Physical Equilibria

8 MCQs9-step worked example
Source: NCERT EquilibriumPYQ coverage: NEET 2023, 2024Official key: NTA-verifiedLast updated: 7 Oct 2026

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Under what condition can equilibrium be reached in a physical process such as evaporation?
  1. A.In a closed system at a given temperature
  2. B.In an open system, if the temperature is kept constant
  3. C.Only when the process is very slow
  4. D.In an open system, if the rate of evaporation is constant
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Answer: A. NCERT states that equilibrium is possible only in a closed system at a given temperature (NCERT Class 11 Chemistry, Chapter 6, page 172).

B is wrong: B is wrong because a constant temperature does not close the system; in an open system the molecules disperse and no equilibrium is reached (NCERT Class 11 Chemistry, Chapter 6, page 170).

C is wrong: C is wrong because speed does not decide it; equilibrium needs a closed system and equal opposing rates (NCERT Class 11 Chemistry, Chapter 6, page 172).

D is wrong: D is wrong because in an open dish the rate of evaporation can be constant, yet condensation stays far slower, so there is no equilibrium (trap: mistaking a constant evaporation rate for equilibrium).

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Physical Equilibria, explained for NEET

The trap is treating an open dish as if it could reach equilibrium, and reading Henry's constant backwards. Physical equilibria are the closed-system balances of a phase change or a dissolution, and each one is summed up by one quantity that stays constant at a given temperature.

Equilibrium is possible only in a closed system at a given temperature. Both opposing processes run at the same rate, the condition is dynamic but stable, and all measurable properties stay constant (NCERT Class 11 Chemistry, Chapter 6, page 172). A watch glass open to the room is an open system and cannot reach equilibrium, because the vapour disperses and condensation stays far slower than evaporation (NCERT Class 11 Chemistry, Chapter 6, page 170).

Solid and liquid: ice and water at 273 K and atmospheric pressure are in equilibrium, with equal rates of transfer in both directions, and the temperature at which this happens is the normal melting point (NCERT Class 11 Chemistry, Chapter 6, page 169).

Liquid and vapour: at equilibrium the vapour pressure is constant at a given temperature, and it increases with temperature. A liquid with a higher vapour pressure is more volatile and has a lower boiling point (NCERT Class 11 Chemistry, Chapter 6, page 170).

Gas in liquid: Henry's law states that the mass of a gas dissolved in a given mass of solvent at any temperature is proportional to the pressure of the gas above the solvent, and that this amount decreases with increase of temperature (NCERT Class 11 Chemistry, Chapter 6, page 171). A soda bottle fizzes when opened because the gas pressure above the liquid falls (NCERT Class 11 Chemistry, Chapter 6, page 171).

Comparing gases: different gases have different Henry's constants K_H at the same temperature, and the higher the K_H, the lower the solubility (NCERT Class 12 Chemistry, Chapter 1, page 7).

Watch out: solubility goes with 1/K_H. The gas with the smallest K_H is the most soluble, not the least.

How do you solve a Physical Equilibria question? A worked example

  1. 1

    Given

    The Henry's law constants K_H of three gases A, B and C in water are 145, 2 × 10⁻⁵ and 35 kbar respectively, at the same temperature. (This is the pattern of a NEET 2024 question.)

  2. 2

    Required

    The order of solubility of the three gases in water.

  3. 3

    Concept

    Different gases have different K_H values at a given temperature, and the higher the K_H at a given pressure, the lower the solubility of the gas (NCERT Class 12 Chemistry, Chapter 1, page 7).

  4. 4

    Formula

    Solubility is proportional to 1/K_H at a given pressure and temperature.

  5. 5

    Substitution

    A: 1/145, B: 1/(2 × 10⁻⁵), C: 1/35.

  6. 6

    Calculation

    1/145 = 6.9 × 10⁻³, 1/(2 × 10⁻⁵) = 5 × 10⁴ and 1/35 = 2.9 × 10⁻². So B is largest, then C, then A. The numbers 1 and 10 are exact and do not limit the significant figures.

  7. 7

    Final answer

    Solubility order: B > C > A. The gas with the smallest K_H, B, is the most soluble.

  8. 8

    Common trap

    Ranking by K_H itself gives A > C > B, which is the reverse of the true solubility order. Reading a large K_H as high solubility is the usual slip.

  9. 9

    Similar NEET-style question

    Gas X has K_H = 0.8 kbar, gas Y has K_H = 62 kbar and gas Z has K_H = 4.5 kbar in water at the same temperature. Arrange them in increasing solubility. (Answer: 1/0.8 = 1.25, 1/4.5 = 0.22 and 1/62 = 0.016, so the solubility order is X > Z > Y; in increasing order Y < Z < X.)

    ---

Can you answer these Physical Equilibria MCQs?

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

Under what condition can equilibrium be reached in a physical process such as evaporation?

Show answer and why every option is right or wrong

Answer: A. NCERT states that equilibrium is possible only in a closed system at a given temperature (NCERT Class 11 Chemistry, Chapter 6, page 172).

Why B is wrong: B is wrong because a constant temperature does not close the system; in an open system the molecules disperse and no equilibrium is reached (NCERT Class 11 Chemistry, Chapter 6, page 170).

Why C is wrong: C is wrong because speed does not decide it; equilibrium needs a closed system and equal opposing rates (NCERT Class 11 Chemistry, Chapter 6, page 172).

Why D is wrong: D is wrong because in an open dish the rate of evaporation can be constant, yet condensation stays far slower, so there is no equilibrium (trap: mistaking a constant evaporation rate for equilibrium).

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MCQ 2Easy RecallPractice

For water and its vapour in equilibrium in a closed vessel at a fixed temperature, which statement is correct?

Show answer and why every option is right or wrong

Answer: C. At equilibrium there is no net evaporation, the rate of evaporation equals the rate of condensation, and the vapour pressure stays constant at that temperature (NCERT Class 11 Chemistry, Chapter 6, page 170).

Why A is wrong: A is wrong because evaporation does not stop; the equilibrium is dynamic, with both processes running at equal rates (trap: treating dynamic equilibrium as static).

Why B is wrong: B is wrong because condensation continues too; the liquid level is constant because the two rates match (trap: treating dynamic equilibrium as static).

Why D is wrong: D is wrong because the pressure rises only until equilibrium is reached and then stays constant at a given temperature (NCERT Class 11 Chemistry, Chapter 6, page 170).

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MCQ 3Easy RecallPractice

According to Henry's law as stated in NCERT, the mass of a gas dissolved in a given mass of solvent at a given temperature is proportional to which quantity?

Show answer and why every option is right or wrong

Answer: B. Henry's law: the mass of a gas dissolved in a given mass of solvent at any temperature is proportional to the pressure of the gas above the solvent (NCERT Class 11 Chemistry, Chapter 6, page 171).

Why A is wrong: A is wrong because the mass is taken for a given mass of solvent, so the amount of solvent is fixed, and the proportionality is to pressure (NCERT Class 11 Chemistry, Chapter 6, page 171).

Why C is wrong: C is wrong because the boiling point of the solvent does not appear in the law.

Why D is wrong: D is wrong because the solubility rises with pressure, so it is proportional to the pressure, not its inverse (trap: confusing proportional to pressure with proportional to 1/K_H).

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MCQ 4Direct ApplicationPractice

Why does carbon dioxide fizz out of a soda bottle when it is opened?

Show answer and why every option is right or wrong

Answer: D. The bottle is sealed under gas pressure, when solubility is high. On opening, the pressure above the liquid falls to the partial pressure of CO₂ in the atmosphere, so dissolved gas escapes to reach a new equilibrium (NCERT Class 11 Chemistry, Chapter 6, page 171).

Why A is wrong: A is wrong because a fall in temperature would raise the solubility of a gas, not lower it; the cause is the pressure drop (NCERT Class 11 Chemistry, Chapter 6, page 171).

Why B is wrong: B is wrong because the open bottle has lower gas pressure above the liquid, so the solubility is lower, not higher (NCERT Class 11 Chemistry, Chapter 6, page 171).

Why C is wrong: C is wrong because the gas leaves the liquid; a faster return into the liquid would stop the fizz (trap: reversing the direction of the net flow).

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MCQ 5Direct ApplicationPractice

Liquid X has a higher equilibrium vapour pressure than liquid Y at the same temperature. Which statement about X is correct?

Show answer and why every option is right or wrong

Answer: A. NCERT says the liquid with the higher vapour pressure is more volatile and has the lower boiling point (NCERT Class 11 Chemistry, Chapter 6, page 170).

Why B is wrong: B is wrong because a liquid with a higher vapour pressure is more volatile, not less (NCERT Class 11 Chemistry, Chapter 6, page 170).

Why C is wrong: C is wrong because high volatility goes with a lower boiling point, not a higher one (trap: pairing high vapour pressure with a high boiling point).

Why D is wrong: D is wrong because it reverses both properties of the liquid with the higher vapour pressure (NCERT Class 11 Chemistry, Chapter 6, page 170).

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MCQ 6Direct ApplicationPractice

Under high pressure a diver's blood dissolves more of any breathing gas. Using this idea, which property makes helium a suitable diluent for oxygen in diving apparatus?

Show answer and why every option is right or wrong

Answer: C. Helium is used as a diluent for oxygen in modern diving apparatus because of its very low solubility in blood (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 7, page 211). Little of it dissolves even at high pressure, in line with Henry's law (NCERT Class 11 Chemistry, Chapter 6, page 171).

Why A is wrong: A is wrong because helium is a noble gas and forms no compound with oxygen here; the reason is its low solubility in blood (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 7, page 211).

Why B is wrong: B is wrong because it reverses the reason; the point is that helium dissolves very little in blood (trap: reading low solubility as high solubility).

Why D is wrong: D is wrong because helium is lighter than oxygen, and density is not the reason NCERT gives for its use in diving apparatus.

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MCQ 7CalculationPractice

The Henry's law constants K_H of three gases P, Q and R in water at the same temperature are 0.5, 120 and 8.0 kbar respectively. What is the order of their solubility in water?

Show answer and why every option is right or wrong

Answer: D. The higher the K_H, the lower the solubility, so solubility follows 1/K_H: P = 1/0.5 = 2.0, R = 1/8.0 = 0.125 and Q = 1/120 = 0.0083. The order is P > R > Q (NCERT Class 12 Chemistry, Chapter 1, page 7).

Why A is wrong: A is wrong because it ranks the gases by K_H itself, which is the reverse of the solubility order (trap: reading Henry's constant as solubility).

Why B is wrong: B is wrong because it puts R above P although R has the larger K_H and so the lower solubility.

Why C is wrong: C is wrong because Q has the largest K_H, 120 kbar, so it is the least soluble, not the middle one.

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MCQ 8CalculationPractice

At a fixed temperature, 3.0 g of a gas dissolves in 1.0 kg of water when its pressure above the water is 2.0 bar. How much dissolves in 1.0 kg of water at the same temperature when the pressure is 5.0 bar?

Show answer and why every option is right or wrong

Answer: B. The mass dissolved in a given mass of solvent is proportional to the gas pressure, so the mass is 3.0 g × 5.0 / 2.0 = 7.5 g (NCERT Class 11 Chemistry, Chapter 6, page 171).

Why A is wrong: A is wrong because 6.0 g would follow from pressure doubling; here the pressure rises by a factor of 2.5, not 2 (trap: using the wrong ratio of pressures).

Why C is wrong: C is wrong because 3.0 × 2.0 / 5.0 = 1.2 g inverts the ratio; solubility rises with pressure (trap: inverting the proportionality).

Why D is wrong: D is wrong because 15 g multiplies 3.0 g by 5 and ignores the starting pressure of 2.0 bar (trap: using the new pressure as the multiplier).

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What to remember before solving Physical Equilibria questions

10 NCERT lines
Key Fact

Solid-liquid equilibrium: ice and water at 273 K, equal transfer rates, normal melting point

Ice and water kept in a perfectly insulated thermos flask (no exchange of heat between its contents and the surroundings) at 273K and the atmospheric pressure are in equilibrium state and the system shows interesting characteristic features. We observe that the mass of ice and water do not change with time and the temperature remains constant. However, the equilibrium is not static. The intense activity can be noticed at the boundary between ice and water. Molecules from the liquid water collide against ice and adhere to it and some molecules of ice escape into liquid phase. There is no change of mass of ice and water, as the rates of transfer of molecules from ice into water and of reverse transfer from water into ice are equal at atmospheric pressure and 273 K. It is obvious that ice and water are in equilibrium only at particular temperature and pressure. For any pure substance at atmospheric pressure, the temperature at which the solid and liquid phases are at equilibrium is called the normal melting point or normal freezing point of the substance.

-- NCERT Class 11 Chemistry, Ch. 6, p. 169
Key Fact

Normal boiling point: liquid and vapour in equilibrium at 1.013 bar; lower at high altitude

For any pure liquid at one atmospheric pressure (1.013 bar), the temperature at which the liquid and vapours are at equilibrium is called normal boiling point of the liquid. Boiling point of the liquid depends on the atmospheric pressure. It depends on the altitude of the place; at high altitude the boiling point decreases.

-- NCERT Class 11 Chemistry, Ch. 6, p. 170
Key Fact

General characteristics of equilibria in physical processes (closed system, equal rates, constant properties)

For the physical processes discussed above, following characteristics are common to the system at equilibrium: (i) Equilibrium is possible only in a closed system at a given temperature. (ii) Both the opposing processes occur at the same rate and there is a dynamic but stable condition. (iii) All measurable properties of the system remain constant. (iv) When equilibrium is attained for a physical process, it is characterised by constant value of one of its parameters at a given temperature. Table 6.1 lists such quantities. (v) The magnitude of such quantities at any stage indicates the extent to which the physical process has proceeded before reaching equilibrium.

-- NCERT Class 11 Chemistry, Ch. 6, p. 172
Key Fact

Higher KH means lower gas solubility

Different gases have different KH values at the same temperature, so KH is a function of the nature of the gas. The higher the value of KH at a given pressure, the lower is the solubility of the gas in the liquid.

-- NCERT Class 12 Chemistry, Ch. 1, p. 7
Key Fact

Helium in diving apparatus

In the NEET syllabus; removed from current NCERT.

Helium is used as a diluent for oxygen in modern diving apparatus because of its very low solubility in blood.

-- NCERT Class 12 Chemistry (pre-2023 edition), Chapter 7, p. 211

Physical Equilibria: NEET previous year questions (PYQs) with answers

2 questions from NEET 2023, 2024, answers verified against NTA official keys
NEET 2024Revised key

The Henry’s law constant (K ) values of three gases (A, B, C) in water are 145, 2 × 10⁻⁵ and 35 kbar, H respectively. The solubility of these gases in water follow the order:

1B > A > C
2B > C > A
3A > C > B
4A > B > C
NTA Answer: Option 2(revised_final)

Why the other options are wrong

  • Option 1: A (145 kbar) is placed above C (35 kbar). The higher the value of KH at a given pressure, the lower is the solubility of the gas in the liquid, so C is more soluble than A.
  • Option 3: This is reversed. A has the highest KH, and higher the value of KH at a given pressure, the lower is the solubility; so A is the least soluble and B the most.
  • Option 4: A has the highest KH (145 kbar), so it is the least soluble, not the most: higher the value of KH at a given pressure, the lower is the solubility of the gas.

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NEET 2023

Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R Assertion A : Helium is used to dilute oxygen in diving apparatus. Reason R : Helium has high solubility in O₂. In the light of the above statements, choose the correct answer from the options given below

1Both A and R are true and R is NOT the correct explanation of A
2A is true but R is false
3A is false but R is true
4Both A and R are true and R is the correct explanation of A
NTA Answer: Option 2(final)

Why the other options are wrong

  • Option 1: R is false, so this cannot hold. Helium is used as a diluent for oxygen in modern diving apparatus because of its very low solubility in blood, not high solubility.
  • Option 3: A is true: helium is used as a diluent for oxygen in modern diving apparatus. R is the false one.
  • Option 4: R is false. The reason helium is used is its very low solubility in blood.

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All 15 past-paper questions from Equilibrium →

More in Equilibrium: 8 exam traps and mistakes · 5 formulas · 4 question patterns from its other lessons.

Sources

NCERT refs: Class 11 Chemistry Chapter 6, p.172 | Class 11 Chemistry Chapter 6, p.170 | Class 11 Chemistry Chapter 6, p.169 | Class 11 Chemistry Chapter 6, p.171 | Class 12 Chemistry Chapter 1, p.7

Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.

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