temperature, is called the vapour pressure of the liquid.
Factors affecting vapour pressure:
- Nature of the liquid: Liquids with weaker intermolecular forces of attraction (e.g. ether, acetone) evaporate more readily and have higher vapour pressure. Liquids with strong intermolecular forces (e.g. water, due to hydrogen bonding) have comparatively lower vapour pressure. …
- CBSE 2026Set ANNUAL1 markMCQQ.Which of the following is not a general characteristics of equilibria involving physical processes?(a) Equilibrium is possible only in a closed system at a given temperature.(b) All measurable properties of the system remain constant.(c) All the physical processes stop at equilibrium.(d) The opposing processes occur at the same rate and there is dynamic but stable condition.
›Reveal solutionSolution
Equilibrium is a dynamic balance — forward and reverse processes keep occurring at equal rates, they never stop.
Checking each option against the real characteristics of physical equilibria:
- (a) True — equilibrium can only be established in a closed system at a given temperature (no matter exchange with surroundings).
- (b) True — once equilibrium is reached, all measurable properties (pressure, concentration, colour, etc.) stay constant with time. …
- CBSE 2026Set sz1 markMCQQ.Select the correct one: If pressure is applied to the equilibrium of solid <=> liquid, the melting point of a solid :(a) will not change(b) may increase or decrease depending upon its nature(c) will always increase(d) will always decrease
›Reveal solutionSolution
Applying pressure to a solid <=> liquid equilibrium shifts it toward the denser (smaller-volume) phase; whether that phase is the solid or the liquid depends on the specific substance, so the melting point may go up or down depending on the substance's nature.
According to Le Chatelier's principle, if pressure is increased on a system at equilibrium, the equilibrium shifts in the direction that reduces the total volume, i.e. toward the side (solid or liquid) that is more compact/denser.
- For most substances, the solid phase is denser than the liquid phase. Increasing pressure then favours the solid, so a higher temperature is needed to melt it under the higher pressure — the melting point increases. …
- CBSE 2025Set ANNUAL1 markMCQQ.In a given system, water and ice are in equilibrium. If pressure is increased to the system then(a) More of ice is formed(b) Amount of ice and water remains same(c) More of ice is melted(d) None of these
›Reveal solutionSolution
Increasing pressure on the ice-water equilibrium melts more ice, because water (the denser phase) is favoured.
Unusually, ice is less dense than liquid water (water expands on freezing). By Le Chatelier's principle, increasing pressure on a system at equilibrium shifts the equilibrium in the direction that reduces volume, i.e. towards the denser phase. Since liquid water occupies less volume than the same mass of ice, increased press …
- CBSE 2025Set ANNUAL1 markQ.In N2(g) + 3H2(g) <=> 2NH3(g), the reactant and product are in heterogeneous equilibrium.
›Reveal solutionSolution
The statement is False: this reaction is a homogeneous, not heterogeneous, equilibrium, because every species is a gas.
An equilibrium is called homogeneous when all the reactants and products are present in the same phase, and heterogeneous when they are present in more than one phase (e.g. a solid and a gas). In N2(g) + 3H2(g) <=> 2NH3(g), nitrogen, hydrogen, and ammonia are all gases -- a single phase -- so this is a …
- CBSE 2025Set ANNUAL1 markQ.State Henry's law for equilibrium.
›Reveal solutionSolution
Henry's law states that the partial pressure of a gas over a solution is directly proportional to its mole fraction in the solution: p = KH . x.
Henry's law is used to describe gas-liquid equilibrium (solubility of a gas in a liquid). It states that at constant temperature, the partial pressure (p) of a gas in the vapour phase, in equilibrium with its dissolved form in a solution, is directly proportional to the mole fraction (x) of that gas in the solution: p = KH . x, where KH is the Henry's law constant, which de …
- CBSE 2024Set ANNUAL1 markMCQQ.The Equilibrium system CO2 (gas) = CO2 (in solution), is governed by which law?(a) Kohlrausch's Law(b) Hook's Law(c) Henry's Law(d) Law of Chem. Eqbm.
›Reveal solutionSolution
The dissolution equilibrium of a gas in a liquid (like CO2 gas dissolving in solution) is described by Henry's Law, which relates the partial pressure of the gas above the liquid to the mole fraction of gas dissolved in it.
The equilibrium CO2(gas) rightleftharpoons CO2(in solution) is a physical (gas-liquid) equilibrium — it's how CO2 stays dissolved in a sealed soft-drink bottle, or how oxygen dissolves in water for aquatic life.
Henry's Law states: at a given temperature, the partial pressure (p) of a gas in equilibrium with its solution is directly proportional to the mole fraction (x) of the gas in the solution:
p=KH⋅x
where KH (Henry's law constant) is different for every gas-solvent pair and increases with temperature (which is why a soda bottle fizzes more, i.e. releases more dissolved CO2, when warm than when cold).
…
- CBSE 2024Set ANNUAL1 markMCQQ.Dissociation of acetic acid in the following equilibrium, CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq) is suppressed by(a) A) decreasing the concentration of H+ ions(b) B) increasing the concentration of H+ ions(c) C) decreasing the concentration of CH3COO- ions(d) D) decreasing both concentrations of H+ and CH3COO- ions
›Reveal solutionSolution
[!TLDR]
B) increasing the concentration of H+ ions
Why
By Le Chatelier's principle / common ion effect, increasing [H+] (e.g., by adding a strong acid) shifts the equilibrium backward, suppressing fu …
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