Why is fire blue

Why is fire red-yellow?

Heat makes soot particles glow

The red and yellow that we see in the flames by the fireplace or in the candle is nothing but glowing soot. So the color only comes from the heat that makes the soot particles glow. It has to do with the combustion process as such only insofar as the combustion provides the heat.

Because what happens when we burn wood or - in a candle - wax? In both cases, the carbon it contains is oxidized, i.e. it combines with the oxygen in the air. This creates either the notorious carbon dioxide or carbon monoxide - i.e. gases - as a waste product. It's a chemical reaction. But because the oxygen supply is restricted in a normal flame, the combustion is always incomplete - there is always a lot of carbon left over that does not oxidize - that is the soot.

During the combustion, however, heat is released at the same time. And so temperatures of 1200 to 1400 degrees arise in the flame. In this heat, the soot particles start to glow - that's the yellow-red thing we see.

Why does something turn red when it gets hot and glowing?

As a rule, this is the so-called black-body radiation. That sounds contradicting, because the body is not black, but red. The expression is supposed to say, however, that this radiation is independent of the color that an object otherwise has. This means that a body emits a certain basic radiation at any temperature. Most of the time we don't see it because it is not in the range of visible light. But when the body gets hotter - i.e. at around 1200 degrees - this basic radiation takes on a red-yellow color. And when it gets even hotter, more and more bluish parts appear, so that the body becomes whiter overall.

What if the flame is blue?

Blue flames are more a sign that the fuel is completely burned and that no soot is produced. However, some fabrics also take on a typical color when they burn. We know this from fireworks, where different metals are used to create certain colors in the sky. But then this is not a “glow color”, but there the color arises from a chemical reaction.

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