change of a quantity

The increment of the value of a quantity $Q$ with time. The change may be expressed either infinitesimally at time $t$ by the differential $d Q$ or $d Q t$ , or in practice it may be expressed by a finite increment over the time interval $t 1 t 2$ , i.e. $Q t 2 − Q t 1$ , which may be written $Δ Q$ or $Δ Q t 1 t 2$

$Δ Q = Δ Q t 1 t 2 = Q t 2 − Q t 1$

Examples are: mass change, $Δ m$ ; amount of substance change, $Δ n$ ; volume change, $Δ V$ ; substance concentration change, $Δ c$ .

Source:

PAC, 1992, 64, 1569 (Quantities and units for metabolic processes as a function of time (IUPAC Recommendations 1992)) on page 1571