reactions

Reactions refer to processes that produce change, or responses to stimuli. The term is used across disciplines to denote specific processes such as chemical changes, as well as everyday language to describe how organisms or people respond to events. The meaning depends on context.

Chemical reactions: A chemical reaction is a process that transforms reactants into products, often accompanied by energy release or absorption. Reactions are described by stoichiometry, balance, and rate laws. Factors affecting rate: temperature, concentration, surface area, catalysts. Equilibrium occurs when forward and reverse rates balance; Le Chatelier's principle explains shifts with stress. Common types: synthesis, decomposition, single-displacement, double-displacement, combustion. Energetics: exothermic vs endothermic.

Biochemical and biological reactions: In biology, reactions occur in metabolism, signaling, and gene regulation. Enzymes act as catalysts, increasing reaction rates under physiological conditions. Substrate binding specificity and regulation by cofactors control pathways.

Physics and engineering: The term reaction also appears in Newton's third law: every action has an equal and opposite reaction. Reaction forces are central to motion, propulsion, and structures. In materials science, reactions at surfaces influence corrosion, catalysis, and adhesion.

Human and social reactions: Humans and animals show behavioral reactions to stimuli. Reaction time is the latency between a stimulus and response, and reflexes are automatic, involuntary reactions mediated by the nervous system. Reactions can be conscious decisions or automatic responses.