What is the function of a mordant in gram staining?

The function of mordant in the gram staining is to expose grams positive cell to the decolorizer which dissolves the lipids in the cell wall thus allowing the crystals violent-iodine to leach out of the cell. This facilitate the cell subsequently be stained by with safranin.

The function that a mordant has is to form a complex with the primary stain so that it becomes trapped with the bacteria that have structural properties. In Gram stain, the gram of iodine is used to form a complex with crystal violet, which becomes trapped in gram-positive cells. For this reason, its color is purple.