Why is candle flame generally yellow?

Why commercial HNO_3 is generally yellow in colour ?

A student wants to project the image of a candle flame on the walls of the school laboratory by using a mirror. Can he use this mirror to project a diminished image of the candle flame on the same wall? State 'how' if your answer is 'yes' and 'why not' if your answer is 'no'.

A student wants to project the image of a candle flame on the walls of the school laboratory by using a mirror. At what distance, in terms of focal length 'f' of the mirror, should he place the candle flame to get the magnified image on the wall?

Name the process by which air above the candle flame gets heated.