Open System
A system that can exchange both energy and matter with its surroundings.
Unit
Dimensionless
In depth
Most real chemical processes are open, since reagents are added and gases escape. Mass balance must be tracked alongside energy balance, which makes open systems the harder case to analyse but the more realistic one for industrial and biological chemistry.
Examples in the real world
A reaction in an open beaker releasing gas; a living organism taking in food and expelling waste and heat.