If two systems are each in thermal equilibrium with a third, they are in thermal equilibrium with each other. Defines temperature operationally.
-- NCERT Class 11 Physics, Ch. 11, p. 228Thermal Equilibrium Zeroth Law
8 MCQs9-step worked example
Source: NCERT ThermodynamicsOfficial key: NTA-verifiedLast updated: 27 Sep 2026
Thermal Equilibrium Zeroth Law, explained for NEET
The trap you're walking into: NEET doesn't just ask "state the zeroth law." It tests whether you understand why thermal equilibrium is transitive and what the zeroth law actually establishes — the logical foundation for temperature as a measurable quantity. Aspirants who memorise the statement but miss its consequence (that temperature is a valid state variable) lose marks on conceptual questions.
What thermal equilibrium means. Two systems are in thermal equilibrium when, connected by a diathermic (heat-conducting) wall, no net heat flows between them. Their temperatures are equal. An adiabatic wall, by contrast, prevents heat exchange entirely — systems separated by an adiabatic wall are thermally isolated regardless of their temperatures (NCERT Class 11 Physics Chapter 11, page 227).
The zeroth law — and why it matters. If system A is in thermal equilibrium with system C, and system B is also in thermal equilibrium with system C, then A and B are in thermal equilibrium with each other. This transitivity property is not obvious — it is a physical law, not a mathematical tautology. Without it, you could not assign a single number (temperature) to characterise a system's thermal state and then use a thermometer (system C) to compare two other systems.
Why "zeroth"? The first and second laws of thermodynamics were formulated earlier. When physicists realised that the transitivity of thermal equilibrium was a logically prior assumption — required before the other laws made sense — they numbered it zero.
What NEET targets here:
- Distinguishing diathermic vs adiabatic walls and their role in defining equilibrium.
- Recognising that the zeroth law is the basis for the concept of temperature, not just a statement about heat flow.
- Understanding that thermal equilibrium is about net heat flow being zero, not about systems being identical.
Can you answer these Thermal Equilibrium Zeroth Law 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
The zeroth law of thermodynamics is the basis for the concept of:
Show answer and why every option is right or wrong
Answer: B. The zeroth law establishes that thermal equilibrium is transitive, which is the logical foundation for defining temperature as a state variable (NCERT Class 11 Physics Chapter 11, page 227).
Why A is wrong: A is wrong because heat is a mode of energy transfer defined independently of the zeroth law; the zeroth law doesn't define heat — it uses the concept of no net heat flow to define equilibrium.
Why C is wrong: C is wrong because work is defined through mechanics and the first law of thermodynamics, not the zeroth law.
Why D is wrong: D is wrong because entropy is introduced via the second law of thermodynamics, not the zeroth law.
MCQ 2Easy RecallPractice
Two systems A and B are separated by an adiabatic wall. System A is at 400 K and system B is at 300 K. Which statement is correct?
Show answer and why every option is right or wrong
Answer: D. An adiabatic wall prevents any heat exchange between systems, regardless of their temperature difference (NCERT Class 11 Physics Chapter 11, page 227).
Why A is wrong: A is wrong because an adiabatic wall blocks heat transfer; heat would flow from A to B only through a diathermic wall.
Why B is wrong: B is wrong because thermal equilibrium requires equal temperatures (or zero net heat flow through a connecting wall); here temperatures differ at 400 K and 300 K, and the adiabatic wall simply prevents the systems from interacting thermally.
Why C is wrong: C is wrong because heat spontaneously flows from higher to lower temperature, not the reverse — and in any case, the adiabatic wall prevents all heat flow.
MCQ 3Easy RecallPractice
A diathermic wall is one that:
Show answer and why every option is right or wrong
Answer: A. A diathermic wall permits heat exchange between systems in contact. When systems reach the same temperature through a diathermic wall, they are in thermal equilibrium (NCERT Class 11 Physics Chapter 11, page 227).
Why B is wrong: B is wrong because that describes an adiabatic wall, not a diathermic wall. Aspirants commonly swap these two definitions.
Why C is wrong: C is wrong because diathermic refers specifically to heat conduction, not matter transfer. In thermodynamics, a closed system doesn't exchange matter regardless of wall type.
Why D is wrong: D is wrong because a perfect thermal insulator is the definition of an adiabatic wall — the exact opposite of a diathermic wall.
MCQ 4Direct ApplicationPractice
System A is in thermal equilibrium with system C, and system B is also in thermal equilibrium with system C. According to the zeroth law, which conclusion follows?
Show answer and why every option is right or wrong
Answer: A. The zeroth law states exactly this transitivity: if A is in equilibrium with C, and B is in equilibrium with C, then A and B are in equilibrium with each other — meaning they have the same temperature (NCERT Class 11 Physics Chapter 11, page 227).
Why B is wrong: B is wrong because thermal equilibrium implies equal temperatures, not equal internal energies. Two systems at the same temperature can have very different internal energies if they differ in mass, composition, or phase.
Why C is wrong: C is wrong because the zeroth law is about temperature equality, which is independent of the material composition of the systems.
Why D is wrong: D is wrong because it overstates the conclusion. The zeroth law says A and B are currently at the same temperature. If conditions change (e.g., one system is heated), heat could flow between them through a diathermic wall.
MCQ 5Direct ApplicationPractice
A thermometer works on the principle of:
Show answer and why every option is right or wrong
Answer: C. A thermometer (system C) is placed in contact with a body (system A) until thermal equilibrium is reached. The zeroth law guarantees that if another body (system B) also reads the same on the thermometer, then A and B are at the same temperature. This transitivity is what makes thermometric measurement valid (NCERT Class 11 Physics Chapter 11, page 227).
Why A is wrong: A is wrong because the first law deals with energy conservation (ΔU = Q − W), not with the logical basis for temperature measurement.
Why B is wrong: B is wrong because the second law deals with the direction of spontaneous processes and entropy, not with the definition of temperature via thermal equilibrium.
Why D is wrong: D is wrong because Newton's law of cooling describes the rate of heat loss, not the foundational principle that allows a thermometer to compare temperatures of two systems.
MCQ 6Direct ApplicationPractice
Two bodies at different temperatures are brought into contact through a diathermic wall. At thermal equilibrium:
Show answer and why every option is right or wrong
Answer: C. Thermal equilibrium is defined by the condition that net heat flow between the systems is zero, which occurs when they reach the same temperature. The final common temperature lies between the two initial temperatures (NCERT Class 11 Physics Chapter 11, page 227).
Why A is wrong: A is wrong because the hotter body loses only enough internal energy to equalise temperatures; it does not lose all its internal energy.
Why B is wrong: B is wrong because the final equilibrium temperature is intermediate between the initial temperatures of the two bodies, not equal to the initially cooler body's temperature.
Why D is wrong: D is wrong because equal temperature does not imply equal internal energy. Internal energy depends on mass, specific heat, and phase in addition to temperature.
MCQ 7Concept TrapPractice
System A is connected to system C by a diathermic wall and is left in contact until A reaches thermal equilibrium with C. Later, system B is connected to C by a diathermic wall and is left in contact until B also reaches thermal equilibrium with C. A and B are never joined directly except by a permanent adiabatic wall between them. According to the zeroth law, are A and B in thermal equilibrium with each other, and does the adiabatic wall between them contradict this?
Show answer and why every option is right or wrong
Answer: A. The zeroth law states that if A is in thermal equilibrium with C, and B is also in thermal equilibrium with C, then A and B are in thermal equilibrium with each other — i.e., they share the same temperature. This conclusion follows purely from each system's separate history with C; it does not depend on what kind of wall (if any) later separates A and B. An adiabatic wall placed between A and B afterwards only prevents heat from flowing between them — since they are already at the same temperature, no heat needs to flow anyway, so the adiabatic wall is fully consistent with, not a contradiction of, their equilibrium (NCERT Class 11 Physics Chapter 11, page 227).
Why B is wrong: B is wrong because the wall directly between A and B has no bearing on whether they share a temperature — that fact is already fixed by each one's separate equilibrium with C. An adiabatic wall only blocks heat flow; it cannot un-equalise temperatures that were established independently, so it does not block the zeroth law's conclusion.
Why C is wrong: C is wrong because an adiabatic wall is defined precisely as one that prevents heat exchange — it cannot be the mechanism that equalises A and B's temperatures. Their equal temperature is already guaranteed by each being in equilibrium with C, before the adiabatic wall is even considered.
Why D is wrong: D is wrong because the zeroth law is explicitly a three-body statement: it takes two separate equilibrium relations (A with C, and B with C) and concludes a third (A with B). Relating three systems is exactly what the law does.
MCQ 8Concept TrapPractice
The zeroth law of thermodynamics was numbered 'zero' because:
Show answer and why every option is right or wrong
Answer: B. The first and second laws were formulated and named earlier. The zeroth law was later recognised as a more fundamental assumption — the logical prerequisite for defining temperature — so it was placed before the first law as 'zeroth' (NCERT Class 11 Physics Chapter 11, page 227).
Why A is wrong: A is wrong because the zeroth law was actually formulated after the first and second laws, not before all laws of physics. It was numbered zero retroactively due to its logical priority.
Why C is wrong: C is wrong because while thermal equilibrium involves zero net heat flow, the name 'zeroth' refers to the law's logical ordering, not to zero heat transfer.
Why D is wrong: D is wrong because the zeroth law applies at all temperatures, not only at absolute zero. It is a general statement about the transitivity of thermal equilibrium.
Free NEET study resources
Get a structured 30-day Mechanics plan and a complete formula booklet — delivered to your inbox instantly.
How do you solve a Thermal Equilibrium Zeroth Law question? A worked example
- 1
Given
Three metal blocks P, Q, and R are insulated from the environment. Block P (at an unknown temperature Tₚ) is placed in contact with block R through a diathermic wall and they reach thermal equilibrium at 350 K. Block Q (at an unknown temperature T_Q) is then placed in contact with block R through a diathermic wall and they also reach thermal equilibrium at 350 K.
- 2
Required
Determine whether blocks P and Q are in thermal equilibrium with each other, and state the law that justifies your answer.
- 3
Concept
The zeroth law of thermodynamics: if system P is in thermal equilibrium with system R, and system Q is in thermal equilibrium with system R, then P and Q are in thermal equilibrium with each other.
- 4
Formula
No algebraic formula is needed. The zeroth law is the governing principle: thermal equilibrium is transitive.
- 5
Substitution
• P is in thermal equilibrium with R (both at 350 K after contact).• Q is in thermal equilibrium with R (both at 350 K after contact).• By the zeroth law: P is in thermal equilibrium with Q.
- 6
Calculation
No numerical calculation required. The zeroth law directly yields the conclusion.
- 7
Final answer
Yes, P and Q are in thermal equilibrium with each other. Both are at 350 K. The zeroth law of thermodynamics justifies this conclusion.
- 8
Common trap
Aspirants sometimes confuse "same temperature" with "same internal energy." Two blocks at 350 K are in thermal equilibrium — but they may have very different internal energies if they differ in mass or specific heat capacity. The zeroth law guarantees equal temperature, not equal energy content.
- 9
Similar NEET-style question
Three gas containers X, Y, and Z are connected pairwise through diathermic walls. X and Z reach equilibrium; Y and Z reach equilibrium. Without the zeroth law, could you conclude X and Y are at the same temperature? (Answer: No — the zeroth law is precisely the physical assertion that this transitivity holds.)
---
What to remember before solving Thermal Equilibrium Zeroth Law questions
More in Thermodynamics: 2 exam traps and mistakes · 4 formulas · 2 question patterns from its other lessons.
Thermal Equilibrium Zeroth Law questions from past NEET papers
No question in our NEET 2020–2025 set targets this topic directly.
Sources
NCERT refs: Class 11 Physics Chapter 11, p.227
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 →