Is lightning flash chemical or physical change?
It is a physical change. I can tell you the whole story about lightning but all you need to know is- it has nothing to do with chemicals or chemical reactions.
You see a bright flash and then flames during a class demonstration Is this an example of physical change or a chemical change explain?
It is an example of a chemical change in that fire is an oxidation reaction. It is also an example of a physical change: if the bright flash and then flames were not the intended result of the experiment as approved by your teacher, the student responsible will be displaced from the science room to the office to explain to the principal why he tried to blow up the science lab.