Which principle states that mass cannot be created or destroyed?

Which principle states that mass cannot be created or destroyed?

• A. Conservation of Energy
• B. Duct Area
• C. Conservation of Mass
• D. Volumetric Flow Rate

Answer: (C)

Mass cannot be created or destroyed is about keeping track of what actually enters and leaves a system. In any process, you can imagine a control volume and write a mass balance: the mass flow into the volume must equal the mass flow out plus any change in stored mass. At steady state, there’s no accumulation, so the mass flow rate in equals the mass flow rate out. Since mass flow rate is density times volumetric flow rate, the same amount of mass can move through a pipe even if the density (and thus the volume) changes along the path. That’s why this principle is the best answer: it applies universally to fluids and other substances, regardless of changes in temperature, pressure, or phase. Energy conservation describes how energy moves or converts, not how much matter moves. Duct area is just a geometric property and doesn’t express a fundamental balance law. Volumetric flow rate alone can vary with density, so it isn’t a universal statement about matter conservation.

Mass cannot be created or destroyed is about keeping track of what actually enters and leaves a system. In any process, you can imagine a control volume and write a mass balance: the mass flow into the volume must equal the mass flow out plus any change in stored mass. At steady state, there’s no accumulation, so the mass flow rate in equals the mass flow rate out. Since mass flow rate is density times volumetric flow rate, the same amount of mass can move through a pipe even if the density (and thus the volume) changes along the path.

That’s why this principle is the best answer: it applies universally to fluids and other substances, regardless of changes in temperature, pressure, or phase. Energy conservation describes how energy moves or converts, not how much matter moves. Duct area is just a geometric property and doesn’t express a fundamental balance law. Volumetric flow rate alone can vary with density, so it isn’t a universal statement about matter conservation.