An open system exchanges both energy and matter with its surroundings, whereas a closed system exchanges energy but does not normally exchange matter. This distinction is part of IB ESS subtopic 1.2 Systems at both SL and HL.
A system is a set of interacting or interdependent components. Its boundary separates the system from its surroundings, while inputs enter across that boundary and outputs leave it.
| System type | Exchange across the boundary | ESS example |
|---|---|---|
| Open system | Energy and matter | An ecosystem receives sunlight, water and migrating organisms, while releasing heat, water and organisms. |
| Closed system | Energy, but not matter | Earth is often modelled as approximately closed for matter: solar radiation enters and heat leaves, but very little matter enters or escapes. |
Most ecosystems are open systems. For example, a forest receives solar energy through radiation and matter through precipitation, atmospheric gases and animal migration. It loses energy as heat and matter through runoff, leaching, gas emissions and movement of organisms.
Earth is treated as an approximately closed system for matter because exchanges such as meteorites entering and spacecraft leaving are extremely small compared with Earth's total mass. However, it is open to energy: short-wave solar radiation enters, and long-wave infrared radiation leaves.
The common misconception is that a closed system exchanges nothing with its surroundings. This is incorrect: a closed system can exchange energy, but not matter. A system exchanging neither matter nor energy would be isolated, although perfectly isolated systems are theoretical.
In a systems diagram, draw storages as rectangular boxes and flows as arrows. The arrow direction must show the direction of the flow, and the system boundary should make clear which inputs and outputs cross it.
Exam technique: If asked to “distinguish” open and closed systems, refer to both throughout and identify exactly what crosses the boundary. Support the distinction with an accurate example rather than only stating two definitions.