Reaction

Common categories

Reactions are often grouped by discipline. Typical categories include:

• Chemical reactions — rearrangements of atoms and electrons that form new substances.
• Physical reactions / mechanical — forces and motions produced as a consequence of other forces (e.g., Newtonian reaction force).
• Biological and physiological reactions — reflexes, immune responses, and metabolic changes.
• Psychological and social reactions — mental or collective responses such as emotion, opinion, or political backlash.

Key characteristics and examples

Chemical reactions are characterized by reactants, products, energy changes (exothermic or endothermic), and rates governed by kinetics and conditions like temperature and catalysts. Physical reactions include the equal-and-opposite forces described by classical mechanics and conservation laws. Biological reactions range from immediate reflex arcs to slower adaptive processes such as immune activation. Psychological reactions encompass perception, judgment and emotion; social reactions describe how groups respond to news, policy, or events.

History and development

The modern scientific understanding of reactions grew with the development of chemistry and physics in the 18th and 19th centuries, and later with advances in physiology and psychology. The conceptual unifier is causality: reactions link an antecedent cause (stimulus, collision, or interaction) to a consequent change.

Importance and distinctions

Recognizing which type of reaction applies in a situation is important for prediction and control: chemists manipulate reactants and conditions, engineers design systems to manage mechanical reactions, physicians treat pathological physiological reactions, and communicators shape social reactions. Distinctions to note include reversible versus irreversible processes, spontaneous versus induced responses, and immediate versus delayed reactions.

Notable facts

• Reaction time is a measurable property in physiology and human factors, distinct from the outcome of a reaction.
• Catalysts speed chemical reactions without being consumed.
• In mechanics, "reaction force" is not a separate cause but a consequence of interactions governed by Newton's third law.