reaction

Everyday

1. What someone says, does or feels because of something that has happened: her reaction to the news was a big smile. tree →
2. In science, a chemical reaction is when substances change into new substances, like vinegar and bicarbonate of soda fizzing. tree →
3. An allergic reaction is when the body responds badly to something, such as nuts or a bee sting. tree →
4. How quickly you respond: fast reactions. tree →
5. In forces, a reaction is the push back in the opposite direction. tree →
6. A reaction can also be a movement that turns back against a change or an earlier way of doing things. tree →

In the lessons

In science, signs of a chemical reaction include fizzing (a gas), a colour change, heat or light, or a new solid forming; burning and rusting are reactions that cannot be undone. In health, an allergic reaction can cause a rash, swelling or trouble breathing; a severe one (anaphylaxis) is an emergency — call for help and an adult may use an adrenaline pen. In sport and games, quick reactions help you catch a ball. In stories, a character's reaction shows how they feel. Phrases: "chemical reaction", "allergic reaction", "a quick reaction", "reaction time", "chain reaction".

Examples

• What was your reaction when you saw the surprise? tree →
• Mixing vinegar and baking soda causes a fizzy reaction. tree →
• She had an allergic reaction to the peanuts. tree →
• Rusting is a slow chemical reaction. tree →
• Goalkeepers need fast reactions. tree →
• His reaction was to burst out laughing. tree →
• We measured our reaction times with a falling ruler. tree →
• The fizzing told us a reaction was happening. tree →
• When the Hindu reaction against Buddhism came, the Brahmins adapted these, with the omission of Buddha as the central figure. tree →

Note

Countable: a reaction, reactions. Verb: react; adjective: reactive. Melting and dissolving are usually changes, not chemical reactions, because no new substance is made. Any sign of a bad allergic reaction (swelling of lips or tongue, wheezing) needs an adult and emergency help at once.