Question

Is carbon dioxide always formed when a hydrocarbon is burnt?

Answer 1

yess
because all the hydrocarbon have carbon which rects with oxygen in air to form carbon dioxide.
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Answer 2

yes,If there is insufficient air for complete combustion, incomplete combustion (also called partial combustion) happens. Hydrogen is still oxidised to water, but carbon monoxide formsinstead of carbon dioxide. ... They reduce the amount of sunlight reaching the Earth's surface.