Q.Define the following terms -
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Types of Thermodynamic Systems
Imagine you are holding a cup of hot tea. The tea inside the cup is what we call the system — the part of the universe we are focusing on. Everything outside the cup — the air in the room, the table, you — is the surroundings. The boundary between them is the cup’s wall and the open top.
Now, the question is: what can cross that boundary? Can steam escape from the cup? Can heat flow out into the room? The answer to that question decides what kind of system you are dealing with.
There are exactly three types, and they are defined by two simple yes/no questions: Does matter cross the boundary? and Does energy cross the boundary?
1. Open System — both matter and energy can cross
Think of a pot of boiling water on a stove. The pot is open at the top. Steam (matter) escapes into the air, and heat (energy) flows from the stove into the water. Both are crossing the boundary freely.
An open system exchanges both matter and energy with its surroundings.
Real-life examples are everywhere: a cup of coffee losing heat and water vapour, a living cell taking in nutrients and releasing waste, an engine cylinder taking in fuel and exhausting gases. In an open system, the boundary is permeable to everything.
2. Closed System — only energy can cross
Now imagine the same pot, but this time you put a tight lid on it. No steam can escape — the amount of water inside stays the same. But heat from the stove still flows through the metal and into the water. Matter stays in; energy goes in and out.
A closed system exchanges energy but not matter with its surroundings.
A common exam example is a gas trapped inside a cylinder with a movable piston. The gas cannot leak out (matter is fixed), but you can heat the cylinder or compress the gas — energy crosses the boundary as heat or work. The mass of the gas remains constant.
A closed system does not mean the boundary is rigid. The piston can move, changing the volume. "Closed" refers only to matter, not to shape or size.
3. Isolated System — neither matter nor energy can cross
Take that lidded pot and wrap it in a perfect thermal insulator — something that no heat can pass through. Now nothing gets in or out: no steam escapes, no heat enters or leaves. The system is completely cut off from the universe.
An isolated system exchanges neither matter nor energy with its surroundings.
In reality, a perfect isolated system does not exist — some heat always leaks. But we approximate it. A thermos flask (vacuum flask) is the classic example: hot coffee stays hot for hours because the vacuum between the walls stops heat transfer, and the sealed cap stops matter from escaping. In thermodynamics, we often imagine an isolated system to study processes that happen without any outside interference.
The Big Picture — a single table
| System type | Matter exchange? | Energy exchange? | Example |
|-------------|-----------------|------------------|---------| …
Thermodynamics begins by dividing the universe into the part being studied and everything else, and then tracks quantities that either do or do not depend on the path taken between states. …
System = the part under study; surroundings = everything else; state function = a property depending only on initial/final state, not the path (e.g. U, H, S).
- System: That specific part of the universe which is under observation/study is called the system. Systems can be open (exchange both matter and energy with surroundings), closed (exchange only energy, not matter), or isolated (exchange neither).
- Surroundings: Everything else in the universe, outside the system, that can interact with (exchange matter/energy with) the system is called the surroundings. System + Surroundings = Universe. …
- CBSE 2026Set ANNUAL1 markMCQQ.A system which can neither exchange matter nor energy with the surrounding is called(a) Open system(b) Isolated system(c) Closed system(d) Both (A) and (B)
›Reveal solutionSolution
Thermodynamic systems are classified by what they can exchange with the surroundings: open (both matter and energy), closed (only energy, not matter), and isolated (neither matter nor energy).
- Open system: can exchange both matter and energy with the surroundings (e.g. an open beaker of hot water).
- Closed system: can exchange energy but NOT matter with the surroundings (e.g. a sealed, non-insulated container). …
- CBSE 2026Set ANNUAL1 markMCQQ.There is an exchange of energy and matter between system and surroundings in:(a) Isolated system(b) Closed system(c) Open system(d) Adiabatic process
›Reveal solutionSolution
An open system exchanges both matter and energy with its surroundings; a closed system exchanges only energy; an isolated system exchanges neither.
In thermodynamics, systems are classified by what they can exchange with their surroundings:
- Isolated system: no exchange of matter or energy (e.g., a substance in a perfectly insulated, sealed thermos flask).
- Closed system: exchanges only energy, not matter (e.g., a sealed container that can conduct heat).
- Open system: exchanges BOTH matter and energy (e.g., hot tea in an open cup, which loses both heat and water vapour to the surroundings). …
- CBSE 2023Set annual31 markQ.Define the term 'Closed System'.
›Reveal solutionSolution
A closed system exchanges energy but not matter with its surroundings.
In thermodynamics, the universe is divided into the system (the part under study) and the surroundings, separated by a boundary. Systems are classified by what can cross this boundary:
- Open system — both matter and energy can be exchanged (e.g. an open beaker of hot water).
- Closed system — only energy (heat or work) can be exchanged; matter cannot cross the boundary.
- Isolated system — neither matter nor energy can be exchanged (e.g. an ideal thermos flask). …
- CBSE 2022Set ANNUAL1 markQ.System + Surroundings = ................. (Fill in the blank)
›Reveal solutionSolution
System + Surroundings = Universe.
In thermodynamics, the specific part of the physical world we choose to study (a beaker of reactants, a gas in a cylinder, etc.) is called the system. Everything else outside the system that can interact with it by exchanging heat, work, or matter is the surroundings. The system and its surroundings together make …
- CBSE 2021Set annual1 markQ.Define Isolated System.
›Reveal solutionSolution
An isolated system exchanges neither matter nor energy with its surroundings.
In thermodynamics, systems are classified by what they can exchange with the surroundings:
- Open system: exchanges both matter and energy (e.g., hot tea in an open cup).
- Closed system: exchanges energy but not matter (e.g., a sealed, non-insulated container).
- Isolated system: exchanges neither matter nor energy (e.g., reactants enclosed in a thermally insulated, perfectly sealed rigid vessel, an idealised thermos flask). …
- CBSE 2020Set ANNUAL1 markQ.Fill in the blank: A cup of tea (kept out in the open) is an example of ___. (choose: open system / closed system)
›Reveal solutionSolution
A cup of tea kept in the open is an open system because it loses heat AND water vapour to the surrounding air.
In thermodynamics, systems are classified by what they can exchange with their surroundings:
- Open system: exchanges both matter and energy with surroundings.
- Closed system: exchanges only energy, not matter.
- Isolated system: exchanges neither matter nor energy. …
- CBSE 2019Set ANNUAL1 markQ.Fill in the blank: A tightly closed thermos flask is an example of a ______ system.
›Reveal solutionSolution
A tightly closed thermos flask approximates an isolated system: no matter can enter/leave (sealed), and its insulated double walls minimise heat exchange with the surroundings.
In thermodynamics, systems are classified by what they can exchange with the surroundings:
- Open system: exchanges both matter and energy.
- Closed system: exchanges only energy, not matter.
- Isolated system: exchanges neither matter nor energy. …
- CBSE 2018Set ANNUAL1 markQ.What is the difference between an open system and a closed system?
›Reveal solutionSolution
The distinction is what crosses the system boundary: matter+energy for an open system, energy only for a closed system.
Step 1: An open system is one that can exchange both matter (mass) and energy with its surroundings across its boundary. Example: hot tea in an open cup -- it loses heat (energy) to the surroundings and also loses water vapour (matter). …
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