Why is heat necessary in gram staining?

Answer 1

Heat is needed to allow the sample to be fixed to the slide (ethanol can also be used to fix the sample to slide as well). By heating the slide, you not only allow the specimen to dry on the slide it also allows for the staining reagents to properly incorporated into the species membrane that is being stained. If you don't, everything would run off and not stain correctly during your washing steps with deionized or saline water.