为什么火焰通常是黄色的？

人们形容红色时总是习惯拿火来做比喻：红如火，艳似霞。然而，我们通常看到的火大都是黄色和桔红色的，最多只有火芯一点接近红色。这又是为什么呢？

The typical, bright, yellowish-orange upper part of a flame is due to the heating of unburned carbon particles.

The temperature of the fire and the material being burned are the factors that determine the color of the flame. The various colors of flames in a wood fire are due to the different substances in the flames.

The strong orange color of most wood flames results when sodium¹ contained in the wood is heated.

The temperature of wood flames is lower than that of candle flames, which colors the wood flames orange, not yellow. If, however, some of the carbon particles in the fire are very hot, the color will be yellow. The product of the burnt carbon, when it has cooled, is black soot².

Since fire needs oxygen to burn, and since the bottom of a candle flame does not get much oxygen, it is the hottest spot in the flame and is blue in color.

The flame cools and changes color as it moves away from the source of the flame, because it is exposed to more oxygen. The temperature change causes the color of the flame to change from blue, at the hottest, lower portion of the flame, to the typical, bright, yellowish-orange or bright orange color with which most people are familiar with. Which shade of orange is seen at the upper portion of the flame, where the flame is the coolest, depends upon the material being burned.

sodium: 钠

soot: 烟灰