After burning your candle for a few hours you and your friend notice that it's mass has decreased. Your friend claims that the mass "disappeared". Why do you know he is wrong (Hint: it's a Scientific Law)? What actually happened to the mass? What form is that mass in now? What is this process called?

The law of conversion of mass is a scientific law that says that mass or matter can not be created or destroyed like energy but changes its form in a different type of state. So, by the law of conversion of mass, you can say that mass is not disappeared but change in different forms of states of matter like gas, soot, or melted residues of candle and ash of thread.

Due to this, the mass of the solid or waxy state due to burning changes to gas released by burning and residue of wax that will be equal to the mass of the original mass.

dynamic equilibrium is the answer.