18. an experiment in which only one variable is changed is a(

controlled experiment

Explanation:controlled experiment/experiment in which only one variable is changed

in a chemical reaction, the atoms and molecules produced by the reaction are called products. no new atoms are created, and no atoms are destroyed. in a chemical reaction, reactants contact each other, bonds between atoms in the reactants are broken, and atoms rearrange and form new bonds to make the products.

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