Why do sometimes cooking vessels get blackened on a gas or kerosene stove?

Why do some metals not get rusted ?

While cooking, if the bottom of the vessel is getting blackened on the outside, it means that

Why do we get rains?

Why do subcutaneous blood vessels bulge on the side away form the heart when the hand is tied?

Why some metals are used for making cooking vessels?

Why do statues and vessels made up of copper become dull green after certain time ?

Why do the number of organisms get decreased as we move from producer to different level of consumes?

(A) : In isobaric process, during an expansion of a gas both volume and temperature of the gas increase (R ): Heat added to a gas partly gets converted into internal energy and remaining to do external work