Where does reaction collide?

A process in which reactant substances are transformed into products through the breaking and forming of chemical bonds, rearranging atoms into new molecules. Eyring's 1935 The Activated Complex in Chemical Reactions gave the transition-state account of how fast such a change proceeds, treating the rate as set by an activated complex poised at the top of an energy barrier. The referent is a transformation of matter at the molecular level, measured by its rate and described by a mechanism, and it is not a mechanical push-back, a person's response delay, or the body's defence against a foreign substance.[1]

Unit

One reactant-to-product transformation, scaled by rate and yield

Recovery means

A reversible reaction can run back toward reactants until it settles at equilibrium

Measured by

Followed by reaction rate, by the change in concentration over time, and by the energy barrier inferred from how the rate varies with temperature

The equal and opposite force a body exerts back on whatever pushes or pulls on it. Newton's 1687 Principia stated the third law: to every action there is always an equal and opposite reaction, so when a book presses down on a table the table presses up with the normal reaction, and a support holds a load by reacting against it. The referent is a mechanical force paired with an applied action, measured in newtons, and it is not a transformation of matter, a delay in responding, or an immune defence.[2]

Unit

A force in newtons, equal and opposite to an applied action

Recovery means

Not applicable

Measured by

Computed from the applied load through Newton's third law, or read directly from a force sensor placed at a support

The interval between the onset of a stimulus and the start of a person's response to it, used as a measure of how long mental processing takes. Donders's work, published in translation in 1969 as On the speed of mental processes, introduced the subtraction method: by comparing simple and choice tasks he estimated the duration of hidden mental steps. The referent is a measured duration of human response, timed in milliseconds, and it is not a chemical transformation, a mechanical push-back, or a bodily defence.[3]

Unit

An elapsed time in milliseconds between stimulus and response

Recovery means

Not applicable

Measured by

Recorded as the latency from stimulus onset to a keypress or spoken response, averaged across many trials

The body's defensive response to a foreign substance, mounted by antibodies and immune cells, and in its harmful form an allergic or hypersensitivity reaction. Rajan's 2003 re-interpretation of the Gell and Coombs classification set out four types of hypersensitivity reaction, ranging from immediate antibody-driven responses to delayed cell-mediated ones. The referent is a biological defence unfolding in living tissue, graded by type and severity, and it is not a rearrangement of chemical bonds, a mechanical force, or a stopwatch reading.[4]

Unit

A defensive immune response to an antigen, graded by type and severity

Recovery means

The response subsides as the antigen is cleared, though prior sensitisation can leave the body primed to react faster on later exposure

Measured by

Classified by mechanism and timing, and assessed clinically by skin tests, antibody levels, and the response to a controlled challenge