"Black carbon powder" typically refers to finely divided carbon particles that are black in color. It is often produced through the incomplete combustion of carbon-rich materials, such as fossil fuels, biomass, or waste. Black carbon consists of elemental carbon in the form of soot or other similar materials.
However, the term "soluble" in relation to black carbon powder is a bit contradictory. Generally, carbon particles, including black carbon, are not soluble in water or other common solvents. Carbon is a nonpolar material, and as a result, it doesn't readily dissolve in polar solvents like water. Instead, it tends to disperse and form suspensions.
If you are referring to a specific product or substance labeled as "soluble black carbon powder," it might be a product that has been chemically modified or treated to increase its solubility in certain solvents. Such modifications could involve adding functional groups to the carbon particles that make them more compatible with certain solvents.