The molecular weight of candle oil (hydrocarbon) is too large,

Too much carbon,

Incomplete combustion of oxygen in the air,

Produce all kinds of low-grade organic matter,

If the candle burns completely, there will only be water and carbon dioxide, and it will not be black.

The beauty of mathematics is happy to answer for you, please ask questions if you don't understand! Please accept it if you are satisfied, thank you! O(∩_∩)O~