Carbon burns in oxygen to form carbon dioxide gas. The reaction is highly exothermic. That is why different forms of coal are used as fuels. The most important compounds of carbon are hydrocarbons. Just like carbon, hydrogen also readily burns in oxygen or air to form water a heat. The hydrocarbon methane (CH₄) is the major constituent of natural gas. Propane (C₃H₈) and butane (C₄H₁₀) are present in liquid petroleum gas (LpG). Petrol and kerosene also contain different hydrocarbons. Therefore, these are used as fuels.