Why does the outer cone of a Bunsen flame have oxidizing properties?

The outer cone of a Bunsen flame exhibits oxidizing properties primarily due to the presence of excess oxygen from the surrounding atmosphere.1

In a standard Bunsen burner, the flame consists of two distinct zones: an inner core and an outer cone. While the inner core is the site of primary combustion, the outer cone acts as a zone of diffusion flame. As the remaining fuel gases from the inner core move outward, they encounter and mix with the surrounding air. This diffusion process ensures that any residual fuel is completely oxidized by the available atmospheric oxygen. Consequently, this region is characterized as an oxidizing zone, contrasting with the inner core, where different chemical reactions—often reducing in nature—take place.12