A company's accountant tracks every dollar entering and leaving a business, because money doesn't simply vanish — it has to end up somewhere, as revenue, expense, or retained earnings. Chemical engineers run essentially the same accounting discipline on matter and energy: a material or energy balance tracks everything entering a process and requires that it all be accounted for as product, waste, or accumulation within the system, resting on the basic physical principle that mass and energy are conserved, not created or destroyed, within an ordinary chemical process.
What goes in has to come out, in some form
A material balance, applied to a chemical process or a specific piece of equipment within it, states that the total mass entering must equal the total mass leaving, plus any mass accumulating within the system if the process isn't at a steady state. If a reactor takes in a certain mass of raw material per hour, that same total mass has to leave as product, unreacted material, or waste byproduct — engineers can't simply lose track of a portion of the input mass, because physically, it has to end up somewhere. This isn't a design choice or convention; it follows directly from the conservation of mass, a fundamental physical constraint that holds regardless of how a specific process happens to be designed.
Energy balances apply the same logic to heat and work
Energy balances apply the identical accounting principle to energy rather than mass: energy entering a process, whether as heat, work, or chemical energy contained in reactants, has to be accounted for as energy leaving the process or accumulating within it, following the first law of thermodynamics. Combined, material and energy balances are the fundamental design tool chemical engineers use to size equipment, determine operating conditions, and predict how a process will actually behave before it's ever built — a proposed plant design that doesn't balance, where the accounted-for outputs don't match the tracked inputs, indicates either a genuine error in the design or some overlooked pathway the mass or energy is actually taking, exactly the way an accounting ledger that doesn't balance signals a genuine bookkeeping error somewhere in the underlying records.
What we're still unsure about
The conservation principles underlying material and energy balances — that mass and energy are neither created nor destroyed in an ordinary chemical process — are fundamental, rigorously established physics, not subject to genuine dispute in this context. What requires real engineering skill, rather than being automatic, is correctly identifying every actual pathway mass and energy can take through a genuinely complex real industrial process, since a real plant can have far more inlet and outlet streams, recycle loops, and side reactions than a simplified textbook example, and setting up a balance that correctly captures every one of those pathways for a complicated real system is a skill chemical engineers develop through extensive practice, not something the underlying conservation principle alone automatically guarantees getting right on the first attempt.
This sits inside Material & Energy Balances, one of eight topics in Chemical Engineering, one of four domains in Engineering, one of seventeen subjects the app can quiz you on.