When carbon is burned in the presence of oxygen, the primary substance produced is carbon dioxide (CO2).

Here's the chemical reaction:

C + O2 → CO2

This is a combustion reaction, where carbon reacts with oxygen to release energy in the form of heat and light.

However, the specific substance produced can vary depending on the conditions of the burning process:

* Incomplete combustion: If there's not enough oxygen present, carbon monoxide (CO) can also be produced. This is a toxic gas.

* Other impurities: Depending on the source of the carbon and the impurities present, small amounts of other substances like soot (carbon particles) or sulfur dioxide (SO2) can also be formed.

So, the most common and primary product of burning carbon is carbon dioxide.