A chemical substance is a material with a specific chemical composition. 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.
About fifteen miles above New Orleans the river goes very slowly. It has broadened out there until it is almost a sea and the water is yellow with the mud of half a continent. Where the sun strikes it, it is golden. Frank Yerby
All through the night, like the tumult of a river when it races between the cliffs of a canyon, in my sleep I could hear the steady roar of the passing army. Richard Harding Davis