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react

/riˈækt//riˈækt/·verb
to act or change in response to something that happens
Fig. 1 — A sprung plate sits at rest, doing absolutely nothing, because nothing has touched it yet.
01Definition

To react is to act back — to respond to a stimulus, an event, or a force. The prefix re- ('back') joined to act gives the whole idea: nothing reacts until something first acts upon it. People react to news with joy or alarm; markets react to a single rumour; iron reacts with damp air and slowly begins to erode. A reaction carries no plan of its own — it is shaped entirely by what triggered it, which is why a measured response is so much harder than a reflex.

02In use
  • iInvestors reacted to the rumour before anyone could confirm whether it was true.
  • iiWise leaders anticipate a crisis instead of waiting to react once the damage is done.
  • iiiWhen the two solutions were mixed, they reacted at once, fizzing and changing colour.
03Collocations
  • react to a situation
  • react badly
  • quick to react
  • react with alarm
  • an instinct to react

Family reaction (noun) · reactive (adjective) · reactionary (adjective) · reactor (noun)

04Relations

=respond, answer, reply, retaliate, counter

≠ignore, anticipate

06TOEFL & IELTS

React is everyday vocabulary, so in Writing the move is to upgrade around it — 'respond to' or 'address' often read as more academic. Still, react is precise for involuntary or immediate responses (in TOEFL science lectures two chemicals react; in IELTS, public opinion reacts). Mind the preposition — you react TO something — and the noun reaction, which collocates with provoke, trigger, and chain.

07Asked
Is it react to, react with, or react against?
Three different jobs. React to takes the trigger and is the default: markets react to news. React with takes the emotion shown — she reacted with alarm — or, in chemistry, the partner substance: iron reacts with oxygen. React against is deliberate opposition: a generation reacting against its parents' taste. Only the first is neutral; the other two each add their own story.
What's the difference between reacting and responding?
Speed and authorship. A reaction is immediate and shaped by the trigger; a response is considered and shaped by the responder. The scene above is all reaction: the size of the blow sets the size of the answer, and the sprung plate contributes nothing of its own. A response, by contrast, implies an agent who weighed the moment and chose — which is why respond and response read as more deliberate and more academic, the shade exams reward.
What does react mean in chemistry?
Substances meeting and becoming different substances: the reactants interact and turn into products, as vinegar and baking soda fizz into something neither was. The verb takes with for the partner (sodium reacts violently with water) and carries no hint of intention — chemical reacting is pure cause and effect, which is why the sciences adopted the word so comfortably.
What's the difference between reactive and reactionary?
False friends inside one family. Reactive is neutral-to-technical: responding to events rather than anticipating them — a reactive strategy, a reactive chemical. Reactionary is political and pointed: opposed to change and wanting an earlier order restored. Calling a manager reactive critiques their planning; calling them reactionary accuses them of fighting the future. Exams punish the swap.
What does overreact mean?
To answer with more force than the trigger deserves — tears over a typo, a lawsuit over a joke. The prefix does the measuring: over- says the response outweighed the stimulus. Its mirror, underreact, exists and is rarer, mostly in analysis of crises ('officials underreacted at first'). Between them they mark the scale on which every reaction gets judged.