Why are elements and compounds classified as pure substances?

Pentane with molecular formula c5h12, exists in three isomeric forms. one shows linear carbon chains, another has one -ch3 groups present on the third carbon atom, and the third has two -ch3 groups present on the second carbon atom. what types of isomers are these? a. geometric isomers b. structural isomers c. halotropic isomers

Reactants undergo chemical reaction to form products. this chemical equation represents one such reaction. the coefficient for one of the reactants or products is incorrect. which part of the chemical equation is incorrect?

In general, are there any major differences that you can see? explain