During the starting of a vehicle, the exhaust smoke appears white. Why?

There are a number of reasons why this happens. One of the main ones is water which has condensed in the cylinders and exhaust pipes is being turned into steam from the hot exhaust gasses

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because when carburreters work to mix fuels with air during injection
molding most part fuel mixers get compressed n remaining part of mixers that
is unburne gases get free by exhaust outlet nd its appears white

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When vehicle start, den more quanity of fuel is cnvrt in smoke as cmpared to d latr....whch is initialy white and colour change of smoke latr.