Petrol is naturally a colourless liquid. But the refineries add dyes to the finished product. This is done for a variety of reasons, mainly to identify the product as their petrol, to differentiate different grades of petrol. In some countries, diesel fuel in particular, that is bought for agricultural use, has a reduced taxation application. In order to prevent this from being used for non-agricultural purposes, it is dyed a distinctive colour,
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Petrol is a mixture of different hydrocarbons which usually forms a transparent solution. This is mainly due to carbon and hydrogen atoms within linear carbon chains which form a clear liquid.