Amass balance, also called amaterial balance, is an application ofconservation of massto the analysis of physical systems. By accounting for material entering and leaving a system,mass flowscan be identified which might have been unknown, or difficult to measure without this technique. The exactconservation lawused in the analysis of the system depends on the context of the problem, but all revolve around mass conservation, i.e. thatmattercannot disappear or be created spontaneously.[1]:5962Therefore, mass balances are used widely inengineeringandenvironmental analyses. For example, mass balance theory is used to designchemical reactors, to analyse alternative processes to produce chemicals, as well as to modelpollutiondispersion and other processes of physical systems. Closely related and complementary analysis techniques include thepopulation balance,energy balanceand the somewhat more complexentropybalance. These techniques are required for thorough design and analysis of systems such as therefrigeration cycle.

In environmental monitoring the termbudget calculationsis used to describe mass balance equations where they are used to evaluate the monitoring data (comparing input and output, etc.) In biology thedynamic energy budgettheory for metabolic organisation makes explicit use of mass and energy balances.

Chemical engineering[edit]Chemical engineering unit operations consist of five classes:

1. Fluid flow processes, includingfluids transportation,filtration, andsolids fluidization.

2. Heat transferprocesses, includingevaporationandheat exchange.

3. Mass transferprocesses, includinggas absorption,distillation,extraction,adsorption, anddrying.

4. Thermodynamic processes, includinggas liquefaction, andrefrigeration.

5. Mechanical processes, includingsolids transportation,crushing and pulverization, andscreening and sieving.

Chemical engineering unit operations also fall in the following categories which involve elements from more than one class:

Combination (mixing)

Separation (distillation,crystallization,chromatography)

Reaction (chemical reaction)

Furthermore, there are some unit operations which combine even these categories, such asreactive distillationand stirred tankreactors. A "pure" unit operation is a physical transport process, while a mixed chemical/physical process requires modeling both the physical transport, such as diffusion,andthe chemical reaction. This is usually necessary for designingcatalytic reactions, and is considered a separate discipline, termedchemical reaction engineering.

Chemical engineering unit operations and chemical engineeringunit processingform the main principles of all kinds of chemical industries and are the foundation of designs of chemical plants, factories, and equipment used.

In general, unit operations are designed by writing down the balances for the transported quantity for each elementary component (which may be infinitesimal) in the form of equations, and solving the equations for the design parameters, then selecting an optimal solution out of the several possible and then designing the physical equipment. For instance, distillation in a plate column is analyzed by writing down the mass balances for each plate, wherein the known vapor-liquid equilibrium and efficiency, drip out and drip in comprise the total mass flows, with a sub-flow for each component. Combining a stack of these gives the system of equations for the whole column. There is a range of solutions, because a higher reflux ratio enables fewer plates, and vice versa. The engineer must then find the optimal solution with respect to acceptable volume holdup, column height and cost of construction.

The Law of Conservation of Mass

The Law of Conservation of Mass states that matter can be changed from one form into another, mixtures can be separated or made, and pure substances can be decomposed, but the total amount of mass remains constant. We can state this important law in another way. The total mass of the universe is constant within measurable limits; whenever matter undergoes a change, the total mass of the products of the change is, within measurable limits, the same as the total mass of the reactants.

The formulation of this law near the end of the eighteenth century marked the beginning of modern chemistry. By that time many elements had been isolated and identified, most notably oxygen, nitrogen, and hydrogen. It was also known that, when a pure metal was heated in air, it became what was then called acalx(which we now call an oxide) and that this change was accompanied by an increase in mass. The reverse of this reaction was also known: Many calxes on heating lost mass and returned to pure metals. Many imaginative explanations of these mass changes were proposed. Antoine Lavoisier (1743-1794), a French nobleman later guillotined in the revolution, was an amateur chemist with a remarkably analytical mind. He considered the properties of metals and then carried out a series of experiments designed to allow him to measure not just the mass of the metal and the calx but also the mass of the air surrounding the reaction. His results showed that the mass gained by the metal in forming the calx was equal to the mass lost by the surrounding air.

With this simple experiment, in which accurate measurement was critical to the correct interpretation of the results, Lavoisier established the Law of Conservation of Mass, and chemistry became an exact science, one based on careful measurement. For his pioneering work in the establishment of that law and his analytical approach to experimentation, Lavoisier has been called the father of modern chemistry.

Note that this step forward, like so many others in science, depended on technology - in this instance, on the development of an accurate and precise balance (see Figure 1.5).