A system in thermodynamics refers to that part of universe in which observations are made and remaining universe constitutes the surroundings. The surroundings include everything other than the system. System and the surroundings together constitute the universe. The universe = The system + The surroundings
-- NCERT Class 11 Chemistry, Ch. 5, p. 137Fundamentals Thermodynamics
8 MCQs9-step worked example
Source: NCERT ThermodynamicsPYQ coverage: NEET 2024Official key: NTA-verifiedLast updated: 8 Oct 2026
Try this first
Which kind of system exchanges neither energy nor matter with its surroundings?
- A.Open system
- B.Closed system
- C.Isolated system
- D.Adiabatic system
Tap to see the answer
Answer: C. An isolated system has no exchange of energy or matter with the surroundings; reactants in a thermos flask are the NCERT example (NCERT Class 11 Chemistry, Chapter 5, page 138).
A is wrong: A is wrong because an open system exchanges both energy and matter (NCERT Class 11 Chemistry, Chapter 5, page 137).
B is wrong: B is wrong because a closed system cannot exchange matter but can exchange energy (NCERT Class 11 Chemistry, Chapter 5, page 137).
D is wrong: D is wrong because an adiabatic system only blocks the transfer of heat; work can still change its internal energy (NCERT Class 11 Chemistry, Chapter 5, page 138).
Fundamentals Thermodynamics, explained for NEET
Start with a question: a thermos flask, a beaker and a closed steel tank all hold a reaction. Which of them can lose heat, which can lose matter, and why does that decide how we describe the energy change? Thermodynamics begins by naming what we are watching and what it can exchange.
A system is the part of the universe in which observations are made, and the remaining universe is the surroundings. System and surroundings together make the universe (NCERT Class 11 Chemistry, Chapter 5, page 137). For practical purposes the surroundings are only the portion of the remaining universe that can interact with the system, usually the region next to it (NCERT Class 11 Chemistry, Chapter 5, page 137).
Systems are classified by what crosses the boundary:
- Open: energy and matter can both be exchanged; reactants in an open beaker (NCERT Class 11 Chemistry, Chapter 5, page 137).
- Closed: no exchange of matter but energy can be exchanged; reactants in a closed vessel of conducting material such as copper or steel (NCERT Class 11 Chemistry, Chapter 5, page 137).
- Isolated: no exchange of energy or matter; reactants in a thermos flask or any closed insulated vessel (NCERT Class 11 Chemistry, Chapter 5, page 138).
State variables such as pressure p, volume V and temperature T are also called state functions, because their values depend only on the state of the system and not on how it is reached (NCERT Class 11 Chemistry, Chapter 5, page 138). The internal energy U is the total energy of the system, and it changes when heat passes into or out of the system, work is done on or by it, or matter enters or leaves (NCERT Class 11 Chemistry, Chapter 5, page 138). Joule's experiments show that adiabatic work depends only on the two states, so U is a state function (NCERT Class 11 Chemistry, Chapter 5, page 139). V, p and T are other familiar state functions: a temperature change from 25°C to 35°C is +10°C whatever route is taken (NCERT Class 11 Chemistry, Chapter 5, page 139).
An adiabatic process is one in which there is no transfer of heat between the system and the surroundings (NCERT Class 11 Chemistry, Chapter 5, page 138). Heat, q, is the energy exchanged because of a temperature difference (NCERT Class 11 Chemistry, Chapter 5, page 139). At constant volume, ΔV = 0, the heat supplied equals the change in internal energy: ΔU = qV (NCERT Class 11 Chemistry, Chapter 5, page 142).
Sign convention (IUPAC): q is positive when heat flows into the system; w is positive when work is done on the system and negative when the system does work (NCERT Class 11 Chemistry, Chapter 5, page 139). The general case is ΔU = q + w (NCERT Class 11 Chemistry, Chapter 5, page 140).
Watch out: an adiabatic wall blocks heat, not work. Work done on an adiabatic system still changes its internal energy.
How do you solve a Fundamentals Thermodynamics question? A worked example
- 1
Given
A gas absorbs 75 J of heat and does 25 J of work on its surroundings.
- 2
Required
The change in internal energy, ΔU.
- 3
Concept
The first law connects ΔU to heat and work, ΔU = q + w. Heat absorbed is positive, and work done by the system is negative in the IUPAC convention (NCERT Class 11 Chemistry, Chapter 5, pages 139 and 140).
- 4
Formula
ΔU = q + w
- 5
Substitution
q = +75 J (absorbed) and w = −25 J (done by the system), so ΔU = (+75) + (−25).
- 6
Calculation
75 − 25 = 50. Both inputs have two significant figures, so the result is 50 J.
- 7
Final answer
ΔU = +50 J. The internal energy rises.
- 8
Common trap
Entering w as +25 J gives 100 J. That is correct only if 25 J of work were done on the system.
- 9
Similar NEET-style question
A system loses 40 J of heat and has 15 J of work done on it. What is ΔU? (Answer: q = −40 J and w = +15 J, so ΔU = −40 + 15 = −25 J.)
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Can you answer these Fundamentals Thermodynamics 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
Which kind of system exchanges neither energy nor matter with its surroundings?
Show answer and why every option is right or wrong
Answer: C. An isolated system has no exchange of energy or matter with the surroundings; reactants in a thermos flask are the NCERT example (NCERT Class 11 Chemistry, Chapter 5, page 138).
Why A is wrong: A is wrong because an open system exchanges both energy and matter (NCERT Class 11 Chemistry, Chapter 5, page 137).
Why B is wrong: B is wrong because a closed system cannot exchange matter but can exchange energy (NCERT Class 11 Chemistry, Chapter 5, page 137).
Why D is wrong: D is wrong because an adiabatic system only blocks the transfer of heat; work can still change its internal energy (NCERT Class 11 Chemistry, Chapter 5, page 138).
MCQ 2Easy RecallPractice
In thermodynamics, the part of the universe in which observations are made is called the
Show answer and why every option is right or wrong
Answer: A. A system is that part of the universe in which observations are made; the remaining universe is the surroundings (NCERT Class 11 Chemistry, Chapter 5, page 137).
Why B is wrong: B is wrong because the surroundings are everything other than the system, or in practice the part of it that can interact with the system (NCERT Class 11 Chemistry, Chapter 5, page 137).
Why C is wrong: C is wrong because the boundary is the wall that separates the system from the surroundings, not the system itself (trap: confusing the wall with the region it encloses).
Why D is wrong: D is wrong because the universe is the system plus the surroundings (NCERT Class 11 Chemistry, Chapter 5, page 137).
MCQ 3Easy RecallPractice
An adiabatic process is one in which
Show answer and why every option is right or wrong
Answer: C. An adiabatic process is a process in which there is no transfer of heat between the system and the surroundings (NCERT Class 11 Chemistry, Chapter 5, page 138).
Why A is wrong: A is wrong because constant temperature describes an isothermal process, not an adiabatic one (trap: mixing up isothermal and adiabatic).
Why B is wrong: B is wrong because at constant volume ΔV = 0 and ΔU = qV, so heat does flow (NCERT Class 11 Chemistry, Chapter 5, page 142).
Why D is wrong: D is wrong because constant pressure describes an isobaric process; an adiabatic process is defined by the absence of heat transfer (NCERT Class 11 Chemistry, Chapter 5, page 138).
MCQ 4Direct ApplicationPractice
Which pairing of situation and system type is correct?
Show answer and why every option is right or wrong
Answer: D. A sealed steel vessel conducts heat but lets no matter through, so energy can be exchanged and matter cannot: a closed system (NCERT Class 11 Chemistry, Chapter 5, page 137).
Why A is wrong: A is wrong because an open beaker lets both energy and matter cross, so it is an open system (NCERT Class 11 Chemistry, Chapter 5, page 137).
Why B is wrong: B is wrong because copper conducts heat, so energy is still exchanged; the sealed vessel is closed, not isolated (NCERT Class 11 Chemistry, Chapter 5, page 137).
Why C is wrong: C is wrong because a thermos flask is the example of an isolated system (NCERT Class 11 Chemistry, Chapter 5, page 138).
MCQ 5Direct ApplicationPractice
A sample of water is taken from 25°C to 35°C once by heating straight up, and once by first cooling a few degrees and then heating to 35°C. The change in temperature is
Show answer and why every option is right or wrong
Answer: A. Temperature is a state function, so its change depends only on the initial and final states: 35°C − 25°C = +10°C by either route (NCERT Class 11 Chemistry, Chapter 5, page 139).
Why B is wrong: B is wrong because a route through a cooler temperature does not add to the net change; only the end states count (NCERT Class 11 Chemistry, Chapter 5, page 139).
Why C is wrong: C is wrong because the detour does not subtract from the net change either; the net change is fixed by the end states (NCERT Class 11 Chemistry, Chapter 5, page 139).
Why D is wrong: D is wrong because the change in a state function needs only the two end states, not the heat supplied (trap: treating a state function like heat or work).
MCQ 6Direct ApplicationPractice
A gas in a rigid vessel (constant volume) absorbs 85 J of heat. What is the change in its internal energy?
Show answer and why every option is right or wrong
Answer: A. At constant volume ΔV = 0, so no expansion work is done and ΔU = qV = +85 J, positive because the heat flows into the system (NCERT Class 11 Chemistry, Chapter 5, page 142).
Why B is wrong: B is wrong because the sign is reversed; heat absorbed by the system is positive (NCERT Class 11 Chemistry, Chapter 5, page 139).
Why C is wrong: C is wrong because a rigid vessel stops work, not the change in energy; the heat absorbed raises U (NCERT Class 11 Chemistry, Chapter 5, page 142).
Why D is wrong: D is wrong because it doubles the heat; ΔU equals qV exactly at constant volume (NCERT Class 11 Chemistry, Chapter 5, page 142).
MCQ 7CalculationPractice
A system absorbs 60 J of heat and does 25 J of work on its surroundings. What is ΔU? (IUPAC convention)
Show answer and why every option is right or wrong
Answer: A. Heat absorbed is q = +60 J. Work done by the system is negative, w = −25 J. So ΔU = q + w = +60 + (−25) = +35 J (NCERT Class 11 Chemistry, Chapter 5, page 140).
Why B is wrong: B is wrong because +85 J treats the 25 J as work done on the system; work done by the system is negative (NCERT Class 11 Chemistry, Chapter 5, page 139).
Why C is wrong: C is wrong because it flips the overall sign; the heat gain of 60 J outweighs the 25 J lost as work (trap: sign slip on ΔU).
Why D is wrong: D is wrong because it makes both terms negative; the system absorbs heat, so q is positive (NCERT Class 11 Chemistry, Chapter 5, page 139).
MCQ 8CalculationPractice
Match List I (process) with List II (condition). List I: A. Isothermal process, B. Isochoric process, C. Isobaric process, D. Adiabatic process. List II: I. No heat exchange, II. Carried out at constant temperature, III. Carried out at constant volume, IV. Carried out at constant pressure. Choose the correct answer.
Show answer and why every option is right or wrong
Answer: D. Isothermal means constant temperature (II), isochoric means constant volume (III), isobaric means constant pressure (IV), and adiabatic means no heat exchange (I), giving A-II, B-III, C-IV, D-I. The chapter states the adiabatic and constant-volume conditions (NCERT Class 11 Chemistry, Chapter 5, pages 138 and 142); the names isothermal and isobaric come from the NEET 2024 key for this question.
Why A is wrong: A is wrong because it pairs isothermal with constant pressure and isobaric with constant temperature, swapping A and C (trap: swapping isothermal and isobaric).
Why B is wrong: B is wrong because it pairs isochoric with constant temperature and isobaric with constant volume, swapping B and C (trap: swapping isochoric and isobaric).
Why C is wrong: C is wrong because it pairs isothermal with no heat exchange; that is the adiabatic condition (NCERT Class 11 Chemistry, Chapter 5, page 138).
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What to remember before solving Fundamentals Thermodynamics questions
12 NCERT lines
However, the entire universe other than the system is not affected by the changes taking place in the system. Therefore, for all practical purposes, the surroundings are that portion of the remaining universe which can interact with the system. Usually, the region of space in the neighbourhood of the system constitutes its surroundings.
-- NCERT Class 11 Chemistry, Ch. 5, p. 137In an open system, there is exchange of energy and matter between system and surroundings [Fig. 5.2 (a)]. The presence of reactants in an open beaker is an example of an open system*. Here the boundary is an imaginary surface enclosing the beaker and reactants.
-- NCERT Class 11 Chemistry, Ch. 5, p. 137In a closed system, there is no exchange of matter, but exchange of energy is possible between system and the surroundings [Fig. 5.2 (b)]. The presence of reactants in a closed vessel made of conducting material e.g., copper or steel is an example of a closed system.
-- NCERT Class 11 Chemistry, Ch. 5, p. 137In an isolated system, there is no exchange of energy or matter between the system and the surroundings [Fig. 5.2 (c)]. The presence of reactants in a thermos flask or any other closed insulated vessel is an example of an isolated system.
-- NCERT Class 11 Chemistry, Ch. 5, p. 138Definition
State variables / state functions depend only on the state of the system, not on how it is reached
The state of a thermodynamic system is described by its measurable or macroscopic (bulk) properties. We can describe the state of a gas by quoting its pressure (p), volume (V), temperature (T ), amount (n) etc. Variables like p, V, T are called state variables or state functions because their values depend only on the state of the system and not on how it is reached.
-- NCERT Class 11 Chemistry, Ch. 5, p. 138In thermodynamics, we call it the internal energy, U of the system, which may change, when • heat passes into or out of the system, • work is done on or by the system, • matter enters or leaves the system.
-- NCERT Class 11 Chemistry, Ch. 5, p. 138Therefore, internal energy, U, of the system is a state function.
-- NCERT Class 11 Chemistry, Ch. 5, p. 139Can you name some other familiar state functions? Some of other familiar state functions are V, p, and T. For example, if we bring a change in temperature of the system from 25°C to 35°C, the change in temperature is 35°C–25°C = +10°C, whether we go straight up to 35°C or we cool the system for a few degrees, then take the system to the final temperature. Thus, T is a state function and the change in temperature is independent of the route taken.
-- NCERT Class 11 Chemistry, Ch. 5, p. 139We can also change the internal energy of a system by transfer of heat from the surroundings to the system or vice-versa without expenditure of work. This exchange of energy, which is a result of temperature difference is called heat, q.
-- NCERT Class 11 Chemistry, Ch. 5, p. 139Definition
Adiabatic process definition
Adiabatic process is a process in which there is no transfer of heat between the system and the surroundings.
-- NCERT Class 11 Chemistry, Ch. 5, p. 138Key Fact
At constant volume, ΔU=qV
If a process is carried out at constant volume (ΔV=0), then ΔU=qV; the subscript V denotes that heat is supplied at constant volume.
-- NCERT Class 11 Chemistry, Ch. 5, p. 142Fundamentals Thermodynamics: NEET previous year questions (PYQs) with answers
1 question from NEET 2024, answers verified against NTA official keys
NEET 2024Revised key
1A-IV, B-III, C-II, D-I
2A-IV, B-II, C-III, D-I
3A-I, B-II, C-III, D-IV
4A-II, B-III, C-IV, D-I
NTA Answer: Option 4(revised_final)
More in Thermodynamics: 4 exam traps and mistakes · 6 formulas · 2 question patterns from its other lessons.
Sources
NCERT refs: Class 11 Chemistry Chapter 5, p.137 | Class 11 Chemistry Chapter 5, p.138 | Class 11 Chemistry Chapter 5, p.139 | Class 11 Chemistry Chapter 5, p.142 | Class 11 Chemistry Chapter 5, p.140
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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