Amount of substance Amount of substance Amount of substance is a standard-defined quantity that measures the size of an ensemble of particles, such as atoms, molecules, electrons, and other particles. It is sometimes referred to as chemical amount. The International System of Units (SI) defines the amount of substance to be proportional to the number of entities present. The SI unit for amount of substance is the mole. It has the unit symbol mol. The proportionality constant is the inverse of the Avogadro constant. The mole is defined as the amount of substance that contains an equal number of elementary entities as there
Mass flow rate Mass flow rate In physics and engineering, mass flow rate is the mass of a substance which passes per unit of time. Its unit is kilogram per second in SI units, and slug per second or pound per second in US customary units. The common symbol is formula_1 (ṁ, pronounced "m-dot"), although sometimes "μ" (Greek lowercase mu) is used. Sometimes, mass flow rate is termed "mass flux" or "mass current", see for example "Fluid Mechanics, Schaum's et al". In this article, the (more intuitive) definition is used. Mass flow rate is defined by the limit: The reason for the dot