Page "Bunsen burner" Paragraph 8

If the collar at the bottom of the tube is adjusted so more air can mix with the gas before combustion, the flame will burn hotter, appearing blue as a result.

If the holes are closed, the gas will only mix with ambient air at the point of combustion, that is, only after it has exited the tube at the top.

This reduced mixing produces an incomplete reaction, producing a cooler but brighter yellow which is often called the " safety flame " or " luminous flame ".

The yellow flame is luminous due to small soot particles in the flame which are heated to incandescence.

The yellow flame is considered " dirty " because it leaves a layer of carbon on whatever it is heating.

When the burner is regulated to produce a hot, blue flame it can be nearly invisible against some backgrounds.

The hottest part of the flame is the tip of the inner flame, while the coolest is the whole inner flame.

Increasing the amount of fuel gas flow through the tube by opening the needle valve will increase the size of the flame.

However, unless the airflow is adjusted as well, the flame temperature will decrease because an increased amount of gas is now mixed with the same amount of air, starving the flame of oxygen.