When a person is about to die and leave his mortal body it is said that we must put gangajal i.e. water of holy river Ganga and tulsipatra i.e. leaves of sacred Tulsi plant in the person's mouth. What is the significance of this ritual? Is it necessary to do this? Does this have any scientific significance as well?
According to the Brahmavaivata-Purana (Madhya, 26/59)
gangA-narAyanakshetre pivan gangAjalam narah/ Ganga-Gangeti sriNvanto mrito vA kim na sAdhayet//
meaning that taking Ganga-water at the time of death gives all the possible virtues.
In the Agastya-Samhita we get
mukhasthA tulasi yasya nirvANa-padadAyini (7)
meaning that tulasi-leaf kept in the mouth liberates an individual.
So giving ganga-jal and tulasi-patra is advised at the time of death by the holy scriptures.
I do not think science has anything to do here. There is no concept of virtue or liberation in science and that is I hope is known to all!