Definition
Dynamic equilibrium
State in which forward and reverse reactions occur at equal rates; concentrations remain constant. Reaction continues at molecular level despite no net macroscopic change.
-- NCERT Class 11 Chemistry, Ch. 6, p. 168Dynamic Equilibrium
8 MCQs9-step worked example
Source: NCERT EquilibriumOfficial key: NTA-verifiedLast updated: 27 Sep 2026
Dynamic Equilibrium, explained for NEET
Dynamic equilibrium — what NEET actually tests
The concept sounds simple, but NEET uses it to filter students who memorised a definition from those who understand what "dynamic" means at the molecular level.
The core idea (NCERT Class 11 Chemistry Chapter 6, page 171): In a closed system, when a reversible reaction reaches equilibrium, the forward and reverse reactions continue at equal rates. Macroscopic properties — concentrations, pressure, colour — stop changing, but molecular-level transformations never stop.
Three non-negotiable features of dynamic equilibrium:
- Both reactions run simultaneously. Equilibrium is not "reaction stopped." Reactant molecules still convert to products, and products still revert to reactants — at the same rate.
- Closed system required. If products escape (open vessel, gas released), the system cannot reach equilibrium. NEET stems sometimes describe open containers to test whether you notice.
- Observable properties are constant, not zero. Concentrations are constant because the rate of formation equals the rate of consumption — not because nothing is happening.
The distinction that costs marks: Static vs. dynamic. A saturated salt solution with undissolved solid at the bottom is at dynamic equilibrium — dissolution and crystallisation occur at equal rates. A sealed bottle of pure water with no dissolved gas is simply unchanging (no competing process). NEET stems test whether you can identify which scenario is genuinely dynamic.
Watch-out: If a question says "equilibrium is reached," that tells you rates are equal and net change is zero. It does NOT tell you the reaction has stopped, that concentrations are equal, or that the amounts of reactant and product are the same.
Can you answer these Dynamic Equilibrium 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
At dynamic equilibrium in a closed system, which statement is correct?
Show answer and why every option is right or wrong
Answer: D. By definition, dynamic equilibrium means the forward and reverse reaction rates are equal, so there is no net change in concentrations (NCERT Class 11 Chemistry Chapter 6, page 171).
Why A is wrong: A is wrong because at dynamic equilibrium, the forward reaction continues — it has not stopped. Both forward and reverse reactions proceed simultaneously.
Why B is wrong: B is wrong because equal rates do not imply equal concentrations. The equilibrium concentrations depend on the equilibrium constant, not on the rates being equal.
Why C is wrong: C is wrong because both the forward and reverse reactions occur at dynamic equilibrium, not just the reverse reaction.
MCQ 2Easy RecallPractice
Which of the following is an essential condition for a system to attain dynamic equilibrium?
Show answer and why every option is right or wrong
Answer: A. Dynamic equilibrium requires a closed system so that neither reactants nor products can escape. Without containment, the reverse reaction cannot build up to match the forward rate (NCERT Class 11 Chemistry Chapter 6, page 171).
Why B is wrong: B is wrong because an open system allows products (especially gases) to escape, preventing the reverse reaction from reaching the same rate as the forward reaction.
Why C is wrong: C is wrong because dynamic equilibrium can be established at any temperature, not only at 25°C. The equilibrium constant changes with temperature, but equilibrium itself is not restricted to a specific temperature.
Why D is wrong: D is wrong because a catalyst speeds up attainment of equilibrium but is not required for equilibrium to exist. A system reaches equilibrium with or without a catalyst.
MCQ 3Direct ApplicationPractice
A sealed container holds N₂O₄(g) in equilibrium with NO₂(g). The brown colour of NO₂ remains constant over time. This observation indicates that:
Show answer and why every option is right or wrong
Answer: C. Constant colour means constant [NO₂]. At dynamic equilibrium, N₂O₄ still decomposes and NO₂ still recombines, but these rates are equal, so the net concentration (and therefore colour intensity) does not change (NCERT Class 11 Chemistry Chapter 6, page 171).
Why A is wrong: A is wrong because NO₂ is still being formed — the forward reaction (N₂O₄ → 2NO₂) continues at equilibrium. The colour is constant because formation and consumption of NO₂ occur at equal rates, not because formation stopped.
Why B is wrong: B is wrong because N₂O₄ is still decomposing at equilibrium. The constant colour reflects equal rates, not cessation of the forward reaction.
Why D is wrong: D is wrong because if all N₂O₄ had been consumed, no reverse reaction would be possible and the system would not be at equilibrium. Equilibrium requires both species to be present.
MCQ 4Concept TrapPractice
A chemist adds a small amount of radioactively labelled sugar to a saturated sugar solution that already has excess non-radioactive solid sugar at the bottom, at constant temperature, and disturbs the system no more than usual. After enough time has passed, radioactivity is tested in both the solid and the solution. Which result would confirm that this dissolution equilibrium is dynamic, and why?
Show answer and why every option is right or wrong
Answer: A. A is correct. NCERT describes exactly this radioactive-tracer experiment: dropping radioactive sugar into a saturated non-radioactive sugar solution shows radioactivity appearing in the solid sugar over time, even though the mass of solid and the solution concentration stay constant. This is only possible if dissolution and crystallisation continue simultaneously at equal rates, constantly exchanging molecules between the two phases — proving the equilibrium is dynamic, not static (NCERT Class 11 Chemistry, Chapter 6, page 171).
Why B is wrong: B is wrong because the tracer experiment shows the opposite: radioactivity DOES appear in the solid, because solute molecules keep depositing onto it even at equilibrium. If exchange stopped, no radioactive sugar would ever reach the solid.
Why C is wrong: C is wrong because the solution keeps its radioactivity too — molecules continuously re-dissolve from the solid. If crystallisation only ran one way, the solution would empty out, which contradicts the constant equilibrium concentration.
Why D is wrong: D is wrong because dissolution and crystallisation both occur continuously at equal rates at saturation; that two-way exchange is exactly what the appearance of radioactivity in the solid demonstrates.
MCQ 5Easy RecallPractice
Which of the following correctly distinguishes a static situation from dynamic equilibrium?
Show answer and why every option is right or wrong
Answer: B. Dynamic equilibrium is defined by two opposing processes occurring simultaneously at equal rates, producing no net macroscopic change. In a static situation, no opposing process is occurring at all (NCERT Class 11 Chemistry Chapter 6, page 171).
Why A is wrong: A is wrong because at dynamic equilibrium, macroscopic properties (concentration, pressure, colour) are constant, not changing. The molecular-level activity is ongoing, but the observable properties remain unchanged.
Why C is wrong: C is wrong because in a static situation there are no opposing processes at all — nothing is occurring in both directions. Equal-rate opposing processes define dynamic equilibrium, not a static state.
Why D is wrong: D is wrong because a static situation means nothing is actively occurring, not that only a forward process runs. A system with only a forward process and no reverse would be an irreversible reaction proceeding to completion, not a static state.
MCQ 6Direct ApplicationPractice
At equilibrium, the concentration of a reactant in a closed vessel is 0.5 mol/L. A student claims this means 0.5 mol/L of products must also be present. This claim is:
Show answer and why every option is right or wrong
Answer: A. Equal rates of forward and reverse reactions do not require equal concentrations. The actual equilibrium concentrations are determined by the value of the equilibrium constant (K), which can favour reactants or products to very different extents (NCERT Class 11 Chemistry Chapter 6, page 171).
Why B is wrong: B is wrong because 'equal rates' and 'equal concentrations' are entirely different claims. The equilibrium constant K dictates the ratio of product to reactant concentrations, and K can be much greater or much less than 1.
Why C is wrong: C is wrong because equal rates do not imply equal concentrations. Rate depends on both the rate constant and concentration. Two reactions can have equal rates with very different concentrations if their rate constants differ.
Why D is wrong: D is wrong because at equilibrium both reactants and products coexist. If all reactants had been consumed, the reaction would have gone to completion — that is not equilibrium.
MCQ 7Concept TrapPractice
A reversible reaction A(g) ⇌ B(g) is carried out in an open flask. After some time, only product B is detected. This observation suggests:
Show answer and why every option is right or wrong
Answer: D. An open flask allows gaseous products (or reactants) to escape. Without containment, the reverse reaction cannot proceed at a rate matching the forward reaction, so true dynamic equilibrium is never established. The disappearance of A may simply reflect its continuous conversion to B, which escapes (NCERT Class 11 Chemistry Chapter 6, page 171).
Why A is wrong: A is wrong because dynamic equilibrium requires a closed system. In an open flask, product B can escape (or accumulate without reverse pressure), so the condition of equal forward and reverse rates is never met.
Why B is wrong: B is wrong because K = 1 would mean comparable amounts of A and B at equilibrium. Finding only B does not indicate K = 1, and regardless, equilibrium cannot be established in an open container.
Why C is wrong: C is wrong because the reaction is stated to be reversible (A ⇌ B). The absence of A in the open flask is due to the container being open, not due to the reaction being irreversible.
MCQ 8Direct ApplicationPractice
Consider the reversible reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) in a sealed vessel at constant temperature. At equilibrium, which of the following must be true?
Show answer and why every option is right or wrong
Answer: B. At dynamic equilibrium, the rate of the forward reaction (forming SO₃) equals the rate of the reverse reaction (decomposing SO₃). This is the defining characteristic. Concentrations are constant but not necessarily equal, and all species coexist (NCERT Class 11 Chemistry Chapter 6, page 171).
Why A is wrong: A is wrong because equilibrium does not require reactant and product concentrations to be equal. Their ratio is fixed by the equilibrium constant K, which may strongly favour one side.
Why C is wrong: C is wrong because the total pressure at equilibrium is nonzero — gases are present and exert pressure. Equilibrium means constant pressure (in a rigid vessel at constant T), not zero pressure.
Why D is wrong: D is wrong because at equilibrium, all species (SO₂, O₂, SO₃) are present. The equilibrium mixture contains both reactants and products; none is completely consumed.
Free NEET study resources
Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.
How do you solve a Dynamic Equilibrium question? A worked example
- 1
Given
A closed container holds liquid water in equilibrium with water vapour at 100°C and 1 atm:
H₂O(l) ⇌ H₂O(g)
The rate of evaporation is measured as 2.5 × 10⁻³ mol/s. - 2
Required
Find the rate of condensation at equilibrium.
- 3
Concept
At dynamic equilibrium, the rate of the forward process (evaporation) equals the rate of the reverse process (condensation). The system is closed, so water vapour cannot escape.
- 4
Formula
Rate_forward = Rate_reverse (at dynamic equilibrium)
- 5
Substitution
Rate_condensation = Rate_evaporation = 2.5 × 10⁻³ mol/s
- 6
Calculation
No arithmetic needed. The equality follows directly from the definition of dynamic equilibrium.
- 7
Final answer
The rate of condensation is 2.5 × 10⁻³ mol/s.
- 8
Common trap
The most common error is claiming the rate of condensation is zero ("the water has finished evaporating") or that it differs from the evaporation rate. At equilibrium, both processes continue at the same rate — that is why the water level and vapour pressure remain constant.
- 9
Similar NEET-style question
In a sealed flask, liquid bromine is in equilibrium with bromine vapour. If the rate of vaporisation is r mol/s, what is the rate of condensation? (Answer: r mol/s, by the definition of dynamic equilibrium in a closed system.)
---
What to remember before solving Dynamic Equilibrium questions
More in Equilibrium: 8 exam traps and mistakes · 5 formulas · 4 question patterns from its other lessons.
Dynamic Equilibrium questions from past NEET papers
No question in our NEET 2020–2025 set targets this topic directly.
Sources
NCERT refs: Class 11 Chemistry Chapter 6, p.171
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.
Test yourself on this topic with real past-paper questions:
Practice this topic →