Question

select all the true statements. molecules of two different substances can be composed of the same chemical elements but have different numbers of atoms. two different substances can be composed of different chemical elements. two different substances with crystal structures can be composed of the same chemical elements in the same ratio and also have the same arrangements of atoms.

Explanation:

1. For the first statement, isomers are examples where different substances have the same elements but different atom - counts. For example, ethanol ($C_2H_6O$) and dimethyl ether ($C_2H_6O$) have the same elements but different structures.
2. The second statement is true as various substances are made of different elements, like water ($H_2O$) and carbon dioxide ($CO_2$).
3. The third statement is false. If two substances have the same chemical elements in the same ratio and the same atom - arrangements, they are the same substance, not different substances.

Answer:

Molecules of two different substances can be composed of the same chemical elements but have different numbers of atoms.
Two different substances can be composed of different chemical elements.