BSL Chemistry Glossary - reactive - definition
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Definition: This sign has two forms: very reactive and not very reactive.
Some chemicals react together very quickly; we say they are very reactive. Other chemicals will react much more slowly; we say they are less reactive. For example, look at the link below to watch magnesium burn with oxygen. It's very reactive and happens quickly.
On the other hand some reactions are very slow. If you leave a nail outside it will get wet in the rain then hot in the sun. If you find it again a long time later you will see that it has rusted. This is the result of a very slow reaction.
reproduced by permission from Hodder