Which of the following is an example of an open system?
2026
Which of the following is an example of an open system?
Answer: A. Hot coffee held in a cup — Concept In thermodynamics the part of the universe chosen for study is called the system, and everything outside it is the surroundings; the imaginary or real…
- A.
Hot coffee held in a cup
- B.
A thermos flask with a tight lid
- C.
A sealed pressure cooker
- D.
A closed bottle of soda
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Show answer & explanation
Correct answer: A
Concept
In thermodynamics the part of the universe chosen for study is called the system, and everything outside it is the surroundings; the imaginary or real surface separating the two is the boundary. Systems are classified only by what that boundary lets pass — matter, energy, both, or neither.
System type | Matter crosses the boundary | Energy crosses the boundary |
|---|---|---|
Open | Yes | Yes |
Closed | No | Yes |
Isolated | No | No |
Applying it here
Take the hot coffee held in a cup as the system and the surrounding room as the surroundings. The boundary is the coffee surface plus the cup walls.
The cup has no lid, so water molecules leave the hot surface as vapour and mix into the room air. Matter therefore crosses the boundary.
The coffee is hotter than the room, so heat flows outward through the cup wall and the open top, and the drink cools. Energy therefore also crosses the boundary.
Both matter and energy cross, which by the table above is exactly an open system. So hot coffee held in a cup is the open system.
Cross-check with the other arrangements
A sealed pressure cooker: the gasket clamps the lid and the weighted valve stays seated while the inside pressure is below its set value, so the water and the steam remain in the pot and no matter leaves, while the metal base conducts heat from the flame into the contents. Energy only crosses the boundary, so it is a closed system. If heating continues past the set pressure the valve lifts and vents steam, and only then would matter start to cross; the option specifies a sealed cooker, that is the non-venting state, so the closed-system reading is the intended one.
A closed bottle of soda: the cap holds the liquid and the dissolved carbon dioxide in, while heat still passes through the bottle wall as it warms in a room or chills in a fridge. Energy only crosses — again a closed system.
A thermos flask with a tight lid: the lid stops matter escaping, the vacuum between the double walls removes conduction and convection across the gap, and the silvered low-emissivity surfaces cut down the radiation that a vacuum cannot block, so hardly any matter or heat crosses. It is the everyday approximation to an isolated system.
Only the open cup lets both matter and energy pass across its boundary, so hot coffee held in a cup is the example of an open system.