---
title: "Open System — Intro to Chemical Engineering Definition"
description: "An open system lets both mass and energy cross its boundaries, the basis for material and energy balances on reactors, distillation columns, and flow processes."
canonical: "https://fiveable.me/introduction-chemical-engineering/key-terms/open-system"
type: "key-term"
subject: "Intro to Chemical Engineering"
unit: "Unit 3"
---

# Open System — Intro to Chemical Engineering Definition

## Definition

In chemical engineering, an open system is one where both matter and energy can cross the system boundary, so you track inlet and outlet flows when writing material and energy balances. Most real processes like reactors and distillation columns are open systems.

## What It Is

An open system is any system whose boundary lets both **mass and energy** flow across it. Stuff comes in, stuff goes out, and heat or work can be exchanged with the surroundings at the same time. Think of a continuous chemical reactor with feed pumping in one end and product leaving the other while a jacket heats it. That's the picture you'll work with most often in this course because nearly every industrial process runs continuously.

When you analyze an open system, you draw a boundary (often called a control volume), label every stream crossing it, and account for what enters, what leaves, and what accumulates inside. This is exactly how you set up material balance equations and energy balances. The contrast is with a closed system (energy can cross but not mass) and an isolated system (nothing crosses at all). Knowing which type you're dealing with tells you which terms in your balance equations survive and which drop to zero.

## Why It Matters

Open systems are the default setting for chemical engineering, so they show up the moment you start material balances (Topic 3.2) and conservation of mass (Topic 3.1). The general balance, in = out + accumulation + generation - consumption, is written for an open system, and you simplify it from there. At steady state, accumulation goes to zero and what flows in must flow out.

The same idea carries into the first law of thermodynamics (Topic 4.1). For an open system, your energy balance has to include the energy carried by the mass streams themselves (enthalpy flow), not just heat and work. Recognizing a process as open is what tells you to keep those flow terms in the equation, which is the foundation for designing reactors, separators, and heat exchangers.

## Connections

### Closed System (Unit 3, Unit 4)

A closed system is the open system with the mass-flow terms turned off: energy can still cross the boundary, but no matter does. Comparing the two is the fastest way to see which terms in your balance equations you actually keep.

### [Control Volume (Unit 3)](/introduction-chemical-engineering/key-terms/control-volume)

When you analyze an open system, the control volume is the boundary you draw around it. Picking a smart control volume is what makes a material balance easy or painful.

### Conservation of Mass (Unit 3)

Conservation of mass is why an open system balance works: mass entering must equal mass leaving plus anything accumulating inside. Open systems are where you apply this law to real flowing processes.

### Internal Energy (Unit 4)

In an open system's energy balance, the mass streams carry energy in and out, so you track enthalpy (internal energy plus flow work) rather than internal energy alone. This is what separates an open-system first law from a closed-system one.

## On the AP Exam

Expect problems that hand you a process diagram and ask you to identify the system type before solving. On material balance homework and quizzes, you'll draw the boundary, label inlet and outlet streams, and write in = out + accumulation for an open system at steady state (accumulation = 0). On thermodynamics problems, you'll set up a first-law energy balance that includes enthalpy carried by flowing streams plus heat and work. The key skill graders look for: correctly recognizing the open system and keeping the mass-flow terms instead of dropping them like you would for a closed system.

## Open System vs Closed System

Both can exchange energy with the surroundings, so the only difference is mass. An open system lets matter cross the boundary (a flowing reactor), while a closed system does not (a sealed batch vessel that you only heat). If a stream enters or leaves, it's open.

## Key Takeaways

- An open system allows both mass and energy to cross its boundary, which makes it the model for most continuous industrial processes.
- When you write a balance on an open system, you must include the inlet and outlet flow terms, not just heat and work.
- At steady state the accumulation term is zero, so mass in equals mass out for an open system.
- A closed system blocks mass but allows energy; an isolated system blocks both. Open systems are the least restrictive.
- Reactors, distillation columns, rivers, and the atmosphere are all open systems because they exchange matter with their surroundings.

## FAQs

### What is an open system in chemical engineering?

It's a system where both matter and energy can cross the boundary, so material flows in and out while heat and work may also be exchanged. A continuous reactor with feed and product streams is the classic example.

### Is a chemical reactor an open or closed system?

A continuous reactor is an open system because feed enters and product leaves while it's running. A batch reactor that's sealed during operation behaves more like a closed system, since no mass crosses the boundary until you open it.

### How is an open system different from a closed system?

Both let energy cross the boundary, but only an open system lets mass cross too. If any stream enters or leaves the boundary you've drawn, it's open; if nothing flows in or out, it's closed.

### Do you ignore mass flow in an open system energy balance?

No. For an open system you have to account for the energy the mass streams carry, which means using enthalpy flow terms in your first-law balance, not just heat and work.

### Why do most material balance problems assume an open system?

Because real industrial processes like distillation, mixing, and continuous reactions run with feed coming in and product going out. The general balance, in = out + accumulation, is written for an open system and simplifies to in = out at steady state.

## Related Study Guides

- [3.1 Conservation of mass](/introduction-chemical-engineering/unit-3/conservation-mass/study-guide/VHpTrdpctR2q4Pv1)
- [4.1 First law of thermodynamics](/introduction-chemical-engineering/unit-4/law-thermodynamics/study-guide/ie43BqbHvbGJn5T3)
- [3.2 Material balance calculations](/introduction-chemical-engineering/unit-3/material-balance-calculations/study-guide/kVVIY9WnwtJkb4QK)

## About This Document

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