Liquid hydrocarbons like on hexane (C6H14), octane (C8H18) heating with some colored salts like NiSO4 produce black amorphous carbon on the surface of these salts turning them entirely black with the salt acting as catalyst by extracting carbon from the corresponding hydrocarbons. Colored cations like Ni2+, Cr3+ & their salts like NiSO4 K2Cr2O7, PbCrO4 can extract carbon from liquid hydrocarbons.
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