In which reaction is a small molecule formed from the atoms removed from a single reactant molecule?
addition
condensation
elimination
substitution

Addition reaction, a chemical reaction which occurs when an atom or group of atoms is added to a molecule.

Condensation reaction is a chemical reaction that takes place when two molecules join to create a larger molecule, as it losses a small molecule.

Substitution reaction, also termed as single displacement reaction or single replacement reaction, happens when one functional group in a chemical compound is replaced or displaced by another functional group.

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