A closed system exchanges energy with its surroundings but not matter. An open system exchanges both energy and matter. There’s also a third category, an isolated system, which exchanges neither. These three classifications show up across science, engineering, and everyday life, and understanding the differences helps you make sense of everything from boiling water to how the planet works.
The Three Types of Systems
Every system in science has a boundary, real or imaginary, that separates it from everything else (the “surroundings”). What can cross that boundary determines the system type.
- Open system: Both energy and matter can cross the boundary. A pot of boiling water without a lid is a classic example. Heat flows in from the stove, and steam (matter) escapes into the air.
- Closed system: Energy can cross the boundary, but matter cannot. A sealed terrarium works this way. Sunlight and heat pass through the glass, but the moisture inside stays put, cycling between evaporation and condensation without leaving.
- Isolated system: Neither energy nor matter crosses the boundary. A perfect thermos would qualify, though truly isolated systems are theoretical. Every real container leaks at least a tiny amount of heat.
Why the Distinction Matters in Science
The difference between open and closed systems is central to the second law of thermodynamics, which says that disorder (entropy) in a system tends to increase over time. Closed systems move toward thermodynamic equilibrium relatively quickly, reaching a state where nothing further changes. Open systems, by contrast, generally operate under non-equilibrium conditions. Because matter and energy keep flowing through them, they can maintain structure and complexity without violating the second law. This is why the distinction is so important in biology: life itself depends on being an open system.
Earth as a Closed System
According to NASA, Earth is mostly a closed system with respect to matter. Apart from the occasional meteor entering the atmosphere and tiny amounts of hydrogen drifting in or out at the top of the atmosphere, the planet holds onto its material. The atoms in your body have been on Earth for billions of years.
Energy, on the other hand, flows freely. The sun delivers solar radiation, and Earth radiates heat and light back into space, maintaining a rough energy balance over time. This makes Earth a closed system overall: energy crosses the boundary, matter stays inside. The ocean, by contrast, is an open system within Earth. It exchanges both energy (solar radiation, heat) and mass (water vapor, precipitation) with the atmosphere.
Living Things Are Open Systems
Every living cell is an open system. Cells take in nutrients and oxygen, run chemical reactions to extract energy, and release waste products. These exchanges with the environment are not optional. They are what make life possible. Even a cell that isn’t growing or dividing is constantly trading substances with its surroundings, and that controlled exchange is one of the markers biologists use to determine whether something is alive.
Maintaining this flow requires a steady input of energy. Cells use that energy to keep their internal chemistry far from equilibrium, building complex molecules, repairing damage, and responding to signals. If the exchange stops, the cell reaches equilibrium, and equilibrium for a living thing means death.
A Simple Lab Example
Imagine heating a beaker of water from 30°C to 110°C. In an open beaker (open system), the water absorbs heat from the burner, but as the temperature rises, water molecules gain enough energy to escape as steam. Dissolved gases like oxygen and nitrogen also leave the liquid. The mass of the beaker and its contents drops over time.
Now picture the same experiment with a sealed container (closed system). The same heat energy enters, but no water molecules can escape. The mass stays constant. The water still changes phase, turning to steam inside the container, but nothing leaves. This is the core practical difference: in a closed system, you always account for the same amount of matter, which makes it easier to track chemical reactions and verify that mass is conserved.
Open and Closed Systems in Engineering
The open/closed distinction also shows up in industrial design, particularly in cooling systems used by factories, data centers, and power plants.
An open loop cooling tower lets the water being cooled come into direct contact with the surrounding air. Heat dissipates partly through direct air-water contact and partly through evaporation of a small portion of the water. This approach can cool water below the ambient air temperature, and the equipment costs less upfront because it skips the extra hardware needed to keep fluids separated. The tradeoff is that the water picks up dust, minerals, and biological contaminants from the air.
A closed loop cooling tower keeps the process water completely sealed off from the atmosphere. Heat transfers through a stainless-steel heat exchanger instead of direct contact. Because the cooling water never touches outside air, it stays clean, which reduces corrosion in the equipment and improves heat transfer efficiency over time. Closed loop systems also conserve water better because they reduce the need to periodically flush contaminated water from the basin. The downside is higher initial cost and the possible need for antifreeze in cold climates.
Choosing between the two comes down to priorities. If low cost and maximum cooling power matter most, open loop systems have the edge. If water purity, reduced maintenance, and water conservation are more important, closed loop systems win out.
How to Identify the System Type
When you encounter a system in a textbook or in real life, ask two questions. First: can matter leave or enter? If yes, it’s open. If no, move to the second question: can energy leave or enter? If energy can cross but matter cannot, it’s closed. If neither can cross, it’s isolated.
A few quick-reference examples to anchor the concept:
- Open: a campfire (fuel burns, smoke and heat escape), the ocean, a car engine (fuel in, exhaust out)
- Closed: Earth as a whole, a sealed terrarium, a sealed pressure cooker (steam stays inside, heat moves through the walls)
- Isolated: an ideal thermos (no real-world system is perfectly isolated, but well-insulated containers come close over short time periods)