Original sentence: At one time, a mixture of water and alcohol was generally classified as a compound.Nowadays it is considered to be a mixture, but there was no reason then to suspect that it was not a compound.Water and alcohol would not separate spontaneously, but they could be separated when heated (see distillation)
Edited sentence: At one time, a combination of water and alcohol was generally classified as a compound.Nowadays it is considered to be a solution, but there was no reason then to suspect that it was not a compound.Water and alcohol would not separate spontaneously, but they could be separated when heated