Why is cng preferred over petrol or diesel?

Compressed natural gas is preferred as it is a very clean-burning fuel as in reality it is the cleanest of all fossil fuels. It consists mainly of methane and when it burns it produces carbon dioxide and water vapor which are exhaled when we breathe. Whereas petroleum produces higher carbon emissions, nitrogen oxides and sulfur dioxide and emits ash that worsen pollution. CNG usage is non-toxic to water levels also and does not contaminate the ground water table also.

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Yes, endothermic means it turns cold. and some chemical changes include change of tempature.