Certain chemicals induce neuromodulation.
 A common example of a chemical substance is pure water; it has the same properties and the same ratio of hydrogen to oxygen whether it is isolated from a river or made in a laboratory
 Likewise, the idea of stereoisomerism  that atoms have rigid three-dimensional structure and can thus form isomers that differ only in their three-dimensional arrangement  was another crucial step in understanding the concept of distinct chemical substances

