Where does reactivity collide?

In chemistry, reactivity is how readily a substance undergoes a chemical reaction: how fast it reacts, with what, and how completely. It covers both the speed of a reaction and the tendency of a species to form or break bonds, and chemists rank elements and compounds in a reactivity series and explain the patterns through electronic structure. Pearson's 1963 Hard and Soft Acids and Bases gave one influential account, predicting which acids and bases react most favourably from the softness or hardness of their electron clouds. The referent here is a physical propensity of matter to transform, quantified by reaction rate and yield, not a body's response to stress, an artefact of being observed, or a reactor's distance from criticality.[1]

Unit

A substance's propensity to react, scaled by reaction rate and yield

Recovery means

Not applicable

Measured by

Assessed by rate constants, activation energies, and position in the reactivity series

In psychology and psychophysiology, reactivity is the magnitude of a person's physiological or emotional response to a stressor, together with how quickly the body returns to baseline afterwards. Researchers measure how far heart rate, blood pressure, or stress hormones rise when someone faces a challenge, and treat large or prolonged responses as a marker of risk. Krantz and Manuck's 1984 Acute psychophysiologic reactivity and risk of cardiovascular disease reviewed the evidence that exaggerated cardiovascular reactivity may contribute to heart disease, and set out how it ought to be measured. The referent is a living body's reaction to demand, scaled by the size of the change from rest, not a chemical propensity, a distortion introduced by measurement, or a nuclear reactor's state.[2]

Unit

The size of a body's response to a stressor, as change from resting baseline

Recovery means

Return toward baseline after the stressor ends is itself part of the construct and is often timed separately

Measured by

Recorded as the rise in heart rate, blood pressure, or cortisol during a standardised laboratory challenge

In research methods, reactivity is a threat to valid measurement: the act of observing or measuring people changes the very behaviour being studied, so the record reflects the study as much as the world. The classic example is the Hawthorne effect, where workers changed their output because they knew they were being watched. Webb, Campbell, Schwartz, and Sechrest's 1966 Unobtrusive Measures: Nonreactive Research in the Social Sciences named this problem and argued for measures that do not disturb what they record, and McCambridge, Witton, and Elbourne's 2014 review re-examined how large such effects really are. The referent is a flaw in the link between a measurement and its target, judged by how far being studied shifts the result, not a property of a chemical, a body's stress response, or a reactor's physics.[3][4]

Unit

The amount that being observed shifts the behaviour or answer being recorded

Recovery means

Not applicable

Measured by

Estimated by comparing observed groups against unobtrusive or control conditions, or across differing levels of visibility

In nuclear engineering, reactivity measures how far a reactor is from criticality, the balanced state in which each generation of neutrons exactly reproduces the last. A positive reactivity means the chain reaction is growing, a negative one means it is dying away, and it is defined from the neutron multiplication factor as the fractional departure from a self-sustaining balance. It is expressed in special units, notably dollars and cents tied to the delayed-neutron fraction, or in per-cent-mille (pcm). Lamarsh's 1966 Introduction to Nuclear Reactor Theory gives the standard treatment of how reactivity governs a reactor's behaviour over time. The referent is a control quantity of a fission system, read from the neutron population and the reactor period, not a chemical propensity, a person's stress response, or a bias in measurement.[5]

Unit

A reactor's fractional departure from criticality, in dollars, cents, or per-cent-mille (pcm)

Recovery means

Not applicable

Measured by

Inferred from the neutron multiplication factor and reactor period, or by control-rod worth measurements