masskonservation

Mass conservation, also called the conservation of mass, is a fundamental principle stating that in an isolated system the total mass remains constant over time. In classical physics and chemistry, mass is neither created nor destroyed; it can change form (solid, liquid, gas) but the total amount of mass is preserved. This principle underpins chemical reactions, material balances, and many engineering calculations.

In chemical reactions, the mass of the reactants equals the mass of the products when all species are accounted for in a closed system. Mass balance takes this idea into account to track inflows, outflows, generation, and consumption of substances in reactors, environmental systems, and process engineering.

In fluid dynamics, the conservation of mass is expressed by the continuity equation. For a density ρ and velocity field v, ∂ρ/∂t + ∇·(ρv) = 0. If the fluid is incompressible, ρ is constant and the equation reduces to ∇·v = 0, meaning the flow has zero net source or sink of mass.

In modern physics, the concept is refined by mass–energy equivalence. While total energy and momentum are conserved in all physical processes, the mass of a system can change due to conversion between mass and energy. The rest mass of individual particles is not always conserved in reactions, but the total energy including kinetic and other forms is conserved.

Limitations include open systems, where mass can cross boundaries, and measurement or identification issues in complex mixtures. Still, mass conservation remains a robust and widely applied principle in science and engineering.

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