# Systems, Surroundings and Balance Equations

## Summary
Chemical Engineers use balance equations to describe, design, and troubleshoot products 
and processes. Balance equations describe the amount of _stuff_ (e.g., mass, the concentration of a particular chemical component, energy, etc.) in a _system_ that interacts with its _surroundings_.

Systems can be physical things like tanks or chemical reactors, or they can also be logical items such as a savings account or a stage in a multistage decision process. Likewise, 
_stuff_ can be a physical thing such as energy or the
mass of a material or a logical quantity such as information or financial value. Regardless of
what the _system_ and _stuff_ are, the basic form of the balance equation remains the same. 
Thus, if we learn that basic form, we can describe, simulate or design all the products and processes, we are interested in.  

At the end of this lecture module, students will be able to:
* [Describe the four terms that appear in open and closed _stuff_ balance equations](./stuff-balances.md)
* [Write balances for the mass or moles of material in an open and closed system](./material-balances.md)
* [Write balances for the energy of an open and closed system](./energy-balances.md)
* [Write balances that describe the financial health or performance of a system](./money-balances.md)